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Thrips Competition and Spatiotemporal Dynamics on Reproductive Hosts

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PAGE 1

THRIPS COMPETITION AND SPAT IOTEMPORAL DYNAMICS ON REPRODUCTIVE HOSTS By TOBIN D. NORTHFIELD A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2005

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Copyright 2005 by Tobin D. Northfield

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This document is dedicated to my wife, Kirsten for all of her support and understanding throughout this process. It is also dedicated to my parents for their guidance and support.

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ACKNOWLEDGMENTS I would like to thank my major professor, Dr. Joe Funderburk, for the guidance and assistance in designing and conducting my experiments, and for helping me develop as a scientist. I would also like to thank Dr. Dean Paini, a committee member, who also spent a great deal of time advising and assisting me whenever possible. My other committee members, Drs. Stuart Reitz and Russ Mizell, were a tremendous source of assistance and knowledge, for which I am grateful. Dr. Michelle Stuckey proofread drafts and assisted in the experimental procedure, as well as offered good advice. I would like to thank Dr. Brian Inouye for his assistance in interpreting the models, and the model-fitting process. I appreciated Dr. Ben Bolkers patience and instruction on the modeling process and using R to fit the models. Marcus Griswold helped in providing an R code to work with. Dr. Todd Jackson also lent assistance and insight in developing my experiments. Rebecca Riddle also assisted in the experimental procedure, for which I am grateful. iv

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TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iv LIST OF TABLES............................................................................................................vii LIST OF FIGURES...........................................................................................................ix ABSTRACT.........................................................................................................................x CHAPTER 1 LITERATURE REVIEW.............................................................................................1 Introduction...................................................................................................................1 Diversity.......................................................................................................................2 Population Attributes....................................................................................................3 Plant Defenses..............................................................................................................4 Host Selection and Nutrition........................................................................................5 Within Plant Distribution..............................................................................................7 Seasonal Dynamics.......................................................................................................7 Biotic Factors................................................................................................................9 North Florida Thrips...................................................................................................10 Reproductive Hosts.....................................................................................................11 Conclusion..................................................................................................................12 2 SPATIOTEMPORAL DYNAMICS ON REPRODUCTIVE HOSTS.......................17 Introduction.................................................................................................................17 Materials and Methods...............................................................................................18 Sampling Procedure.............................................................................................18 Data Analysis.......................................................................................................19 Results.........................................................................................................................21 Discussion...................................................................................................................25 Conclusion..................................................................................................................30 3 INTRASPECIFIC AND INTERSPECIFIC COMPETITION IN THRIPS ON FLOWERING PEPPER PLANTS ............................................................................43 Introduction.................................................................................................................43 v

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Materials and Methods...............................................................................................45 Experimental Design...........................................................................................45 Model Fitting.......................................................................................................47 Results.........................................................................................................................48 Discussion...................................................................................................................49 Effects of Competition on F. occidentalis Population Abundance in Florida....50 Effects of Competition on World-Wide F. occidentalis Spread.........................51 4 CONCLUSION...........................................................................................................56 LIST OF REFERENCES...................................................................................................58 BIOGRAPHICAL SKETCH.............................................................................................69 vi

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LIST OF TABLES Table page 1-1 F. occidentalis Reproductive Hosts.........................................................................13 1-2 F.fusca Reproductive Hosts.....................................................................................14 1-3 F. tritici Reproductive Hosts....................................................................................15 1-4 A list of plants F. bispinosa Reproductive Hosts.....................................................16 2-1 Mean number (SEM) of thrips per 20 Raphanus raphanistrum flowers.................32 2-2 Mean number (SEM) of thrips per 20 Raphanus raphanistrum leaves...................33 2-3 Mean number (SEM) of thrips per 20 Raphanus raphanistrum fruits.....................34 2-4 Mean number (SEM) of thrips per 20 Rubus trivialis flowers.................................34 2-5 Mean number (SEM) of thrips per 20 Rubus trivialis leaves...................................35 2-6 Mean number (SEM) of thrips per 20 Rubus trivialis fruits....................................35 2-7 Mean number (SEM) of thrips per 20 Rubus cuneifolius flowers............................36 2-8 Mean number (SEM) of thrips per 20 Rubus cuneifolius leaves..............................36 2-9 Mean number (SEM) of thrips per 20 Rubus cuneifolius fruits...............................36 2-10 Mean number (SEM) of thrips per 20 Vicia sativa flowers.....................................37 2-11 Mean number (SEM) of thrips per20 Vicia sativa leaves........................................37 2-12 Mean number (SEM) of thrips per 20 Vicia sativa leaves.......................................37 2-13 Mean number (SEM) of thrips per 4 Vicia sativa buds............................................38 2-14 Mean number (SEM) of thrips per 4 Trifolium repens racemes..............................38 2-15 Mean number (SEM) of thrips per 20 Trifolium repens leaves...............................39 2-16 Mean number (SEM) of thrips per 20 Solidago canadensis racemes......................40 vii

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2-17 Mean number (SEM) of thrips per 20 Solidago canadensis leaves.........................40 2-18 Mean number (SEM) of thrips per 20 Chenopodium ambrosioides racemes..........41 2-19 Mean number (SEM) of thrips per 20 Chenopodium ambrosioides leaves.............41 2-20 Mean number of thrips per plant for seven plant species on selected dates.............42 3-1 The mean number (SEM) of larvae per female of adult female F. occidentalis and F. bispinosa at various levels of densities of each species................................52 3-2 Model parameters and confidence intervals measuring effects of competition on the number of larvae produced per F. bispinosa female..........................................53 3-3 Model parameters and confidence intervals measuring effects of competition on the number of larvae produced per F. occidentalis female......................................53 viii

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LIST OF FIGURES Figure page 3-1 Example of an additive design.................................................................................53 3-2 Example of a substitutive design..............................................................................54 3-3 Treatments of varying F. bispinosa and F. occidentalis densities to measure the larvae produced per female at different levels of competition.................................55 3-4 Simulation of intraspecific competition of F. bispinosa and F. occidentalis based on the competition model.........................................................................................55 ix

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Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science THRIPS COMPETITION AND SPATIOTEMPORAL DYNAMICS ON REPRODUCTIVE HOSTS By Tobin D. Northfield August, 2005 Chair: Joe Funderburk Major Department: Entomology and Nematology Frankliniella spp. thrips feed and reproduce on crops, causing a silvering of plant tissue, and spread plant diseases such as Tomato spotted wilt virus into crops. However, little is known about the factors affecting Frankliniella spp. thrips abundance and distribution. Thrips often migrate into cropping systems from surrounding vegetation, but few of these uncultivated plant sources have been studied to determine cycles of thrips abundance to evaluate sources of thrips migration. Furthermore, no research has been conducted on the population effects of competition in thrips to better understand thrips distribution. This study was composed of a field work portion, which focused on evaluating uncultivated plant host use, and a laboratory portion, which focused on competitive interactions between Frankliniella occidentalis and F. bispinosa. For the field work study, samples were collected from Raphanus raphanistrum, Rubus trivialis, Rubus cuneifolius, Trifolium repens, Vicia sativa, Solidago canadensis, and Chenopodium ambrosioides, and thrips spatiotemporal dynamics were determined x

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for each. Thrips preferred flowers to leaves on every plant species sampled. Frankliniella spp. thrips were most abundant on R. raphanistrum, T. repens, and R. cuneifolius in spring months and S. canadensis in the fall. The most abundant thrips species collected were F. tritici and F. bispinosa. These plants may serve as important sources of Frankliniella spp. thrips, and reducing abundance of the available hosts may decrease thrips populations migrating into cropping systems. For the laboratory study, interspecific competition was evaluated between the world-wide crop pest F. occidentalis, and F. bispinosa, a species native to Florida. In addition, intraspecific competition for each species was assessed. Larvae per female of F. bispinosa and F. occidentalis were counted at varying densities of each species, using a factorial response surface design. A competition model was fit to the data for each species to evaluate effects of interspecific and intraspecific competition on the number of larvae per female produced. Significant interspecific and intraspecific competition affected F. bispinosa, and the effect of interspecific competition from F. occidentalis was over four times greater than the effect from intraspecific competition. Interspecific competition did not affect F. occidentalis, but statistically significant intraspecific competition occurred. Furthermore, intraspecific competition had a greater effect on F. bispinosa than on F. occidentalis. This superior competitive ability may enhance the spread of F. occidentalis, and competition between F. occidentalis and native species must be assessed when considering the world-wide spread of F. occidentalis. xi

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CHAPTER 1 LITERATURE REVIEW Introduction Debate over the factors that affect thrips population dynamics have persisted for more than 50 years, since Davidson and Andrewartha (1948a, b) first claimed there were no density dependent factors affecting populations. Several researchers subsequently refuted their conclusions (Smith 1961, Orians 1962), and more recently both density dependent and density independent factors have been shown to influence thrips population dynamics. These factors include host defenses (deJager et al. 1996), plant selection and nutrition (Brodbeck et al. 2002), climate (Brdsgaard 1993), predation (Baez et al. 2004), and parasitism (Funderburk et al. 2002). Despite these studies, little is known of the temporal and spatial dynamics of thrips, particularly outside cropping systems. The purpose of this chapter is to discuss recent research on density dependent and independent factors that affect thrips spatial and temporal dynamics. The population dynamics of thrips are strongly influenced by their small body size (0.5-5.0 mm in length), which confers both advantages and disadvantages. Such disadvantages include large fluctuations in body temperature and water loss, due to a high surface area to volume ratio (Kirk 1997a). Conversely, thrips small size permits escape from predators to small, secure areas on the host (Sabelis and Van Rijn 1997), and also may result in extensive wind dispersal as gusts of wind can disrupt thrips flight patterns (Pearsall and Myers 2001). 1

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2 Diversity An appreciation of the natural history and diversity within the Thysanoptera order can reveal similarities in the population characteristics between phylogenetically related species, thereby enhancing the current understanding of thrips population dynamics. The diversity of the thrips order is exemplified by the diversity in feeding habits, which have evolved from that of a fungus feeding ancestor related to Hemiptera, Psocoptera, and Pthiraptera (Mound 1997, Moritz et al. 2001) to species that have adapted to feed on leaves, flowers and small arthropods (see Kirk 1997b for review). The order Thysanoptera is divided into two sub-orders, the Tubulifera and Terebrantia. The Tubulifera consists of a single family, the Phlaeothripidae, which consists of over three thousand species, mostly living on fungus in wet tropics (Moritz et al. 2001). Tubulifera use a U-shaped ovipositor, rather than a straight ovipositor like the Terebrantia. The U-shaped ovipositor is used to deposit eggs on the surface of, rather than into the host tissue, as the fungus provides adequate protection for the eggs (Terry 1997). In contrast to the predominantly fungivorous Tubulifera, the sub-order Terebrantia includes eight families of thrips that display a wide variety of food preferences and use a saw-like ovipositor to insert one egg at a time into the host tissue (Terry 1997). The largest and most diverse family of Terebrantia is the Thripidae, which are represented by over 1,750 species in 260 genera. Species of Thripidae range from Greenland to the sub-Antarctic islands (Moritz et al. 2001). One sub-family of Thripidae, the leaf-feeding Pancheatothripinae, comprising 120 species in 35 genera, is found throughout the tropics and sub-tropics, and includes some crop pests. Thripinae, a more diverse sub-family of Thripidae, consists of approximately 1,400 species in over 200 genera. Many feed and

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3 oviposit in leaves, and some of the more recently evolved species feed and oviposit in flowers. This group exhibits a wide variety of feeding habits and includes thrips species that are predaceous, anthophagous, phytophagous, or even associated with mosses (Mound 1997). The wide variety of food preferences of many Thripinae species includes commercial crops, and some of these species can cause direct damage to crops by feeding and oviposition as well by vectoring plant viruses (Mound 1997, Moritz et al. 2001). Population Attributes Thysanoptera are opportunistic and r-selected, utilizing a high reproductive rate and short generation time to enhance population growth rates under favorable conditions (Mound 1997) and the exploitation of ephemeral resources (Mound and Tuelon 1995). Parthenogenesis, a form of asexual reproduction, is a strategy used to enhance reproduction, and in combination with F 1 back-crossing, can result in a single female forming an entirely new population (Mound and Tuelon 1995). Furthermore, parthenogenesis and back-crossing of an insecticide-resistant female can lead to rapid growth of an insecticide-resistant population, further aiding in adaptation. Vagility enhances the opportunistic, r-selected strategy, improving location and exploitation of new environments and food resources (Mound and Tuelon 1995). A moderately broad food tolerance enhances vagility by enabling invasive thrips to survive in new environments of limited host diversity and increases population stability during seasonal decline of preferred host availability (Mound 1997). The extent of vagility and opportunism in thrips varies by feeding group (Mound and Tuelon 1995). Polyphagous thrips are more vagile than monophagous thrips, which often develop periodicity with the cycle of food availability. Anthophagous thrips include many polyphagous, vagile species that can exploit ephemeral resources.

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4 Alternatively, foliage-feeding thrips include few polyphagous, vagile species, as many develop a cyclic lifestyle in line with the host. The most damaging and opportunistic crop pests are those that feed on both flowers and leaves, and move to new food sources as hosts become inadequate or unavailable without developing host-correlated periodicity. Predaceous thrips are also opportunistic, due to the opportunistic nature of their prey, but few fungal spore or hyphae feeders are vagile or opportunistic because their food source is stable (Mound and Tuelon 1995). Exceptions include those in the tropics, where dry fungi hanging in trees are preferred, and thrips must feed before the fungi falls to the ground. Gall-forming thrips are monophagous, and feed in a stable environment, and are therefore less opportunistic than other thrips species. Plant Defenses A generalist strategy allows phytophagous thrips to feed on a number of hosts and gain access to a variety of available nutrients (Mound and Tuelon 1995). However, a generalist strategy usually includes constraints in plant defense adaptation. Theoretically, a generalist cannot coevolve with a range of hosts as well as a specialist can with a single host plant, due to evolutionary constraints (Ananthakrishnan and Gopichandran 1993). Although the vagility and opportunistic nature of phytophagous thrips enhances adaptability, plant defenses may cause one host to be less suitable than others. Morphological plant defenses, such as dense trichomes, limit phytophagous thrips host suitability. For example, nectarines and peaches are the same species (Prunus persica (L.) Batsch), but Frankliniella occidentalis prefer nectarine hosts due to the smoother tissue of the developing fruit (Felland et al. 1995). Other examples include surface wax and epidermal cell wall thickness, which reduce leaf host quality (Zeier and

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5 Wright 1995), and pollen stickiness, which contributes to a plants defense due to extra handling and grooming time (Kirk 1985). Plant chemical defenses, or allelochemicals, reduce thrips' host preference as well (deJager et al. 1995a, deJager et al. 1995b, Kumar et al. 1995). Allelochemical precursors may include acetyl coenzyme A, mevalonic acid, and shikimic acid and are grouped into quantitative or qualitative defenses (Lowman and Morrow 1998). The effect of quantitative defenses, or digestive reducers, varies by concentration. These immobile, carbon-based chemicals accumulate with tissue age, and passively or actively decrease thrips nutrition. Qualitative defenses are mobile chemicals that affect essential functions, such as respiration or DNA repair, in small chemical concentrations and degrade quickly. Resistance to thrips may be caused by a single chemical present in the plant tissue or by a synergistic effect from a number of chemicals (deJager et al. 1996). These chemicals may be found in a number of plant parts, including leaves or flowers; however constitutive (continuously present) defenses may be more concentrated in flower tissue than in leaves (Strauss et al. 2004). Host Selection and Nutrition Thrips feeding and oviposition choices may be due in part to thrips host location cues. Host location cues may include visual cues such as the colors blue, white and yellow (Frey et al. 1994, Cho et al. 1995a, Childers and Brecht 1996, deKogel and Koschier 2002). Olfactory host location cues may also be important, but the level of importance is unclear. In choice tests, F. occidentalis individuals were attracted to volatile chemicals extracted from chrysanthemum flowers, but could not locate whole flowers without visual cues (Koschier et al. 2000, deKogel and Koschier 2002). Flower type also affects host selection in plant species that exhibit more than one flower type. In

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6 chrysanthemums, F. occidentalis prefer cultivars with disc florets over spider-type flowers that do not include disc florets (Broadbent and Allen 1995, deJager et al. 1995a). Another important factor in thrips host selection is the nutritional quality of the plant (deJager et al. 1995b, Mollema and Cole 1996), though little is known about thrips nutritional ecology (Brodbeck et al. 2002). Mass determinations for growth rate and tissue samples for nutrient retention are difficult to obtain due to thrips' small size, and short development time and non-feeding prepupae and pupae make the organism difficult to observe. Therefore, population experiments are often easier to conduct than individual tests. Past population experiments have shown that population growth is correlated with nitrogen concentration. Because immature thrips molt four times in a short time span, and only eat in two instar stages, they must consume large amounts of amino acids to build new proteins to support the rapid growth (Kirk 1995, Brodbeck et al. 2001). Thrips may therefore prefer hosts with essential amino acids, especially those most rarely found in plants: tryptophan, phenylalanine, and methionine (Ananthakrishnan and Gopichandran 1993). Recent studies have shown a strong correlation between thrips crop damage and concentration of aromatic amino acids, especially phenylalanine, which enhances cuticle production and hardening, reducing the occurrence of dessication or entomopathogenic fungal infection (Mollema and Cole 1996). Glutamine, which can be converted to other essential amino acids, may also stimulate thrips feeding (Andersen et al. 1992). In addition, carbohydrates are important to thrips nutrition and may stimulate feeding. There has been some success in adding sugars to insecticide to increase

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7 insecticide consumption (Parrella 1995) and plant carbohydrate concentration increases F. occidentalis feeding rates, though not as strongly as plant protein concentration (Scott Brown et al. 2002). Within Plant Distribution Thrips small size enables access to recessed areas of a plant, which provide small microclimates that inhibit the desiccation or freezing of thrips (Kirk 1997a). These crevices also enhance protection from predation and being washed off the plant by rain. Vertical distribution of thrips within a plant appears to vary by host plant. In tomatoes most adult thrips feed in the upper portions of the plant, especially in the spring, while larvae are found in the lower portions (Navas et al. 1994, Reitz 2002). In cucumbers, F. occidentalis prefer higher, younger leaves for oviposition, and in a non-choice experiment, oviposition on younger cucumber leaves produced more offspring than on older leaves (deKogel et al. 1997b). In British Columbia nectarine orchards, F. occidentalis adults are more common in the lower portions of the trees, possibly due to a preference of low lying plant hosts (Pearsall 2002). Within plant distribution and movement also varies with season and thrips species. For example, F. occidentalis fly higher in the summer than in the spring in nectarine orchards (Pearsall and Myers 2001). In addition, F. tritici and F. bispinosa are more locally mobile than F. occidentalis (Ramachandran et al. 2001, Hansen et al. 2003), and F. fusca is generally considered more of a foliage feeder than F. occidentalis (Chellemi et al. 1994, Pearsall and Myers 2000). Seasonal Dynamics Low temperatures can reduce host availability, making overwintering difficult for thrips (Kirk 1997a). F. occidentalis have a developmental threshold of 10C (Robb

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8 1989, Cho et al. 1995b), and can survive at -5C for approximately 56-63 hours (Brdsgaard 1993). Frankliniella species either overwinter actively or are dormant in soil, while some species are capable of both, depending on environmental conditions. For example, in higher latitudes, such as British Columbia, Canada, F. occidentalis overwinter as dormant, mated females in soil (Pearsall and Myers 2000), while in lower latitudes such as North Carolina, F. occidentalis survive winters actively on hosts (Cho et al. 1995b). Soil type and host plant species also influence thrips overwintering, as larvae burrow further into lighter soil than heavier soil (MacGill 1929, 1930) and do not burrow into sand (Bailey 1933). Furthermore, thrips with cold resistant hosts may be less likely to overwinter in soil than those without (Kirk 1997a). Thrips populations increase dramatically in early spring, in the presence of increased temperatures and host bloom (Childers et al. 1990, Chellemi et al. 1994, Pearsall and Myers 2000, 2001). This rapid increase varies temporally with latitude. For example, in South Florida F. bispinosa increases in March (Childers et al. 1990), whereas in North Florida F. bispinosa displays greatest increase in April and May (Chellemi et al. 1994). Similarly, in North Florida F. occidentalis population densities rapidly increase in early spring (Chellemi et al. 1994), but show greatest increase in late April and May in British Columbia, Canada (Pearsall and Myers 2000, 2001). Summer usually includes a significant decrease in thrips populations (Childers et al. 1990, Chellemi et al. 1994, Reitz 2002, Nault et al. 2003). Reasons for this population decrease are unclear, as it does not appear due to temperature levels increasing above thrips range. In North Florida, Reitz (2002) found a decrease in F. occidentalis in May, although the temperatures never rose above F. occidentalis optimal temperature range

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9 (29-31C) throughout the experiment (van Rijn et al. 1995, Florida Automated Weather Network UF/IFAS 2003). One possible reason for the summer decline in thrips populations is the presence of natural enemies. Orius insidiosus (Say), a natural predator of F. occidentalis, is most abundant in May and June and may decrease thrips densities in summer months (Ramachandran et al. 2001, Reitz et al. 2003). Increased host resistance may also influence thrips summer decline. DeKogel et al. ( 1997a) found a negative correlation between thrips damage and solar radiation. The authors suggested that summer plant hosts might be more resistant to thrips, increasing thrips population decline. Populations increase slightly in the fall, possibly due to additional host bloom and/or reduced predation by natural enemies (Cho et al. 1995b, Reitz 2002, Groves et al. 2003). Although O. insidiosus is present during the fall, it is less abundant than during early summer months, and this decline may enable thrips populations to increase again (Ramachandran et al. 2001). Biotic Factors Predation by O. insidiosus affects thrips population size, and may affect spatial and temporal dynamics (Funderburk et al. 2000). Thrips populations may migrate to alternative hosts in the presence of O. insidiosus (Funderburk 2002) causing less preferred hosts to gain higher populations in the presence of predators. Predation from O. insidiosus may also affect thrips species ratios by predating one thrips species preferentially (Baez et al. 2004).

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10 Thrips spatial and temporal dynamics are also affected by parasitism and possibly competition. Nematodes (Thripinema spp.) infect thrips, leaving females sterile and reducing population growth (Funderburk et al. 2002). In addition, competition may affect population dynamics. No research has been published on thrips competition, but distribution, fecundity and mortality appear density-dependent in greenhouse thrips (Puche and Funderburk 1992, Kirk 1994 as cited by Kirk 1997a). Competition may be a cause of the density dependent distribution and decreased fecundity and survivorship. North Florida Thrips Three native thrips species Frankliniella bispinosa, F. fusca and F. tritici, and an introduced species, F. occidentalis damage crops in North Florida. Feeding and reproduction of these pests causes chlorosis, deformation of leaves and leaflets, stunting of plants, reduction of photosynthesis, and induction of air pockets in cells, causing fruit malformation and scarring (Chamberlin et al. 1992, Funderburk et al. 1998, Fung et al. 2002, Hao et al. 2002). In addition, F. bispinosa causes premature cellular evacuation, cellular collapse, necrosis and plasmolysis, leading to premature fruit drop (Childers et al. 1994). Species that vector tospoviruses such as Tomato spotted wilt virus can also indirectly reduce crop yields. Tomato spotted wilt causes plant wilt and fruit malformation, greatly reducing crop yield (see Prins and Goldbach 1998 for review). Vectors of tomato spotted wilt in North Florida include F. occidentalis, F. fusca, and F. bispinosa (Sakimura 1962, 1963, Webb et al. 1997). Thrips tabaci, and Frankliniella schultzei also vector tomato spotted wilt (Sakimura 1963, Cho et al. 1988), but are not common in the area. Although F. tritici is a common crop pest, this species does not vector tomato spotted wilt (Sakimura 1953, 1962, de Assis Filho et al. 2004).

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11 Only larvae acquire transferable virus, when the salivary glands are adjacent to the midgut and movement of the virus from the midgut to the salivary glands is possible (Moritz et al. 2004). The virus replicates in the salivary glands as the thrips matures and is eventually transferred to other plants via the saliva (Ullman et al. 1993). Because only larvae acquire the virus, only those plant species that are reproductive hosts can serve as sources for the virus. Reproductive Hosts Little is known of the range of thrips reproductive hosts. Although reproductive hosts can be identified as those plants with larvae present, there is no key for Frankliniella larvae, so identifying the larvae is difficult. Most publications list adult feeding hosts, or include reproductive hosts, but do not specify the reproducing species, due to the inability to identify larvae (though see Childers et al. 1990, Chamberlin et al. 1992, Childers et al. 1994, Cho et al. 1995b, Groves et al. 2002). Rearing thrips larvae to determine species is often difficult due to high mortality in lab rearing, especially when thrips are reared on an alternative host. However, a molecular technique has been developed to determine Frankliniella larval species, aiding in future experiments (Moritz et al. 2002). Frankliniella fusca, F. occidentalis, F. tritici, and F. bispinosa reproduce on a range of dicots and some monocots. Although knowledge is limited, there are some known taxonomic groups that thrips prefer. For example, F. occidentalis reproductive hosts include three or more host species each in Asteraceae, Fabaceae, Rosaceae, and Solanaceae (Table 1-1), and F. fusca reproduces mostly on Asteraceae, Fabaceae and Poaceae plants (Table 1-2). Little research has been focused on F. tritici and F. bispinosa due to F. triticis inability to vector TSWV, and the geographically limited range of F.

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12 bispinosa (Childers et al. 1990, Childers et al. 1994, Tsai et al. 1996). At present, there are no apparent trends in F. tritici and F. bispinosa reproductive hosts (Tables 1-3 and 1-4). Conclusion There are several density dependent and independent factors affecting thrips population dynamics. However, most of these factors are poorly understood. More research is needed to understand aspects of thrips ecology such as competition, predation, parasitism, and utilization of uncultivated hosts. Understanding these factors will allow a more complete knowledge of thrips population dynamics, enabling development of better pest management programs. This thesis will present research conducted on utilization of uncultivated plant hosts and competition between thrips species.

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Table 1-1. F. occidentalis Reproductive Hosts. F. occidentalis Reproductive Host Common Name Family Source Arctium lappa Burdock Asteraceae Bautista et al. 1995 Chrysanthemum morifolium Florist's Daisy Asteraceae Monteiro 2002 Lactuca serriola Prickly Lettuce Asteraceae Stewart et al. 1989 Lactuca sativa var. longifolia Romaine Lettuce Asteraceae Bautista et al. 1995 Verbesina encelioides Crownbeard Asteraceae Stewart, et al. 1989, Mitchell and Smith 1996 Impatiens walleriana Buzzy Lizzy Balsaminaceae Chen et al. 2004 Raphanus raphanistrum* Wild Radish Brassicaceae Buntin and Beshear 1995 Cucumis sativus Cucumber Cucurbitaceae deKogel et al. 1997b Trifolium vesiculosum* Arrowleaf Clover Fabaceae Chamberlin et al. 1992 Medicago polymorpha Bur Clover Fabaceae Stewart et al. 1989 Medicago sativa* Alfalfa Fabaceae Monteiro2002 Trifolium repens White Clover Fabaceae Heagle 2003 Trifolium vesiculosum* Arrowleaf Clover Fabaceae Chamberlin et al. 1992 Vicia villosa Hairy Vetch Fabaceae Toapanta et al.. 1996 Saintpaulia ionantha African Violet Gesneriaceae Monteiro 2002 Alstroemeria sp. Allstroemeria Liliaceae Monteiro2002 Rosa sp. Rose Rosaceae Chamberlin et al. 1992, Monteiro 2002 Prunus persica Peach Rosaceae Monteiro2002 Prunus Persica var. nucipersica Nectarines Rosaceae Monteiro2002 Prunus Serotina* Black Cherry Rosaceae Chamberlin et al. 1992 Capsicum annuum Pepper Solanaceae Scott Brown et al. 2002, Reitz et al. 2003 Datura stramonium Jimson Weed Solanaceae Bautista et al. 1995 Lycopersicon esculentum Tomato Solanaceae Navas et al. 1994 Solanum niagrum Amer. Black Nightshade Solanaceae Stewart et al. 1989 13 *Not confirmed, assumed due to high abundance of adults in the presence of larvae

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Table 1-2. F.fusca Reproductive Hosts. F. fusca Reproductive Host Common Name Family Source Emilia sonchifolia Lilac Tasselflower Asteraceae Stumpf and Kennedy 2005 Gnaphalium obtusifolium* Rabbit Tobacco Astaraceae Cho et al. 1995b Gnaphalium purpureum Spoonleaf Purple Ever. Asteraceae Groves et al. 2002 Hypochaeris radicata Hairy Catsear Asteraceae Groves et al. 2002 Lactuca scariola Prickly Lettcuce Asteraceae Groves et al. 2002 Lactuca floridana Woodland lettuce Asteraceae Johnson et al. 1995 Sonchus asper Spiny Sowthistle Asteraceae Johnson et al. 1995, Groves et al. 2002 Taraxacum officinale Dandelion Asteraceae Cho et al. 1995b, Groves et al. 2002 Verbesina encelioides Crownbeard Astaraceae Mitchell and Smith 1996 Raphanus raphanistrum Wild Radish Brassicaceae Cho et al. 1995b, Groves et al. 2002 Cerastium vulgatum* Mouseear Chickweed Caryophyllaceae Cho et al. 1995b Scleranthus annuus German knotgrass Caryophyllaceae Groves et al. 2002 Stellaria media Common Chickweed Caryophyllaceae Groves et al. 2002 Arachis hypogaea Florunner Peanut Fabaceae Funderburk et al. 1998, Tipping et al. 1998 Arachis hypogaea Volunteer Peanut Fabaceae Chamberlin et al. 1992 Trifolium campestre Field Clover Fabaceae Groves et al. 2002 Geranium carolinianum* Carolina Geranium Geraniaceae Cho et al. 1995b Allium vineale* Wild Garlic Lilliaceae Cho et al. 1995b Lamium amplexicaule Henbit Deadnettle Lamiaceae Groves et al. 2002 Plantago rugeli Blackseed Plantain Plantaginaceae Groves et al. 2002 Plantago lanceolate* Buckhorn Plantain Plantaginaceae Cho et al. 1995b Agropyron repens* Quackgrass Poaceae Cho et al. 1995b Secale cereale Winter Rye Poaceae Buntin and Beshear 1995 Triticum aestivum Winter Wheat Poaceae Buntin and Beshear 1995 Ranunculus sardous Hairy Buttercup Ranunculaceae Johnson et al. 1995, Groves et al. 2002 Capsicum annuum var. camelot Cheyenne Pepper Solanaceae Toapanta et al. 1996, Reitz et al. 2003 Datura stramonium Jimsonweed Solanaceae Stumpf and Kennedy 2005 14 Not confirmed, assumed due to high abundance of adults in the presence of larvae

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Table 1-3. F. tritici Reproductive Hosts. F. tritici Reproductive Host Common Name Family Source Raphanus raphanistrum* Wild Radish Brassicaceae Buntin and Beshear 1995 Trifolium vesiculosum* Arrowleaf Clover Fabaceae Chamberlin et al. 1992 Vicia villosa Hairy Vetch Fabaceae Toapanta et al. 1996 Ranunculus sardous Hairy Buttercup Ranunculaceae Johnson 1995 Capsicum annuum var. camelot Cheyenne Pepper Solanaceae Reitz et al. 2003 Not confirmed, assumed due to high abundance of adults in the presence of larvae 15

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Table 1-4. A list of plants F. bispinosa Reproductive Hosts. F. bispinosa Reproductive Host Common Name Family Source Alocasia cucullata^ Chinese taro Araceae Tsai et al. 1996 Bidens pinosa^ Spanish needle Asteraceae Tsai et al. 1996 Phoenix roebelenii^ Pygmy date palm Arecaceae Tsai et al. 1996 Raphanus raphanistrum* Wild Radish Brassicaceae Eger et al. 1998 Capsicum annuum Cheyenne Pepper Solanaceae Childers et al. 1994, Reitz et al. 2003 Pinus elliottii* Pinaceae Childers et al. 1994 var. densa^ Pinaceae Tsai et al. 1996 Pinus taeda* Loblolly Pine Pinaceae Childers et al. 1994 Prunus Caroliniana* Carolina Laurelcherry Rosaceae Childers et al. 1994 Citrus paradisi* Grapefruit Rutaceae Childers et al. 1990 Citrus sinensis var. navel* Navel Oranges Rutaceae Childers et al. 1990 Citrus sinensis var. valencia* Valencia Oranges Rutaceae Childers et al. 1990 Salix caroliniana* Coastal Plain Willow Salicaceae Childers et al. 1994 Typha domingensis^ Cattail Typhaceae Tsai et al. 1996 16 Not confirmed, assumed due to high abundance of adults in the presence of larvae ^Reproduced on lab Pollen

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CHAPTER 2 SPATIOTEMPORAL DYNAMICS ON REPRODUCTIVE HOSTS Introduction Frankliniella spp. thrips cause extensive economic damage to many types of crops through feeding and oviposition (Kirk 2002). Damage is either direct, from feeding that causes a silvering of plant tissue, or indirect, via the transmission of tospoviruses, including Tomato spotted wilt virus, one of the most damaging worldwide plant viruses (Prins and Goldbach 1998). The most researched of the Frankliniella thrips is Frankliniella occidentalis (Pergande) (see Kirk and Terry 2003 for review). This worldwide crop pest, native to California (Kirk and Terry 2003), has a broad host range and feeds on many types of crops, including many present in North Florida (Buntin and Beshear 1995, Puche et al. 1995, Funderburk et al. 2000, Funderburk et al. 2002). Other Frankliniella species occurring in North Florida are F. fusca (Hinds), F. bispinosa (Morgan) and F. tritici (Fitch), all three of which are native to the southeastern United States. Tomato spotted wilt virus vectors include F. occidentalis, F. fusca, and F. bispinosa, but not F. tritici (Sakimura 1953 as cited by Sakimura 1962, Sakimura 1962, 1963, Webb et al. 1997, de Assis Filho et al. 2004). In order to make better predictions of thrips population abundance, it is necessary to study cycles of thrips abundance (Funderburk 2002). These cycles have been studied extensively in crops such as tomatoes (Reitz 2002, Nault et al. 2003), citrus (Childers et al. 1990), nectarines (Felland et al. 1995, Pearsall and Myers 2000), and small grains (Buntin and Beshear 1995), but little research has been conducted in uncultivated plant 17

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18 hosts (though see Chamberlin et al. 1992, Chellemi et al. 1994, Cho et al. 1995, Toapanta et al. 1996). Furthermore, past studies either did not monitor thrips populations over the entire year (Chamberlin et al. 1992, Cho et al. 1995, Toapanta et al. 1996), or did not present thrips numbers for each plant host (Chellemi et al. 1994). Thrips often migrate from uncultivated hosts into cropping systems (Pearsall and Myers 2001), so cycles of abundance on uncultivated hosts must be understood to locate potential sources of thrips populations and Tomato spotted wilt virus. Often adult thrips feed on a host, but do not reproduce on the plant (Chamberlin et al. 1992), so a distinction must be made between feeding hosts and reproductive hosts. Reproductive hosts have a more direct connection to population growth than feeding hosts, and are the only sources of Tomato spotted wilt virus, since a transferable virus can only be acquired by a larva (Ullman et al. 1993, Wijkamp et al. 1993). Therefore, it is important to focus on reproductive hosts rather than plants where only adults occur (i.e. feeding hosts). The objective of this study was to determine the cycles of abundance of Frankliniella species on several species of potential reproductive plant hosts growing in field margins. The leaves, fruits and flowers of each plant were sampled to compare plant parts inhabited by the larvae and adults. Materials and Methods Sampling Procedure The study was conducted at the North Florida Research and Education Center in Quincy, Gadsden County. The plants sampled were Solidago canadensis L., Chenopodium ambrosioides L., Rubus trivialis Michx., R. cuneifolius Pursh., Raphanus raphanistrum L., Trifolium repens L., and Vicia sativa L. These species were selected based on the work of Chamberlin et al. (1992), Cho et al. ( 1995), Groves et al. (2002),

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19 Heagle ( 2003) and Dean Paini (Personal Communication). Each species was sampled biweekly, when available, between November 19, 2003 and November 5, 2004 On each sample date, 10 different sites were selected, and one plant was sampled from each site. For each plant, 20 leaves, 20 flowers, 20 fruits, and 20 racemes were placed, as appropriate for each plant species, in vials containing 70-95% ethanol. For clover, only four racemes were sampled per plant due to the low number and large size of the racemes. For V. sativa, which has prominent terminal buds, four buds were sampled per plant, in addition to the flowers, fruits and leaves. The total numbers of flowers, fruits and leaves per plant were also estimated. Because thrips were highly aggregated in the flowers, the number of each thrips species per flower and the number of flowers per plant were used to estimate the total number of each thrips species per plant on dates when thrips were common on the plant host. In the laboratory, the contents of each vial were placed in a Petri dish, and the plant parts were dissected to extract thrips. Adult thrips were identified under a microscope using 6.5-40x magnification. Larvae were counted, but not identified, because no morphological keys were available. Data Analysis Repeated measures ANOVA analyses and Tukeys tests were used to determine the effect of plant part and date on combined thrips densities for data collected when all plant parts were present. A one-way ANOVA was conducted to analyze plant part means of R. cuneifolius, as all three parts were only present on one sample date. Separate repeated measures ANOVA analyses were conducted on the number of Frankliniella spp. thrips per flower on each sample date, and the interaction of Frankliniella species by date was used to compare the patterns of abundance of the different thrips species (Littell et al. 1996). An unidentified non-Frankliniella species

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20 was abundant on C. ambrosioides, so it was included in the analysis. Effects were considered significant when p 0.05. In order to compare the patterns of abundance of the two most abundant species present, contrast procedures were conducted on the interaction between date and species. Specificially, contrast procedures were conducted on the interaction between date and the means of F. tritici and F. bispinosa on each host: R. raphanistrum, R. cuneifolius and T. repens. Contrast procedures were conducted on the interaction between date and the means of F. tritici and F. fusca on each host: R. trivialis, and V. sativa. For S. canadensis, no contrast procedure was conducted due to the low numbers of F. fusca, F. bispinosa, and F. occidentalis. For C. ambrosioides, a contrast procedure was conducted on the interaction between date and the means of F. tritici and the non-Frankliniella species. Data were only analyzed when thrips were present. For R. raphanistrum these dates were April 14 through July 20, 2004. For R. trivialis, these dates were March 29 through April 29, 2004. For R. cuneifolius, these dates were March 29 through April 29, 2004. For V. sativa, these dates were March 29 to April 14, 2004. For T. repens, these dates were April 14 to May 24, 2004. For S. canadensis, these dates were September 9 to October 21, 2004. For C. ambrosioides, these dates were August 19 to October 21, 2004. All analyses were conducted using SAS (SAS Institute 2000). A Cochrans test for homogeneity showed there was significant heterogeneity in the data, so the data were log-transformed using the formula log(x+1) to increase homogeneity. There was still significant heterogeneity in the data for comparing abundances of different thrips species, but the Ftests were considered robust enough to remain unaffected due to the number of treatments and sample size (Underwood 1999).

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21 Results There were 8,112 thrips extracted from 2,068 samples of the seven hosts, and 62% were adults. The adult thrips collected were 75.9% F. tritici, 14.7% F. bispinosa, 3.5% F. fusca, 1.1% F. occidentalis, and 4.8% non-Frankliniella spp. thrips. Raphanus raphanistrum flowered from December 5, 2003 to July 20, 2004. Thrips were most abundant from April 14 to July 20 (Tables 2-1 through 2-3). There was a significant interaction between plant part and date (F = 4.44; df = 18, 184; p<0.0001). There was a significant difference between thrips densities on plant parts (F = 183.01; df = 2, 34; p<0.0001), with more thrips on flowers than on leaves or fruits (Tukeys p<0.0001). The most abundant thrips species were F. tritici and F. bispinosa, comprising 74.5% and 19.9% of adults, respectively. There was a significant difference between the densities of thrips species (F = 65.97; df = 3, 36; p<0.0001), and a significant date effect (F = 25.76; df = 6, 184; p<0.0001) and interaction between species and date (F = 7.29; df = 18, 184; p<0.0001). There was a significant interaction between date and the means of F. tritici and F. bispinosa (F = 7.17; df = 6, 184; p<0.0001). Both species were abundant from April 14 to July 20, and there were more F. tritici than F. bispinosa collected on all dates. However, there were low abundances of both species on June 9, and since F. tritici was more abundant on the previous date than F. bispinosa, there was a greater relative decrease in F. tritici than in F. bispinosa, causing a difference in the patterns of abundance. Larvae consisted of 34.6% of total thrips present and were most abundant from April 14 to July 20. Rubus trivialis flowered from March 2 to April 29, 2004 and thrips were most abundant March 29 through April 14 (Tables 2-4 through 2-6). There was a significant interaction between plant part and date (F = 3.13; df = 4, 35; p<0.05). There was a

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22 significant difference in thrips densities on plant parts (F = 16.41; df = 2, 38; p<0.0001) from March 29 to April 29, with significantly more thrips on flowers than fruits or leaves (Tukeys p<0.0001). The most abundant thrips species were F. tritici and F. fusca, comprising 54.0% and 14.7% of adults, respectively. There was a significant difference in densities of thrips species (F = 2.70; df = 3, 36; p<0.005), and a significant interaction between species and date (F = 2.70; df = 6, 48; p<0.05), but no significant date effect (F = 2.60; df = 2, 95). There was a significant interaction between date and means of F. tritici and F. fusca (F = 5.58; df = 2, 48; p<0.01). F. tritici was abundant on R. trivialis from March 17 through April 29, but F. fusca was not abundant until April 29. Larvae consisted of 33.9% of thrips collected and were most abundant from March 29 to April 29. Rubus cuneifolius flowered from March 29 to May 12, 2004, and thrips densities were most abundant on April 29 (Tables 2-7 through 2-9). There was a significant difference in thrips densities on plant parts (F = 67.64; df = 2, 14; p<0.0001), with significantly more thrips on flowers than on fruits or leaves (Tukeys p<0.05) on April 29. The most abundant species were F. tritici and F. bispinosa, comprising 87.5% and 7.3% of adults, respectively. There was a significant difference in densities of thrips species (F = 33.52; df = 3, 36; p<0.0001), and a significant date effect (F = 19.06; df = 2, 68; p<0.0001) and interaction between date and species (F = 6.91; df = 6, 68, p<0.0001). There was a significant interaction between date and the means of F. tritici and F. bispinosa (F = 5.78; df = 2, 68; p<0.005). F. tritici were abundant from April 14 to April 29, but F. bispinosa were not present until April 29. Larvae were most abundant on R. cuneifolius on April 29, and consisted of 27.5% of thrips collected.

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23 Vicia sativa flowered from December 5 to April 14, and thrips were collected from March 29 to April 14 (Tables 2-10 through 2-13). There was a significant interaction between date and plant part (F = 4.01; df = 6, 52; p<0.005), and a significant difference in thrips densities on plant parts (F = 6.43, df = 3, 40; p<0.005). There were significantly more thrips on flowers than on the leaves or fruits (Tukeys p<0.05), but not buds (Tukeys p>0.05). The most abundant species were F. tritici, and F. fusca, comprising 81.9% and 15.3% of adults, respectively. There was no significant difference in densities of thrips species (F = 1.77; df = 3, 36), or significant difference in thrips densities on March 29 and April 14 (F = 0.00; df = 1, 8). There was no significant interaction between date and species (F = 0.00; df = 1, 8), indicating that there was no difference in the patterns of abundance of any thrips species. Larvae consisted of 47.1% of thrips collected and were present from March 29 to April 14. Trifolium repens flowered from December 12, 2003 to July 7, 2004, and thrips were most abundant April 29 through May 24 (Tables 2-14 and 2-17). There was a significant interaction between date and plant part (F = 6.32; df = 5, 88; p<0.0001), and there were significantly more thrips on racemes than leaves (F = 337.44; df = 1, 18; p<0.0001). The most abundant thrips species were F. tritici and F. bispinosa, comprising 79.4% and 12.0% of adults, respectively. There was a significant difference in densities of thrips species (F = 52.49; df = 3, 36; p<0.0001), and a significant date effect (F = 9.36; df = 3, 108; p<0.0001) and interaction between date and species (F = 4.61; df = 9, 108; p<0.0001). There was no significant interaction between date and the means of F. tritici and F. bispinosa (F = 1.97; df = 3, 108), indicating that there was no difference in the

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24 patterns of abundance of the two species. Larvae consisted of 27.4% of thrips collected, and were abundant from April 14 to May 12. Solidago canadensis flowered from August 19 to November 5, 2004. Thrips were present on S. canadensis from September 24 to October 21, although 82% of thrips were observed on October 21 (Tables 2-16 and 2-17). There was a significant interaction between plant part and date (F = 9.16; df = 2, 22; p<0.005), and significantly more thrips on racemes than leaves (F = 84.63; df = 1, 27; p<0.0001). Of the adults collected, 81.5% were F. tritici and 17.7% were a combination of non-Frankliniella species. There was a significant difference in thrips species (F = 19.72; df = 3, 36; p<0.0001), and a significant date effect (F = 6.50; df = 3, 72; p<0.001) and interaction between date and species (F = 6.23; df = 9, 72; p<0.0001). This interaction was due to the absence of F. bispinosa, F. fusca, and F. occidentalis during F. tritici abundance on October 21, 2004. Larvae consisted of 68.5% of thrips collected, and were most abundant on October 21. Chenopodium ambrosioides flowered from November 19 to December 5, 2003 and from June 24 to November 5, 2004. Thrips were collected on November 19, 2003 and from August 2 to October 21, 2004 (Tables 2-18 and 2-19). There was a significant interaction between date and plant part (F = 3.27, df = 5, 66; p<0.05), and there were significantly more thrips on racemes than leaves (F = 79.45; df = 1, 20; p<0.0001). The most abundant species were an unidentified non-Frankliniella sp. and F. tritici, comprising 68.0% and 14.6% of adults, respectively. There was a significant difference in densities of thrips species (F = 13.92; df = 4, 45; p<0.0001), and a significant interaction between date and species (F = 1.97; df = 16, 170; p<0.05), but no significant difference in the means of different sample dates (F = 0.60; df = 4, 170). There was a

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25 significant interaction between date and the means of F. tritici and non-Frankliniella sp. (F = 6.98, df = 4, 170; p<0.0001). F. tritici were only abundant on August 19, while non-Frankliniella sp. were abundant from September 2 through November 5, 2004. Larvae consisted of 67.3% of thrips collected, and were abundant from August 19 to November 5. There were 332 adult thrips and 116 larvae per R. raphanistrum plant from May 12 to June 24, 2004 (Table 2-20). There were 7 adult thrips and 8 larvae per R. trivialis plant from March 29 to April 14, 2004 (Table 2-20). There were 43 adult thrips and 11 larvae per R. cuneifolius plant on April 29, 2004 (Table 2-20). There were 3 adult thrips and 3 larvae per V. sativa plant on April 14, 2004 (Table 2-20). There were 142 adult thrips and 49 larvae per T. repens plant from April 29 to May 24, 2004 (Table 2-20). There were 574 adult thrips and 2,142 larvae per S. canadensis plant on October 21, 2004 (Table 2-20). There were 378 adult thrips and 765 larvae per C. ambrosioides plant from August 19 to October 21, 2004. Discussion More thrips were found on the flowers than leaves or fruits of all sampled plants, suggesting there is a nutritional or morphological preference for flowers. Thrips may prefer flowers to leaves because of the higher nitrogen content of pollen (Brodbeck et al. 2002), or for the microclimates flowers provide, that reduce desiccation, freezing, and access by predators (see Kirk 1997 for review). The statistically significant interaction between date and plant part on all plants was not considered biologically significant, because thrips were so highly aggregated in the flowers when flowers were present. The abundance of larvae on R. cuneifolius fruits when fruits were first present was probably

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26 due to the inability of larvae to move to new flowers quickly, as there were very few adults collected from fruits on the same date. There were 2.2 larvae per female collected on S. canadensis, and F. tritici was the only Frankliniella sp. collected, suggesting that S. canadensis is a good reproductive host for F. tritici. The high number of larvae per plant, and the abundance of plants throughout the country (USDA, NRCS 2004) suggest that S. canadensis may be an important source of F. tritici larvae that migrate into fall crops as adults. In addition, S. canadensis could be a source of larvae that overwinter as pupae in the soil. When temperatures rise, these developing adults may initiate the build up in thrips population numbers in early spring. If S. canadensis were not available to thrips, there may be a reduction in fall thrips populations and a delay in the spring population growth of thrips. There were many F. tritici and F. bispinosa collected from R. raphanistrum, suggesting that R. raphanistrum is a good feeding host for both species. There were only 0.53 larvae per female collected, suggesting that it was not as good a reproductive host as S. canadensis. However, the mean numbers of F. tritici and F. bispinosa per plant were high for R. raphanistrum, indicating that utilization of R. raphanistrum as a feeding and reproductive host may still be an important part of each species ecology. Furthermore, R. raphanistrum is a host for Tomato spotted wilt virus and may therefore be a source of virus infection in thrips populations (Parrella et al. 2003). R. raphanistrum is common in most of the United States (USDA, NRCS 2004) and may be an important factor in thrips ecology throughout the country. The most common thrips species on R. cuneifolius and T. repens were F. tritici and F. bispinosa. There were 0.39 larvae per female collected from each plant species,

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27 suggesting that neither species is a preferred host. Fewer thrips per plant were collected from T. repens and R. cuneifolius than from S. canadensis and R. raphanistrum. Reduced abundance per plant may be partially due to a lower number of flowers per plant during thrips abundance and the frequent mowing of T. repens during spring and summer. There were high numbers of thrips per plant on C. ambrosioides, but a majority of adults were not Frankliniella species, indicating that it may not support as many crop pests as other plants sampled. Low numbers of thrips per plant were collected from R. trivialis and V. sativa, suggesting that neither is a preferred feeding or reproductive host. Reasons for high Frankliniella species abundance on S. canadensis, R. raphanistrum, R. cuneifolius, and T. repens are unclear, since they are all from different taxonomic families (Asteraceae, Brassicaceae, Fabaceae and Rosaceae). Flowering time does not appear to be a major factor, since all plants were flowering when thrips were abundant. Nutritional differences among the plant species may cause thrips to prefer R. raphanistrum, R. cuneifolius, T. repens and S. canadensis. Thrips are known to prefer aromatic amino acids, which enhance cuticle production and hardening, and these amino acids may be more common in the more suitable hosts (Mollema and Cole 1996, Brodbeck et al. 2002). Furthermore, there may be a difference in chemical or morphological defenses, such as primary or secondary metabolites and trichomes that cause some plant species to be more attractive than others (Felland et al. 1995, deJager et al. 1996). More research must be conducted on the physical and chemical characteristics of plant hosts to understand why there were more Frankliniella spp. thrips on R. raphanistrum, R. cuneifolius, and S. canadensis than other plants surveyed.

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28 In addition to plant host charachterisitics, there may be competitive interactions between thrips species on R. trivialis, R. cuneifolius, and C. ambrosioides that affect species abundance. In R. trivialis, F. fusca was only abundant on April 29, when F. tritici was reduced. However, only one plant flowered on April 29, so this difference in patterns of abundance may be due to sampling error and the low abundance of F. fusca on most sample dates. In R. cuneifolius, F. tritici may benefit from an early abundance, allowing a competitive advantage over F. bispinosa. If interspecific competition is occurring, F. bispinosa may find the plant host less desirable if there are established interspecific competitors present. There may be climatic or plant physiological changes altering the species ratios on C. ambrosioides. For example, the cooler fall temperatures may benefit non-Frankliniella sp. populations more than F. tritici, or the plant may accumulate defenses to which only non-Frankliniella sp. have adapted. In addition, competitive displacement may have occurred locally, limiting abundance of F. tritici in the presence of non-Frankliniella sp. Reasons for the low abundance of thrips on R. raphanistrum on June 9, that caused the interaction between F. tritici and F. bispinosa, are unclear. Populations of F. tritici on T. repens were also reduced on that date, indicating that reduced thrips abundances were not limited to R. raphanistrum. Climatic effects, such as heavy rain or wind, prior to the sample date may have reduced thrips densities on R. raphanistrum. Although R. raphanistrum flowered from December 5 to August 2, there were no thrips collected from the flowers until April 14. The reasons for the delay in abundance of thrips on R. raphanistrum are unclear. Highest numbers of thrips were not found in R.

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29 raphanistrum until after R. cuneifolius finished flowering on May 24 and larvae to adult ratios were low, suggesting that R. raphanistrum may not be a preferred host, and is only utilized when alternative hosts are not available. Raphanus raphanistrum could also be utilized as an enemy-free niche as there was only one Orius insidiosus, an important thrips predator, collected from this host and thrips presence on this host corresponds to the seasonal increase of O. insidiosus in crops, as presented in past studies. For example, Reitz et al. (2003) collected most O. insidiosus during May and June in peppers, reaching abundances of one individual per 1.4 flowers during the same season as peak thrips densities on R. raphanistrum in my study. Reasons for the delay in abundance of larvae on R. raphanistrum are also unclear. The slight delay in abundance from April 29 to May 24 could be due to naturally occurring oviposition and incubation times. However, it is unclear why there were much higher larvae per female numbers from June 24 to July 20 than from April 29 to May 24. There may be a physiological change occurring in the plant during this time that makes the host more suitable for reproduction. Conversely, thrips may choose to oviposit in R. raphanistrum during these dates due to an abundance of O. insidiosus in cropping systems (Reitz et al. 2003). O. insidiosus preferentially feeds on larvae (Baez et al. 2004), and there may be selective pressure to reproduce in areas with less predation during this time of high predator abundance. Fewer F. occidentalis were collected than were collected previously in North Florida tomatoes and peppers (Funderburk et al. 2000, Reitz et al. 2003), and fewer F. fusca than were collected from North Florida peanuts (Funderburk et al. 2002). Frankliniella fusca and F. occidentalis may have different nutritional requirements to

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30 those of F. tritici, and this may partially explain their general absence from these hosts. Furthermore, there may be competitive interactions occurring on uncultivated hosts that are different from those in crops. Although F. occidentalis are excellent competitors on fertilized pepper plants (see chapter 3), they may not be able to compete as well on unfertilized wild hosts. More F. occidentalis per flower were found in R. raphanistrum in Georgia than in Florida (Buntin and Beshear 1995). Reasons for the reduced densities of F. occidentalis in Florida are unclear. Because of their abundance on cultivated crops in past studies, climate does not appear to be the only factor limiting F. occidentalis densities on R. raphanistrum. However, there may be an interaction between climate and availability of other reproductive hosts. An increase in alternative host species in Georgia would increase the overall population and may increase the number of thrips migrating into R. raphanistrum. The difference in densities may also be due to an interaction between climate and interspecific competition. Although conditions in Georgia and Florida are similar, the slight change may affect competition in F. occidentalis, decreasing their competitive ability on R. raphanistrum under Florida conditions compared with those in Georgia. Conclusion Seven uncultivated reproductive hosts were sampled to determine the seasonal abundance of Frankliniella tritici, F. bispinosa, F. fusca, and F. occidentalis populations on each plant host. The abundant thrips species on Raphanus raphanistrum, Rubus cuneifolius, and Trifolium repens were F. tritici and F. bispinosa. The abundant thrips species on Rubus trivialis and Vicia sativa were F. tritici and F. fusca. The abundant

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31 thrips species on Solidago canadensis was F. tritici. The abundant thrips species on Chenopodium ambrosioides were F. tritici and an unidentified non-Frankliniella sp. Thrips were highly aggregated in the flowers, rather than leaves or fruits, of every plant species. The spring hosts that supported the largest Frankliniella spp. thrips populations included R. raphanistrum, T. repens, and R. cuneifolius, and the fall host that supported the largest Frankliniella spp. populations was S. canadensis. Reducing the occurrence of these uncultivated hosts in areas surrounding crops may decrease the number of thrips migrating into cropping systems, leading to a reduction in crop damage.

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32 Table 2-1. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F. bispinosa, adult F. occidentalis and Frankliniella species larvae from samples of 20 flowers of Raphanus raphanistrum collected biweekly on 17 dates from December 12, 2003 to August 2, 2004 in Gadsden County, Florida. Date Adult F. tritici Adult F. fusca Adult F. bispinosa Adult F. occidentalis Frankliniella spp. Larvae 2003 12-Dec 0 0 0 0 0 5-Dec 0.7(0.4) 0 0 0 0 2004 2-Jan 0.9(0.3) 0 0 0 0 15-Jan 0.6(0.3) 0 0 0 0 29-Jan 0.1(0.1) 0 0 0 0 17-Feb 0.3(0.2) 0.1(0.1) 0 0 0 2-Mar 0.3(0.2) 0 0 0 0 17-Mar 0.3(0.2) 0 0 0 0 29-Mar 0.2(0.1) 0.1(0.1) 0 0 0 14-Apr 5.3(2.0) 0.6(0.3) 0.2(0.2) 0 0.1(0.1) 29-Apr 21.3(11.2) 1.7(1.6) 4.1(2.1) 0.3(0.2) 6.3(4.0) 12-May 100.3(19.9) 6.1(3.1) 25.6(13.5) 2.7(1.1) 26.9(11.5) 24-May 60.8(14.9) 0.5(0.4) 15.3(2.8) 0.7(0.3) 37.4(11.4) 9-Jun 11.4(3.1) 0.9(0.3) 5(1.4) 0.2(0.1) 7.1(1.5) 7-Jul 19.0(4.9) 0.2(0.2) 10.2(3.0) 1.0(0.6) 72(0.2) 20-Jul 13.8(2.7) 0.3(0.2) 4.6(1.9) 0 27.4(5.6) 2-Aug 2.3(0.9) 0 0.5(0.5) 0 8.5(5.3)

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33 Table 2-2. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F. bispinosa, adult F. occidentalis and Frankliniella species larvae from samples of 20 leaves of Raphanus raphanistrum collected biweekly on 17 dates from December 12, 2003 to August 2, 2004 in Gadsden County, Florida. Date Adult F. tritici Adult F. fusca Adult F. bispinosa Adult F. occidentalis Frankliniella spp. Larvae 2003 12-Dec 0 0 0 0 0 5-Dec 0 0 0 0 0 2004 2-Jan 0 0 0 0 0 15-Jan 0 0 0 0 0 29-Jan 0 0 0 0 0 17-Feb 0 0 0 0 0 2-Mar 0 0 0 0 0 17-Mar 0 0 0 0 0 29-Mar 0 0 0 0 0 14-Apr 0 0 0 0 0.2(0.2) 29-Apr 0.3(0.3) 0.3(0.3) 0.4(0.4) 0 0.4(0.3) 12-May 1.8(1.0) 0.4(0.2) 0 0.1(0.1) 3.5(1.2) 24-May 1.7(0.7) 0.3(0.2) 0.9(0.4) 0.1(0.1) 4.6(1.5) 9-Jun 0.1(0.1) 0 0 0 1.3(1.7) 7-Jul 0.2(0.2) 0 0 0 0.2(0.2) 20-Jul 0.3(0.2) 0.2(0.1) 0 0 0.5(0.2) 2-Aug 0 0 0 0 0

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34 Table 2-3. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F. bispinosa, adult F. occidentalis and Frankliniella species larvae from samples of 20 fruits of Raphanus raphanistrum collected biweekly on 17 dates from December 12, 2003 to August 2, 2004 in Gadsden County, Florida. Date Adult F. tritici Adult F. fusca Adult F. bispinosa Adult F. occidentalis Frankliniella spp. Larvae 2003 12-Dec 0 0 0 0 0 5-Dec 0.1(0.1) 0 0 0 0 2004 2-Jan 0 0 0 0 0 15-Jan 0 0 0 0 0 29-Jan 0 0 0 0 0 17-Feb 0 0 0 0 0 2-Mar 0 0 0 0 0 17-Mar 0 0 0 0 0 29-Mar 0 0 0 0 0 14-Apr 0.1(0.1) 0 0 0 0 29-Apr 0 0.1(0.1) 0.1(0.1) 0 0.1(0.1) 12-May 0.6(0.3) 0 0 0 3.9(1.7) 24-May 0.8(0.4) 0 0.2(0.1) 0 3.3(1.2) 9-Jun 0.3(0.2) 0 0 0 0.1(0.1) 7-Jul 0 0 0 0 0 20-Jul 0.3(0.2) 0 0 0 0.1(0.1) 2-Aug 0 0 0 0 0 Table 2-4. Mean number (SEM) of adult Franklniella tritici, adult F. fusca, adult F. bispinosa, adult F. occidentalis and Frankliniella species larvae from samples of 20 flowers of Rubus trivialis collected biweekly on 5 dates from March 2 to April 29, 2004 in Gadsden County, Florida. Date Adult F. tritici Adult F. fusca Adult F. bispinosa Adult F. occidentalis Frankliniella spp. Larvae 2-Mar 0 0 0 0 0.3(0.6) 17-Mar 0.5(0.3) 0.2(0.1) 0 0 0.1(0.1) 29-Mar 2.3(1.0) 0.3(0.2) 0 0.2(0.1) 1.1(0.4) 14-Apr 10.0(0.5) 0 0 0 3.5(0.1) 29-Apr* 8 5 0 2 2 *n=1 (Only one plant flowering)

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35 Table 2-5. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F. bispinosa, adult F. occidentalis and Frankliniella species larvae from samples of 20 leaves of Rubus trivialis collected biweekly on 5 dates from March 2 to April 29, 2004 in Gadsden County, Florida. Date Adult F. tritici Adult F. fusca Adult F. bispinosa Adult F. occidentalis Frankliniella spp. Larvae 2-Mar 0 0 0 0 0.1(0.1) 17-Mar 0 0 0 0 0 29-Mar 0 0 0 0 0 14-Apr 0 0 0 0 0 29-Apr 0 0.1(0.1) 0 0 0 Table 2-6. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F. bispinosa, adult F. occidentalis and Frankliniella species larvae from samples of 20 fruits of Rubus trivialis collected biweekly on 3 dates from March 29 to April 29, 2004 in Gadsden County, Florida. Date Adult F. tritici Adult F. fusca Adult F. bispinosa Adult F. occidentalis Frankliniella spp. Larvae 29-Mar 0 0 0 0 0.2(0.2) 14-Apr 0 0.1(0.1) 0 0.1(0.1) 1.4(0.5) 29-Apr 0 0.1(0.1) 0 0 0.2(0.1)

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36 Table 2-7. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F. bispinosa, adult F. occidentalis and Frankliniella species larvae from samples of 20 flowers of Rubus cuneifolius collected biweekly on 4 dates from March 29 to May 12, 2004 in Gadsden County, Florida. Date Adult F. tritici Adult F. fusca Adult F. bispinosa Adult F. occidentalis Frankliniella spp. Larvae 29-Mar 0.2(0.1) 0.1(0.1) 0 0 0 14-Apr 23.0(17.3) 0.1(0.1) 0.4(0.3) 0.1(0.1) 0.8(0.3) 29-Apr 49.6(18.0) 0.2(0.1) 5.5(2.0) 0.3(0.2) 11.0(3.9) 12-May 0 0 0 0 0 Table 2-8. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F. bispinosa, adult F. occidentalis and Frankliniella species larvae from samples of 20 leaves of Rubus cuneifolius collected biweekly on 4 dates from March 29 to May 12, 2004 in Gadsden County, Florida. Date Adult F. tritici Adult F. fusca Adult F. bispinosa Adult F. occidentalis Frankliniella spp. Larvae 29-Mar 0 0 0 0 0 14-Apr 0 0 0 0 0.4(0.3) 29-Apr 0.1(0.1) 0.1(0.1) 0 0 0 12-May 0 0.1(0.1) 0 0 0.2(0.1) Table 2-9. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F. bispinosa, adult F. occidentalis and Frankliniella species larvae from samples of 20 fruits of Rubus cuneifolius collected biweekly on 2 dates from April 29 to May 12, 2004 in Gadsden County, Florida. Date Adult F. tritici Adult F. fusca Adult F. bispinosa Adult F. occidentalis Frankliniella spp. Larvae 29-Apr 1.7(0.7) 0.2(0.2) 0.2(0.2) 0 20.8(4.3) 12-May 0 0.1(0.1) 0 0 0.7(0.4)

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37 Table 2-10. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F. bispinosa, adult F. occidentalis and Frankliniella species larvae from samples of 20 flowers of Vicia sativa collected biweekly on 6 dates from January 29 to April 14, 2004 in Gadsden County, Florida. Date Adult F. tritici Adult F. fusca Adult F. bispinosa Adult F. occidentalis Frankliniella spp. Larvae 29-Jan 0 0 0 0 0 17-Feb 0.1(0.1) 0 0 0 0.1(0.1) 2-Mar 0 0 0 0 0 17-Mar 0 0 0 0 0 29-Mar 0.1(0.1) 0 0 0 0.7(0.6) 14-Apr 3.7(2.7) 0.5(0.3) 0.2(0.2) 0 3.2(1.4) Table 2-11. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F. bispinosa, adult F. occidentalis and Frankliniella species larvae from samples of 20 leaves of Vicia sativa collected biweekly on 6 dates from January 29 to April 14, 2004 in Gadsden County, Florida. Date Adult F. tritici Adult F. fusca Adult F. bispinosa Adult F. occidentalis Frankliniella spp. Larvae 29-Jan 0 0 0 0 0.1(0.1) 17-Feb 0 0 0 0 0 2-Mar 0 0 0 0 0 17-Mar 0 0 0 0 0 29-Mar 0 0 0 0 0 14-Apr 0.1(0.1) 0 0 0 0 Table 2-12. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F. bispinosa, adult F. occidentalis and Frankliniella species larvae from samples of 20 leaves of Vicia sativa collected biweekly on 3 dates from March 17 to April 14, 2004 in Gadsden County, Florida. Date Adult F. tritici Adult F. fusca Adult F. bispinosa Adult F. occidentalis Frankliniella spp. Larvae 17-Mar 0 0 0 0 0 29-Mar 0 0.2(0.2) 0 0 0.1(0.1) 14-Apr 0.1(0.1) 0.1(0.1) 0 0 0.3(0.2)

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38 Table 2-13. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F. bispinosa, adult F. occidentalis and Frankliniella species larvae from samples of 4 buds of Vicia sativa collected biweekly on 6 dates from January 29 to April 14, 2004 in Gadsden County, Florida. Date Adult F. tritici Adult F. fusca Adult F. bispinosa Adult F. occidentalis Frankliniella spp. Larvae 29-Jan 0 0 0 0 0.2(0.2) 17-Feb 0 0 0 0 0 2-Mar 0 0 0 0 0.2(0.2) 17-Mar 0 0 0 0 0 29-Mar 0 0 0 0 0 14-Apr 1.4(1.0) 0.1(0.1) 0 0 0.9(0.4) Table 2-14. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F. bispinosa, adult F. occidentalis and Frankliniella species larvae from samples of 4 racemes of Trifolium repens collected biweekly on 14 dates from December 12, 2003 to July 7, 2004 in Gadsden County, Florida. Date Adult F. tritici Adult F. fusca Adult F. bispinosa Adult F. occidentalis Frankliniella spp. Larvae 2003 12-Dec 0.2(0.2) 0 0 0 0.1(0.1) 2004 2-Jan 0 0 0 0 0 15-Jan 0.8(0.6) 0 0 0 0 29-Jan 0.1(0.1) 0 0 0 0.2(0.1) 17-Feb 0.1(0.1) 0 0 0 0.2(0.1) 2-Mar 0 0 0 0 0.1(0.1) 17-Mar 0.1(0.1) 0 0 0 0 29-Mar 0.2(0.1) 0 0.2(0.1) 0 0.2(0.1) 14-Apr 1.7(0.7) 0.1(0.1) 0.2(0.1) 0 3.1(1.4) 29-Apr 9.0(4.1) 0.6(0.3) 1.5(0.7) 0.2(0.1) 4.4(0.9) 12-May 5.9(2.2) 1.5(0.7) 0.8(0.4) 0.1(0.1) 10.8(6.1) 24-May 21.5(7.6) 0.1(0.1) 3.5(1.3) 0 0.4(0.2) 9-Jun 1.0(0.4) 0.0(0.0) 0.1(0.1) 0 0.6(0.3) 7-Jul 1.9(0.5) 0.1(0.1) 0.1(0.1) 0.1(0.1) 0.1(0.1)

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39 Table 2-15. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F. bispinosa, adult F. occidentalis and Frankliniella species larvae from samples of 20 leaves of Trifolium repens collected biweekly on 14 dates from December 12, 2003 to July 7, 2004 in Gadsden County, Florida. Date Adult F. tritici Adult F. fusca Adult F. bispinosa Adult F. occidentalis Frankliniella spp. Larvae 2003 12-Dec 0 0 0 0 0 2004 2-Jan 0 0 0 0 0 15-Jan 0 0 0 0 0.1(0.1) 29-Jan 0 0 0 0 0 17-Feb 0 0 0 0 0 2-Mar 0 0 0 0 0 17-Mar 0 0 0 0 0 29-Mar 0.1(0.1) 0 0 0 0 14-Apr 0 0.3(0.2) 0 0 0 29-Apr 0.1(0.1) 0 0.1(0.1) 0 0.2(0.1) 12-May 0 0.1(0.1) 0.1(0.1) 0 0.1(0.1) 24-May 0 0.1(0.1) 0 0 0.1(0.1) 9-Jun 0.1(0.1) 0 0 0 0 7-Jul 0 0 0 0 0

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40 Table 2-16. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F. bispinosa, adult F. occidentalis and Frankliniella species larvae from samples of 20 racemes of Solidago canadensis collected biweekly on 5 dates from September 2 to November 5, 2004 in Gadsden County, Florida. Date Adult F. tritici Adult F. fusca Adult F. bispinosa Adult F. occidentalis Frankliniella spp. Larvae 2-Sep 0 0 0 0 0.5(0.3) 24-Sep 5.7(2.7) 0 0.1(0.1) 0 0.4(0.4) 7-Oct 3.2(0.9) 0 0 0 3.2(1.2) 21-Oct 23.8(8.9) 0 0 0.2(0.2) 89.5(44.1) 5-Nov 0 0 0 0 0 Table 2-17. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F. bispinosa, adult F. occidentalis and Frankliniella species larvae from samples of 20 leaves of Solidago canadensis collected biweekly on 5 dates from September 2 to November 5, 2004 in Gadsden County, Florida. Date Adult F. tritici Adult F. fusca Adult F. bispinosa Adult F. occidentalis Frankliniella spp. Larvae 2-Sep 0 0 0 0 0 24-Sep 0 0 0 0 0 7-Oct 0 0 0 0 0 21-Oct 0.2(0.2) 0 0 0 0 5-Nov 0 0 0 0 0

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41 Table 2-18. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F. bispinosa, adult F. occidentalis, adult non-Frankliniella species, and larvae from samples of 20 racemes of Chenopodium ambrosioides collected biweekly on 2 dates from November 19 to December 5, 2003 and 6 dates from August 19 to November 5, 2004 in Gadsden County, Florida. Date Adult F. tritici Adult F. fusca Adult F. bispinosa Adult F. occidentalis Adult non-Frankliniella sp. Larvae 2003 19-Nov 0 0 0 0 0 0.4(0.3) 5-Dec 0 0 0 0 0 0 2004 19-Aug 1.1(0.5) 0 0 0 0.1(0.1) 4.4(0.1) 2-Sep 0.1(0.1) 0 0 0 1.7(1.1) 2.2(0.9) 24-Sep 0.2(0.2) 0 0 0 1.6(0.7) 5.1(2.0) 7-Oct 0.1(0.1) 0 0 0 1.5(0.6) 1.8(0.6) 21-Oct 0.3(0.3) 0 0 0 2.0(1.6) 5.3(1.7) 5-Nov 0 0 0 0 1.6(0.7) 7.4(2.8) Table 2-19. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F. bispinosa, adult F. occidentalis, adult non-Frankliniella species, and larvae from samples of 20 leaves of Chenopodium ambrosioides collected biweekly on 2 dates from November 19 to December 5, 2003 and 6 dates from August 19 to November 5, 2004 in Gadsden County, Florida. Date Adult F. tritici Adult F. fusca Adult F. bispinosa Adult F. occidentalis Adult non-Frankliniella sp. Larvae 2003 19-Nov 0.1(0.1) 0 0 0 0 0.1(0.1) 5-Dec 0 0 0 0 0 0 2004 19-Aug 0 0 0 0 0 0.1(0.1) 2-Sep 0 0 0 0 0 0.1(0.1) 24-Sep 0 0 0 0 0 0 7-Oct 0 0 0 0 0 0 21-Oct 0 0 0 0 0 0 5-Nov 0 0 0 0 0 0

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Table 2-20. Mean number of adult Frankliniella tritici, F. fusca, F. bispinosa, F. occidentalis, larvae and non-Frankliniella sp. per plant for seven plant species on selected dates. Plant Species Dates Adult F. tritici per Plant Adult F. fusca per Plant Adult F. bispinosa per Plant Adult F. occidentalis per plant Larvae per Plant Adult non-Frankliniella spp. per plant R. raphanistrum 12-May to 24-Jun 244.30 11.37 71.51 5.04 115.80 0 R. trivialis 29-Mar to 14-Apr 6.66 0.09 0 0.47 8.13 0 R. cuneifolius 29-Apr 40.79 0.08 1.97 0.12 11.41 0 V. sativa 14-Apr 2.62 0.57 0.12 0.00 2.74 0 T. repens 29-Apr to 24-May 115.17 6.96 18.35 0.95 49.36 0 S. canadensis 21-Oct 570.41 0 0 3.99 2142.03 0 C. ambrosioides 19-Aug to 21-Oct 70.22 0 0 0 764.60 307.90 42

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CHAPTER 3 INTRASPECIFIC AND INTERSPECIFIC COMPETITION IN THRIPS ON FLOWERING PEPPER PLANTS Introduction Competition can be an important factor in determining population size, structure and interactions (Inouye 1999a, b, Hansen et al. 2003, Young 2004). Interspecific competition may be particularly important when assessing the impact of invasive species, which are often good competitors (Mooney and Cleland 2001). Competitive ability enhances invasive species capabilities to increase rapidly and become pests in new environments (Gurnell 1996, Petren and Case 1996, Holway et al. 1998, Callaway and Aschehoug 2000). There have been several examples of invasive species out-competing native species and becoming pests. The Argentine ant Linepithema humile has outcompeted several native species, and become a pest in the southern United States, and many other areas of the world (Holway et al. 1998, Holway and Suarez 2004). Competitive superiority of the invasive fire ant Solenopsis invicta over the native ant Forelius mccooki has enhanced the spread of S. invicta, allowing the species to reach pest status in the southeastern United States (Mehdiabadi et al. 2004). Competitive superiority of the invasive mosquito Aedes albopictus over several native species also appears to have aided in the spread of this invasive pest (Griswold and Lounibos 2005, Juliano and Lounibos 2005). Competition has therefore been an important factor assisting the spread of invasive species and should be considered when assessing any invasive species 43

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44 success. Once the factors affecting the spread of the invasive organism are understood, invasions can be predicted, and pest management programs can be improved (Strong and Pemberton 2000, Yasuda et al. 2004). Interspecific competition studies on animals have often used an additive or substitutive design (e.g. Connell 1961, Moran and Whitham 1990, Forseth et al. 2003). Additive design experiments maintain one species at a constant density, while varying the density of the other (Figure 3-1). Using this design does not distinguish the effect of interspecific competition from intraspecific competition due to the varying number of overall individuals (Damgaard 1998, Inouye 2001, Young 2004). Substitutive designs vary the frequency of the two competing species while maintaining a constant combined density (Figure 3-2), and are therefore useful in comparing interspecific and intraspecific competition. Because the treatments are conducted at the same overall density, interspecific and intraspecific competition can only be measured in relation to each other. The statistical significance of either form of competition can not be determined (Snaydon 1991, Inouye 2001). A third type of design, the response surface design, varies the densities of each species independently and competition models can be used to generate a quantitative value of competition (Inouye 2001). By so doing, empirical data can be fit to a theoretical model, providing a connection between empirical and theoretical approaches that is not possible using an additive or substitutive design experiment (Damgaard 1998, Inouye 2001). Response surface designs have often been used in plant intserspecific competition studies (Law and Watkinson 1987, Rees et al. 1996, Damgaard 1998), but

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45 have been rarely used in animal interspecific competition experiments (though see Inouye 1999a, Young 2004). Frankliniella occidentalis is an invasive crop pest that causes damage to flowers and developing fruits through feeding and ovipositing, as well as by spreading Tomato spotted wilt virus (Sakimura 1962), one of the most damaging worldwide plant viruses (Prins and Goldbach 1998). This species has spread from its native western North America to every continent except Antarctica in the last 30 years (Kirk and Terry 2003). Research has been conducted on some of the factors contributing to the spread of F. occidentalis, including host range (Chellemi et al. 1994), climate (Brdsgaard 1993, Wang and Shipp 2001) and predation (Baez et al. 2004), but no research has been conducted on the population effects of thrips competition. Determining whether or not competition between F. occidentalis and native species occurs may partially explain the spread of this worldwide crop pest. I used a response surface design to test for competition between F. occidentalis (Pergande) and F. bispinosa (Morgan), a native Florida species. A competition model was fit to the data to generate quantitative values measuring the effects of intraspecific and interspecific competition, and make qualitative (presence or absence) determinations of each type of competition. Materials and Methods Experimental Design Female F. bispinosa were collected from perennial peanuts (Arachis glabrata) in Gainesville, FL. Female F. occidentalis were taken from a colony maintained at 21-23C and 50-80% relative humidity, with a 14:10 photophase: scotophase, and regularly supplemented with wild individuals. The experiment was conducted on flowering pepper plants (Capsicum annuum), a known reproductive host for both species (Funderburk et al.

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46 2000, Ramachandran et al. 2001, Hansen et al. 2003). Pepper plants were grown in a greenhouse with no insecticides and were checked regularly for insects, which were killed manually. For the experiment, each pepper plant was enclosed in a plexiglass cylindrical cage 15.5-cm in diameter and 36.5-cm in height. The top of each cage was covered with thrips screen (Green-Tek, Inc., WI), and the bottom was inserted into the soil to prevent thrips escape. Each cage had two 2-cm diameter holes covered with thrips screen to increase ventilation. The experiment was conducted in a climate-controlled room set at 23C with greater than 95% relative humidity within the cages. The densities of female F. bispinosa and F. occidentalis were arranged in a bivariate factorial arrangement from 0 to 30 per plant in increments of ten, with additional single species treatments of 60 (Figure 3-3). These densities reflect those previously recorded in the field (Ramachandran et al. 2001). The single species treatments of 60 were added to increase the chance of including the population carrying capacity in the treatment range, since the carrying capacity was unknown. Each treatment was replicated five times. Female thrips were introduced to the pepper plants and allowed to feed and oviposit. After ten days, plants were destructively sampled, and all larvae were removed. The larvae from each treatment were placed in 30-ml containers with green beans and bee pollen, and the species of each was determined after development to adult. The species ratio of emerged adults was assumed to be the species ratio of larvae produced for the treatment. The larval species ratio and the overall number of larvae produced were multiplied together to estimate the number of larvae produced by each species. Then the number of larvae produced per species for each treatment was divided by the number of

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47 adult females in the treatment to estimate the number of larvae per female of each species. The number of larvae produced per female of each species at the various densities was used to evaluate the effect of competition on female oviposition. Model Fitting Larvae per female of each species were used as the response variable to obtain a measurement of oviposition. The model was fit using maximum log-likelihood estimation, assuming a Poisson error distribution. This method uses the data to estimate the probability of occurrence for each possible value of each parameter. The log of each likelihood (probability) value is then calculated. Then the value for each parameter that had the highest log-likelihood (probability of occurring) was selected. Confidence intervals were determined using log-likelihood ratios. This technique used the 2 distribution of the log-likelihood of each parameter to determine the confidence intervals with the other parameters fixed at the best fit values. All calculations were completed using R (R Development Core Team 2005). Several models were tested, and the best-fit model was the following: R X = (Law and Watkinson 1987) 1+ c(X + XY Y) Where R X is the number of larvae produced per female of species X after ten days, and is generated by the model to predict the larvae per female of species X produced at low densities, in the absence of competition. The parameter c measures intraspecific competition. The parameter XY is the competition coefficient, which measures the relative effect of species Y on the reproduction of species X. This competition coefficient compares the effects of interspecific competition with that of intraspecific competition, which is set at one. For example, if the competition coefficient were estimated as 3, it

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48 would indicate that each interspecific competitor would have 3 times the effect of a conspecific on reproduction. The competition model was fit for both focal species. Model simulation of intraspecific competition was conducted by graphing the models for each species with the number of interspecific competitors set at zero. Results The mean number of larvae per female from the different treatment densities of adult female F. occidentalis and F. bispinosa are presented in Table 3-1. In treatments with only one species present, there were more F. occidentalis larvae produced per female than F. bispinosa. The larvae per F. bispinosa female averaged over all treatments was 0.58 (SE 0.13), and the larvae per F. occidentalis female averaged over all treatments was 2.6 (SE 0.28). The model parameters and confidence intervals measuring effects of competition on the number of larvae produced per F. bispinosa female are presented in Table 3-2. Statistically significant intraspecific competition affected F. bispinosa, as indicated by the confidence interval for c, which did not include zero. The maximum likelihood estimation for XY indicated that the effect of interspecific competition from F. occidentalis was 4.62 times greater than intraspecific competition on F. bispinosa reproduction. The 95% confidence intervals for XY did not include one, proving that interspecific competition from F. occidentalis had a significantly greater effect on the number of F. bispinosa larvae produced per female than intraspecific competition. The model parameters and confidence intervals measuring effects of competition on the number of larvae produced per F. occidentalis female are presented in Table 3-3. Statistically significant intraspecific competition affected F. occidentalis, as indicated by the confidence interval for c, which did not include zero. The estimated value for XY for

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49 F. occidentalis was negative, suggesting that F. occidentalis benefited slightly from the presence of F. bispinosa. Intraspecific competition had a greater effect on F. bispinosa than on F. occidentalis, as indicated by the 95% confidence intervals for c (Tables 3-3 and 3-4), which did not overlap. Model simulation of intraspecific competition also indicated that F. bispinosa were more affected by intraspecific competition than F. occidentalis as the number of larvae per female decreased more rapidly with increased intraspecific competition for F. bispinosa than for F. occidentalis (Figure 3-4). However, the values for were similar, indicating that in the absence of competition, both species would produce similar numbers of larvae per female. Discussion These results indicate that F. occidentalis is competitively superior to F. bispinosa on pepper plants. Being a superior competitor may enhance the spread and abundance of F. occidentalis. The competitive mechanism that occurred between the two species is not clear, although the behavior of these two species indicate that interference occurred. F. bispinosa are more mobile than F. occidentalis in pepper flowers and are more likely to flee in the presence of a predator (Reitz et al. 2002). F. bispinosa may also be more inclined than F. occidentalis to move to another feeding or oviposition site when in the presence of a competitor. This extra time spent locating feeding or oviposition sites would decrease time allotted for feeding and reproducing, reducing fecundity in the presence of intraspecific and interspecific competition. The negative competition coefficient measuring the effect of F. bispinosa on F. occidentalis suggests that F. occidentalis benefited from the presence of F. bispinosa.

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50 This benefit may be due to two effects. First, there may have been intraguild predation, with F. occidentalis increasing fecundity by feeding on F. bispinosa, as F. occidentalis are facultative predators (Faraji et al. 2002). If intraguild predation is occurring, it may increase the spread of F. occidentalis. In addition, there was a higher mortality rate for F. bispinosa than F. occidentalis in the single species larval growth chambers. This differential mortality may have slightly altered the species ratios in interspecific treatments, causing an apparent increase of F. occidentalis due to the misidentification of F. bispinosa larvae. However, if superior larval survivorship is occurring, there may be further characteristics in the relationship between the two species that would increase the spread of F. occidentalis. Effects of Competition on F. occidentalis Population Abundance in Florida Currently F. occidentalis populations are abundant in North America. However, F. occidentalis are much more abundant in North Florida than Central and Southern Florida, while F. bispinosa is the most abundant thrips species in Central and Southern Florida and its range extends to North Florida (Childers et al. 1990, Kirk 2002, Hansen et al. 2003). High effects of interspecific competition on F. bispinosa, and no effects of interspecific competition on F. occidentalis should influence the abundance of F. occidentalis in Central and Southern Florida. If there were no extrinsic factors, F. occidentalis would be able to out-compete F. bispinosa in pepper flowers throughout Florida. Furthermore, there is an abundance of pepper plants available to support populations of F. occidentalis in southern Florida (Kokalis-Burelle et al. 2002, Hansen et al. 2003), and the species has existed in North Florida and Georgia long enough to invade the southern portions of Florida (Beshear 1983). However, F. occidentalis is not common in central and southern pepper plants, indicating some additional factors affect

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51 the abundance of F. occidentalis in Florida. Differential predation may be one of the factors maintaining higher F. bispinosa abundance in central and southern Florida. Differential predation benefits native mosquitoes and ants by limiting population densities of invasive species (Mehdiabadi et al. 2004, Griswold and Lounibos 2005). Similarly, F. bispinosa may be the most abundant species in southern Florida due to a reduction in F. occidentalis from differential predation by Orius insidiosus, a predator of both species (Reitz et al. 2002). Although O. insidiosus is abundant in the eastern United States, it is only able to actively overwinter in central and southern Florida (Bottenberg et al. 1999 as cited by Hansen et al. 2003). This winter predation may limit F. occidentalis numbers in central and southern Florida. Climate may also limit F. occidentalis populations in central and southern Florida, as the species is considered a temperate to subtropical pest (Kirk and Terry 2003). In addition, F. occidentalis may be limited by alternative host availability. Effects of Competition on World-Wide F. occidentalis Spread F. occidentalis has spread to six continents, all of which have native thrips inhabiting the local flora (Moritz et al. 2001, Kirk and Terry 2003). If F. occidentalis is capable of out-competing the native species, as was shown in this study, it will increase the invasive threat of this economically-damaging crop pest. A better understanding of the competitive interactions between F. occidentalis and native species may lead to new methods of controlling the world-wide spread of F. occidentalis by adjusting environmental or ecological conditions to decrease the competitive advantage of F. occidentalis when it is competing against less damaging species.

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52 Table 3-1 The mean number (SEM) of larvae per female 10 days after different treatment densities of adult female F. occidentalis and F. bispinosa were introduced in cages containing a pepper plant. Number of Adult Females per Pepper Plant Mean number of larvae per female (SEM) F. occidentalis F. bispinosa F. occidentalis F. bispinosa 0 10 1.48(0.51) 0 20 2.16(1.26) 0 30 0.81(0.28) 0 60 0.42(0.09) 10 0 3.18(1.41) 10 10 3.15(0.29) 0.03(0.03) 10 20 5.10(1.41) 0.37(0.20) 10 30 4.48(1.28) 0.25(0.14) 20 0 0.72(0.12) 20 10 3.57(1.39) 0.44(0.18) 20 20 1.33(0.56) 0.68(0.36) 20 30 3.11(1.23) 0.03(0.03) 30 0 1.93(0.54) 30 10 2.60(0.66) 0.17(0.11) 30 20 2.32(0.41) 0.13(0.08) 30 30 1.69(0.76) 0.55(0.27) 60 0 1.03(0.18)

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53 Table 3-2. Model parameters and confidence intervals measuring effects of competition on the number of larvae produced per F. bispinosa female after 10 days on a pepper plant (see text for model). Where c is the value of intraspecific competition, is the competition coefficient, and is the larvae per female produced in the absence of competition. Parameter Value 95% Confidence Interval c 0.266 0.111 to 4.86 4.62 3.58 to 5.97 8.46 4.11 to 134.82 Table 3-3. Model parameters and confidence intervals measuring effects of competition on the number of larvae produced per F. occidentalis female after 10 days on a pepper plant (see text for model). Where c is the value of intraspecific competition, is the competition coefficient, and is the larvae per female produced in the absence of competition. Parameter Value 95% Confidence Interval c 0.0741 0.0543 to 0.104 -0.161 -0.274 to -0.0574 6.16 4.98 to 7.93 Density of Species A 020406 0 Density of Species B 05101520253035 Figure 3-1. Example of an additive design. Each point represents a density treatment, which includes a combination of densities of species A and B per unit area.

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54 Density of Species A 05101520253035 Density of Species B 05101520253035 Figure 3-2. Example of a substitutive design. Each point represents a density treatment, which includes a combination of densities of species A and B per unit area.

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55 Density of F. bispinosa 020406 0 Density of F. occidentalis 0204060 Figure 3-3. Treatments of varying F. bispinosa and F. occidentalis densities to measure the larvae produced per female at different levels of competition. Density 010203040506070 Larvae per Female 0246810 F. bispinosa F. occidentalis Figure 3-4. Simulation of intraspecific competition of F. bispinosa and F. occidentalis based on the competition model (refer text) predicting the number of larvae per female produced after 10 days on pepper plants.

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CHAPTER 4 CONCLUSION Developing a better understanding of the factors affecting thrips abundances in cropping systems may lead to new methods of limiting damage from thrips feeding and the vectoring of Tomato Spotted Wilt Virus (TSWV). Known elements affecting population dynamics include host suitability, migrations from alternative hosts, predation, parasitism, and competition. Migration from uncultivated hosts into crops is known to occur in thrips populations (Pearsall and Myers 2001), but little research has been conducted on the sources of thrips migration (though see Chamberlin et al. 1992, Chellemi et al. 1994, Cho et al. 1995, and Toapanta et al. 1996). My research documents several sources, from which thrips may migrate into cropping systems. The most important of these thrips hosts included R. raphanistrum in the spring and S. canadensis in the fall. Raphanus raphanistrum may serve as a predator free niche, as well as a source of TSWV (Parrella et al. 2003). Solidago canadensis may be an important source of thrips feeding on fall crops. Furthermore, larvae developing on S. canadensis may overwinter as pupae that initiate the establishment of thrips populations in the early spring. Solidago canadensis may also be a TSWV host (Parrella et al. 2003), indicating that it could also be a source of viral infection in fall and spring crops. Plant nutrition and defense influence thrips dynamics, and may influence thrips abundance on these and other reproductive hosts. Furthermore, effects such as predation, parasitism, and competition may be affecting the abundance and distribution on reproductive hosts. 56

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57 Past research has demonstrated effects from predation (Funderberk et al. 2000, Reitz et al. 2003, Baez et al. 2004), and parasitism (Funderberk et al. 2000, 2002), but my study is the first to give evidence of competition occurring in thrips populations. Frankliniella occidentalis is a better competitor than F. bispinosa on peppers in Florida conditions, demonstrating that competitive superiority may be a reason for the invasive ability of this worldwide pest. More research must be conducted on the interactions between the host quality, migration, predation, parasitism and competition. For example, predation by Orius insidiosus may preferentially be feeding on F. occidentalis, limiting the abundance of F. occidentalis in Florida. In addition, research must be conducted on the effect of host plant variation on competition, as host plant quality may influence competition if the thrips species have different nutritional requirements. Conversely, niche displacement and host utilization may be caused by competition or predation. Research on the complex interactions between host quality, migration, predation, parasitism and competition will enable a better understanding of thrips population dynamics, enabling the development of more efficient pest management programs.

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67 Sakimura, K. 1953. Frankliniella tritici, a non-vector of the spotted wilt virus. Journal of Economic Entomology 46:915-916. Sakimura, K. 1962. Frankliniella occidentalis (Thysanoptera: Thripidae), a vector of the Tomato spotted wilt virus, with special reference to the color forms. Annals of the Entomological Society of America 55:387-389. Sakimura, K. 1963. Frankliniella fusca, an additional vector for Tomato spotted wilt virus, with notes on Thrips tabaci, another vector. Phytopathology 53:412-&. SAS Institute 2000. SAS System For Windows 8.01. Cary, NC Scott Brown, A. S., M. S. J. Simmonds, and W. M. Blaney. 2002. Relationship between nutritional composition of plant species and infestation levels of thrips. Journal of Chemical Ecology 28:2399-2409. Smith, F. E. 1961. Density dependence in the Australian thrips. Ecology 42:403-407. Snaydon, R. W. 1991. Replacement or additive designs for competition studies. Journal of Applied Ecology 28:930-946. Stewart, J. W., C. Cole, and P. Lummus. 1989. Winter survey of thrips (Thysanoptera, Thripidae) from certain suspected and confirmed hosts of Tomato spotted wilt virus in south Texas. Journal of Entomological Science 24:392-401. Strauss, S. Y., R. E. Irwin, and V. M. Lambrix. 2004. Optimal defence theory and flower petal colour predict variation in the secondary chemistry of wild radish. Journal of Ecology 92:132-141. Strong, D. R., and R. W. Pemberton. 2000. Ecology Biological control of invading species: risk and reform. Science 288:1969-1970. Stumpf, C. F., and G. G. Kennedy. 2005. Effects of Tomato spotted wilt virus (TSWV) isolates, host plants, and temperature on survival, size, and development time of Frankliniella fusca. Entomologia Experimentalis Et Applicata 114:215-225. Terry, L. I. 1997. Host Selection, Communication and reproductive behavior. Pages 65-118 in T. Lewis, editor. Thrips as crop pests. CAB International, New York. Tipping, C., K. B. Nguyen, J. E. Funderburk, and G. C. Smart. 1998. Thripenema fuscum n. sp (Tylenchida : Allantonematidae), a parasite of the tobacco thrips, Frankliniella fusca (Thysanoptera). Journal of Nematology 30:232-236.

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68 Toapanta, M., J. Funderburk, S. Webb, D. Chellemi, and J. Tsai. 1996. Abundance of Frankliniella spp. (Thysanoptera: Thripidae) on winter and spring host plants. Environmental Entomology 25:793-800. Tsai, J. H., B. S. Yue, J. E. Funderburk, and S. E. Webb. 1996. Effect of plant pollen on growth and reproduction of Frankliniella bisponosa. Acta Horticulturae 431:535-541. Ullman, D. E., T. L. German, J. L. Sherwood, D. M. Westcot, and F. A. Cantone. 1993. Tospovirus replication in insect vector cells: immunocytochemical evidence that the nonstructural protein encoded by the S-RNA of tomato spotted wilt tospovirus is present in thrips vector cells. Phytopathology 83:456-463. Underwood, A. J. 1999. Experiments in ecology: their logical design and interpretation using alalysis of variance. Cambridge University Press, Cambridge. USDA, NRCS. 2004. The PLANTS Database, Version 3.5 (http://plants.usda.gov) Baton Rouge, LA 70874-4490 USA. December, 2004 van Rijn, P. C. J., C. Mollema, and G. M. Steenhuisbroers. 1995. Comparative life-history studies of Frankliniella occidentalis and Thrips tabaci (Thysanoptera, Thripidae) on cucumber. Bulletin of Entomological Research 85:285-297. Wang, K., and J. L. Shipp. 2001. Simulation model for population dynamics of Frankliniella occidentalis (Thysanoptera : Thripidae) on greenhouse cucumber. Environmental Entomology 30:1073-1081. Webb, S. E., M. L. Kok-Yokomi, and J. H. Tsai 1997. Evaluation of Frankliniella bispinosa as a potential vector of Tomato spotted wilt virus. Phytopathology 87:102. Wijkamp, I., J. Vanlent, R. Kormelink, R. Goldbach, and D. Peters. 1993. Multiplication of Tomato spotted wilt virus in its insect vector, Frankliniella occidentalis. Journal of General Virology 74:341-349. Yasuda, H., E. W. Evans, Y. Kajita, K. Urakawa, and T. Takizawa. 2004. Asymmetric larval interactions between introduced and indigenous ladybirds in North America. Oecologia 141:722-731. Young, K. A. 2004. Asymmetric competition, habitat selection, and niche overlap in juvenile salmonids. Ecology 85:134-149. Zeier, P., and M. G. Wright. 1995. Thrips resistance in Gladiolus spp.: Potential for IPM and breeding. Pages 411-416 in B. L. Parker, M. Skinner, and T. Lewis, editors. Thrips biology and management. Plenum Press, New York.

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BIOGRAPHICAL SKETCH Tobin Northfield was born and raised in Enumclaw, WA, where he attended school at Enumclaw High School. He then attended Pacific Lutheran University in Tacoma, WA, where he first decided to pursue a career in entomology during a general entomology course. He earned a Bachelor of Science degree in biology at Pacific Lutheran University. Tobin plans to continue working on insects, and eventually earn a Ph.D. in a related field. 69


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THRIPS COMPETITION AND SPATIOTEMPORAL DYNAMICS ON
REPRODUCTIVE HOSTS















By

TOBIN D. NORTHFIELD


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2005

































Copyright 2005

by

Tobin D. Northfield

































This document is dedicated to my wife, Kirsten for all of her support and understanding
throughout this process. It is also dedicated to my parents for their guidance and support.















ACKNOWLEDGMENTS

I would like to thank my major professor, Dr. Joe Funderburk, for the guidance and

assistance in designing and conducting my experiments, and for helping me develop as a

scientist. I would also like to thank Dr. Dean Paini, a committee member, who also spent

a great deal of time advising and assisting me whenever possible. My other committee

members, Drs. Stuart Reitz and Russ Mizell, were a tremendous source of assistance and

knowledge, for which I am grateful. Dr. Michelle Stuckey proofread drafts and assisted

in the experimental procedure, as well as offered good advice. I would like to thank Dr.

Brian Inouye for his assistance in interpreting the models, and the model-fitting process.

I appreciated Dr. Ben Bolker's patience and instruction on the modeling process and

using R to fit the models. Marcus Griswold helped in providing an R code to work with.

Dr. Todd Jackson also lent assistance and insight in developing my experiments.

Rebecca Riddle also assisted in the experimental procedure, for which I am grateful.
















TABLE OF CONTENTS



A C K N O W L E D G M E N T S ................................................................................................. iv

LIST OF TABLES .................. .................. .................. ........... .............. vii

LIST OF FIGURES ......... ....... .................... .......... ....... ............ ix

A B ST R A C T ......... ..... ............................................................................... ......

CHAPTER

1 L ITER A TU R E R E V IE W ............................................................... .. .....................1

In tro d u ctio n ............................................ ................................ 1
D iv ersity ............................................................ .2
Population A attributes ............................... .................... ............... 3
P la n t D e fe n se s .............................................................................................................. 4
H ost Selection and N nutrition .............................................................................. 5
W within Plant D distribution ................................. ......................... .............
Seasonal D ynam ics ......................................... ..... ................ 7
B io tic F a cto rs ............................................ ................................ 9
N north F lorid a T h rip s .......................................... ... .. ...............................................10
R productive H costs ........................... ................ ................................. 11
C on clu sion .......................................... ... ................................................. 12

2 SPATIOTEMPORAL DYNAMICS ON REPRODUCTIVE HOSTS.......................17

In tro d u ctio n .............. .. ............. ........................................................................... 1 7
M materials and M methods ........................................................................ .................. 18
Sampling Procedure.... ............ ........ ........ .........18
Data Analysis .............. .. .............. ................ ........ 19
R e su lts .............. ..... .......... ................................................................................... 2 1
Discussion .................... ............................ 25
C on clu sion .............. ................. ................................................................3 0

3 INTRASPECIFIC AND INTERSPECIFIC COMPETITION IN THRIPS ON
FLOWERING PEPPER PLANTS ............................... .......................... 43

Introduction ................................................................................................. ....... 43









M materials and M methods ....................................................................... ..................4 5
E x p erim ental D esign ............................................... ......................................4 5
M o d e l F ittin g ................................................................. ..............................4 7
R e su lts .........................................................................................................................4 8
D iscu ssion ............... ..... ........................... ..... ..... ....... .............49
Effects of Competition on F. occidentalis Population Abundance in Florida ....50
Effects of Competition on World-Wide F. occidentalis Spread ......................51

4 CONCLUSION..................... ..................56

LIST OF REFERENCES ....................... .......... ..... ............... 58

B IO G R A PH IC A L SK E T C H ....................................................................................... 69
















LIST OF TABLES


Table page

1-1 F. occidentalis Reproductive Hosts. ............................................. ............... 13

1-2 F.fusca Reproductive H osts. ............................................ ............................ 14

1-3 F. tritici Reproductive H osts .................................. ............... ............... 15

1-4 A list of plants F. bispinosa Reproductive Hosts.................... .................16

2-1 Mean number (SEM) of thrips per 20 Raphanus raphanistrum flowers. ................32

2-2 Mean number (SEM) of thrips per 20 Raphanus raphanistrum leaves ................33

2-3 Mean number (SEM) of thrips per 20 Raphanus raphanistrum fruits...................34

2-4 Mean number (SEM) of thrips per 20 Rubus trivialis flowers..............................34

2-5 Mean number (SEM) of thrips per 20 Rubus trivialis leaves.............................. 35

2-6 Mean number (SEM) of thrips per 20 Rubus trivialis fruits ....................................35

2-7 Mean number (SEM) of thrips per 20 Rubus cuneifolius flowers............................36

2-8 Mean number (SEM) of thrips per 20 Rubus cuneifolius leaves..............................36

2-9 Mean number (SEM) of thrips per 20 Rubus cuneifolius fruits. ............................36

2-10 Mean number (SEM) of thrips per 20 Vicia sativa flowers ...................................37

2-11 Mean number (SEM) of thrips per20 Vicia sativa leaves .....................................37

2-12 Mean number (SEM) of thrips per 20 Vicia sativa leaves ....................................37

2-13 Mean number (SEM) of thrips per 4 Vicia sativa buds.........................................38

2-14 Mean number (SEM) of thrips per 4 Trifolium repens racemes .............................38

2-15 Mean number (SEM) of thrips per 20 Trifolium repens leaves ............................39

2-16 Mean number (SEM) of thrips per 20 Solidago canadensis racemes ....................40









2-17 Mean number (SEM) of thrips per 20 Solidago canadensis leaves .......................40

2-18 Mean number (SEM) of thrips per 20 Chenopodium ambrosioides racemes..........41

2-19 Mean number (SEM) of thrips per 20 Chenopodium ambrosioides leaves ............41

2-20 Mean number of thrips per plant for seven plant species on selected dates.............42

3-1 The mean number (SEM) of larvae per female of adult female F. occidentalis
and F. bispinosa at various levels of densities of each species.............................52

3-2 Model parameters and confidence intervals measuring effects of competition on
the number of larvae produced per F. bispinosa female........................................53

3-3 Model parameters and confidence intervals measuring effects of competition on
the number of larvae produced per F. occidentalis female.................. ........... 53















LIST OF FIGURES


Figure page

3-1 Exam ple of an additive design ........................................... .......................... 53

3-2 Example of a substitutive design................................. ........................ ......... 54

3-3 Treatments of varying F. bispinosa and F. occidentalis densities to measure the
larvae produced per female at different levels of competition ..............................55

3-4 Simulation of intraspecific competition ofF. bispinosa and F. occidentalis based
on the com petition m odel. ...... ........................... ........................................ 55















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

THRIPS COMPETITION AND SPATIOTEMPORAL DYNAMICS ON
REPRODUCTIVE HOSTS

By

Tobin D. Northfield

August, 2005

Chair: Joe Funderburk
Major Department: Entomology and Nematology

Frankliniella spp. thrips feed and reproduce on crops, causing a silvering of plant

tissue, and spread plant diseases such as Tomato spottedwilt virus into crops. However,

little is known about the factors affecting Frankliniella spp. thrips abundance and

distribution. Thrips often migrate into cropping systems from surrounding vegetation,

but few of these uncultivated plant sources have been studied to determine cycles of

thrips abundance to evaluate sources of thrips migration. Furthermore, no research has

been conducted on the population effects of competition in thrips to better understand

thrips distribution. This study was composed of a field work portion, which focused on

evaluating uncultivated plant host use, and a laboratory portion, which focused on

competitive interactions between Frankliniella occidentalis and F. bispinosa.

For the field work study, samples were collected from Raphanus raphanistrum,

Rubus trivialis, Rubus cuneifolius, Trifolium repens, Vicia sativa, Solidago canadensis,

and Chenopodium ambrosioides, and thrips spatiotemporal dynamics were determined









for each. Thrips preferred flowers to leaves on every plant species sampled.

Frankliniella spp. thrips were most abundant on R. raphanistrum, T. repens, and R.

cuneifolius in spring months and S. canadensis in the fall. The most abundant thrips

species collected were F. tritici and F. bispinosa. These plants may serve as important

sources of Frankliniella spp. thrips, and reducing abundance of the available hosts may

decrease thrips populations migrating into cropping systems.

For the laboratory study, interspecific competition was evaluated between the

world-wide crop pest F. occidentalis, and F. bispinosa, a species native to Florida. In

addition, intraspecific competition for each species was assessed. Larvae per female of

F. bispinosa and F. occidentalis were counted at varying densities of each species, using

a factorial response surface design. A competition model was fit to the data for each

species to evaluate effects of interspecific and intraspecific competition on the number of

larvae per female produced. Significant interspecific and intraspecific competition

affected F. bispinosa, and the effect of interspecific competition from F. occidentalis was

over four times greater than the effect from intraspecific competition. Interspecific

competition did not affect F. occidentalis, but statistically significant intraspecific

competition occurred. Furthermore, intraspecific competition had a greater effect on F.

bispinosa than on F. occidentalis. This superior competitive ability may enhance the

spread ofF. occidentalis, and competition between F. occidentalis and native species

must be assessed when considering the world-wide spread ofF. occidentalis.














CHAPTER 1
LITERATURE REVIEW

Introduction

Debate over the factors that affect thrips population dynamics have persisted for

more than 50 years, since Davidson and Andrewartha (1948a, b) first claimed there were

no density dependent factors affecting populations. Several researchers subsequently

refuted their conclusions (Smith 1961, Orians 1962), and more recently both density

dependent and density independent factors have been shown to influence thrips

population dynamics. These factors include host defenses (deJager et al. 1996), plant

selection and nutrition (Brodbeck et al. 2002), climate (Br0dsgaard 1993), predation

(Baez et al. 2004), and parasitism (Funderburk et al. 2002). Despite these studies, little is

known of the temporal and spatial dynamics of thrips, particularly outside cropping

systems. The purpose of this chapter is to discuss recent research on density dependent

and independent factors that affect thrips spatial and temporal dynamics.

The population dynamics of thrips are strongly influenced by their small body

size (0.5-5.0 mm in length), which confers both advantages and disadvantages. Such

disadvantages include large fluctuations in body temperature and water loss, due to a high

surface area to volume ratio (Kirk 1997a). Conversely, thrips' small size permits escape

from predators to small, secure areas on the host (Sabelis and Van Rijn 1997), and also

may result in extensive wind dispersal as gusts of wind can disrupt thrips' flight patterns

(Pearsall and Myers 2001).









Diversity

An appreciation of the natural history and diversity within the Thysanoptera order

can reveal similarities in the population characteristics between phylogenetically related

species, thereby enhancing the current understanding of thrips population dynamics. The

diversity of the thrips order is exemplified by the diversity in feeding habits, which have

evolved from that of a fungus feeding ancestor related to Hemiptera, Psocoptera, and

Pthiraptera (Mound 1997, Moritz et al. 2001) to species that have adapted to feed on

leaves, flowers and small arthropods (see Kirk 1997b for review).

The order Thysanoptera is divided into two sub-orders, the Tubulifera and

Terebrantia. The Tubulifera consists of a single family, the Phlaeothripidae, which

consists of over three thousand species, mostly living on fungus in wet tropics (Moritz et

al. 2001). Tubulifera use a U-shaped ovipositor, rather than a straight ovipositor like the

Terebrantia. The U-shaped ovipositor is used to deposit eggs on the surface of, rather

than into the host tissue, as the fungus provides adequate protection for the eggs (Terry

1997).

In contrast to the predominantly fungivorous Tubulifera, the sub-order Terebrantia

includes eight families of thrips that display a wide variety of food preferences and use a

saw-like ovipositor to insert one egg at a time into the host tissue (Terry 1997). The

largest and most diverse family of Terebrantia is the Thripidae, which are represented by

over 1,750 species in 260 genera. Species of Thripidae range from Greenland to the sub-

Antarctic islands (Moritz et al. 2001). One sub-family of Thripidae, the leaf-feeding

Pancheatothripinae, comprising 120 species in 35 genera, is found throughout the tropics

and sub-tropics, and includes some crop pests. Thripinae, a more diverse sub-family of

Thripidae, consists of approximately 1,400 species in over 200 genera. Many feed and









oviposit in leaves, and some of the more recently evolved species feed and oviposit in

flowers. This group exhibits a wide variety of feeding habits and includes thrips species

that are predaceous, anthophagous, phytophagous, or even associated with mosses

(Mound 1997). The wide variety of food preferences of many Thripinae species includes

commercial crops, and some of these species can cause direct damage to crops by feeding

and oviposition as well by vectoring plant viruses (Mound 1997, Moritz et al. 2001).

Population Attributes

Thysanoptera are opportunistic and r-selected, utilizing a high reproductive rate and

short generation time to enhance population growth rates under favorable conditions

(Mound 1997) and the exploitation of ephemeral resources (Mound and Tuelon 1995).

Parthenogenesis, a form of asexual reproduction, is a strategy used to enhance

reproduction, and in combination with Fi back-crossing, can result in a single female

forming an entirely new population (Mound and Tuelon 1995). Furthermore,

parthenogenesis and back-crossing of an insecticide-resistant female can lead to rapid

growth of an insecticide-resistant population, further aiding in adaptation. Vagility

enhances the opportunistic, r-selected strategy, improving location and exploitation of

new environments and food resources (Mound and Tuelon 1995). A moderately broad

food tolerance enhances vagility by enabling invasive thrips to survive in new

environments of limited host diversity and increases population stability during seasonal

decline of preferred host availability (Mound 1997).

The extent of vagility and opportunism in thrips varies by feeding group (Mound

and Tuelon 1995). Polyphagous thrips are more vagile than monophagous thrips, which

often develop periodicity with the cycle of food availability. Anthophagous thrips

include many polyphagous, vagile species that can exploit ephemeral resources.









Alternatively, foliage-feeding thrips include few polyphagous, vagile species, as many

develop a cyclic lifestyle in line with the host. The most damaging and opportunistic

crop pests are those that feed on both flowers and leaves, and move to new food sources

as hosts become inadequate or unavailable without developing host-correlated

periodicity.

Predaceous thrips are also opportunistic, due to the opportunistic nature of their

prey, but few fungal spore or hyphae feeders are vagile or opportunistic because their

food source is stable (Mound and Tuelon 1995). Exceptions include those in the tropics,

where dry fungi hanging in trees are preferred, and thrips must feed before the fungi falls

to the ground. Gall-forming thrips are monophagous, and feed in a stable environment,

and are therefore less opportunistic than other thrips species.

Plant Defenses

A generalist strategy allows phytophagous thrips to feed on a number of hosts and

gain access to a variety of available nutrients (Mound and Tuelon 1995). However, a

generalist strategy usually includes constraints in plant defense adaptation. Theoretically,

a generalist cannot coevolve with a range of hosts as well as a specialist can with a single

host plant, due to evolutionary constraints (Ananthakrishnan and Gopichandran 1993).

Although the vagility and opportunistic nature of phytophagous thrips enhances

adaptability, plant defenses may cause one host to be less suitable than others.

Morphological plant defenses, such as dense trichomes, limit phytophagous thrips

host suitability. For example, nectarines and peaches are the same species (Prunus

persica (L.) Batsch), but Frankliniella occidentalis prefer nectarine hosts due to the

smoother tissue of the developing fruit (Felland et al. 1995). Other examples include

surface wax and epidermal cell wall thickness, which reduce leaf host quality (Zeier and









Wright 1995), and pollen stickiness, which contributes to a plant's defense due to extra

handling and grooming time (Kirk 1985).

Plant chemical defenses, or allelochemicals, reduce thrips' host preference as well

(deJager et al. 1995a, deJager et al. 1995b, Kumar et al. 1995). Allelochemical

precursors may include acetyl coenzyme A, mevalonic acid, and shikimic acid and are

grouped into quantitative or qualitative defenses (Lowman and Morrow 1998). The

effect of quantitative defenses, or digestive reducers, varies by concentration. These

immobile, carbon-based chemicals accumulate with tissue age, and passively or actively

decrease thrips nutrition. Qualitative defenses are mobile chemicals that affect essential

functions, such as respiration or DNA repair, in small chemical concentrations and

degrade quickly. Resistance to thrips may be caused by a single chemical present in the

plant tissue or by a synergistic effect from a number of chemicals (deJager et al. 1996).

These chemicals may be found in a number of plant parts, including leaves or flowers;

however constitutive (continuously present) defenses may be more concentrated in flower

tissue than in leaves (Strauss et al. 2004).

Host Selection and Nutrition

Thrips feeding and oviposition choices may be due in part to thrips host location

cues. Host location cues may include visual cues such as the colors blue, white and

yellow (Frey et al. 1994, Cho et al. 1995a, Childers and Brecht 1996, deKogel and

Koschier 2002). Olfactory host location cues may also be important, but the level of

importance is unclear. In choice tests, F. occidentalis individuals were attracted to

volatile chemicals extracted from chrysanthemum flowers, but could not locate whole

flowers without visual cues (Koschier et al. 2000, deKogel and Koschier 2002). Flower

type also affects host selection in plant species that exhibit more than one flower type. In









chrysanthemums, F. occidentalis prefer cultivars with disc florets over spider-type

flowers that do not include disc florets (Broadbent and Allen 1995, deJager et al. 1995a).

Another important factor in thrips host selection is the nutritional quality of the

plant (deJager et al. 1995b, Mollema and Cole 1996), though little is known about thrips

nutritional ecology (Brodbeck et al. 2002). Mass determinations for growth rate and

tissue samples for nutrient retention are difficult to obtain due to thrips' small size, and

short development time and non-feeding prepupae and pupae make the organism difficult

to observe. Therefore, population experiments are often easier to conduct than individual

tests.

Past population experiments have shown that population growth is correlated with

nitrogen concentration. Because immature thrips molt four times in a short time span,

and only eat in two instar stages, they must consume large amounts of amino acids to

build new proteins to support the rapid growth (Kirk 1995, Brodbeck et al. 2001). Thrips

may therefore prefer hosts with essential amino acids, especially those most rarely found

in plants: tryptophan, phenylalanine, and methionine (Ananthakrishnan and

Gopichandran 1993). Recent studies have shown a strong correlation between thrips crop

damage and concentration of aromatic amino acids, especially phenylalanine, which

enhances cuticle production and hardening, reducing the occurrence of dessication or

entomopathogenic fungal infection (Mollema and Cole 1996). Glutamine, which can be

converted to other essential amino acids, may also stimulate thrips feeding (Andersen et

al. 1992).

In addition, carbohydrates are important to thrips nutrition and may stimulate

feeding. There has been some success in adding sugars to insecticide to increase









insecticide consumption (Parrella 1995) and plant carbohydrate concentration increases

F. occidentalis feeding rates, though not as strongly as plant protein concentration (Scott

Brown et al. 2002).

Within Plant Distribution

Thrips' small size enables access to recessed areas of a plant, which provide small

microclimates that inhibit the desiccation or freezing of thrips (Kirk 1997a). These

crevices also enhance protection from predation and being washed off the plant by rain.

Vertical distribution ofthrips within a plant appears to vary by host plant. In

tomatoes most adult thrips feed in the upper portions of the plant, especially in the spring,

while larvae are found in the lower portions (Navas et al. 1994, Reitz 2002). In

cucumbers, F. occidentalis prefer higher, younger leaves for oviposition, and in a non-

choice experiment, oviposition on younger cucumber leaves produced more offspring

than on older leaves (deKogel et al. 1997b). In British Columbia nectarine orchards, F.

occidentalis adults are more common in the lower portions of the trees, possibly due to a

preference of low lying plant hosts (Pearsall 2002).

Within plant distribution and movement also varies with season and thrips

species. For example, F. occidentalis fly higher in the summer than in the spring in

nectarine orchards (Pearsall and Myers 2001). In addition, F. tritici and F. bispinosa are

more locally mobile than F. occidentalis (Ramachandran et al. 2001, Hansen et al. 2003),

and F. fusca is generally considered more of a foliage feeder than F. occidentalis

(Chellemi et al. 1994, Pearsall and Myers 2000).

Seasonal Dynamics

Low temperatures can reduce host availability, making overwintering difficult for

thrips (Kirk 1997a). F. occidentalis have a developmental threshold of 100C (Robb









1989, Cho et al. 1995b), and can survive at -50C for approximately 56-63 hours

(Br0dsgaard 1993). Frankliniella species either overwinter actively or are dormant in

soil, while some species are capable of both, depending on environmental conditions.

For example, in higher latitudes, such as British Columbia, Canada, F. occidentalis

overwinter as dormant, mated females in soil (Pearsall and Myers 2000), while in lower

latitudes such as North Carolina, F. occidentalis survive winters actively on hosts (Cho et

al. 1995b). Soil type and host plant species also influence thrips overwintering, as larvae

burrow further into lighter soil than heavier soil (MacGill 1929, 1930) and do not burrow

into sand (Bailey 1933). Furthermore, thrips with cold resistant hosts may be less likely

to overwinter in soil than those without (Kirk 1997a).

Thrips populations increase dramatically in early spring, in the presence of

increased temperatures and host bloom (Childers et al. 1990, Chellemi et al. 1994,

Pearsall and Myers 2000, 2001). This rapid increase varies temporally with latitude. For

example, in South Florida F. bispinosa increases in March (Childers et al. 1990), whereas

in North Florida F. bispinosa displays greatest increase in April and May (Chellemi et al.

1994). Similarly, in North Florida F. occidentalis population densities rapidly increase in

early spring (Chellemi et al. 1994), but show greatest increase in late April and May in

British Columbia, Canada (Pearsall and Myers 2000, 2001).

Summer usually includes a significant decrease in thrips populations (Childers et

al. 1990, Chellemi et al. 1994, Reitz 2002, Nault et al. 2003). Reasons for this population

decrease are unclear, as it does not appear due to temperature levels increasing above

thrips range. In North Florida, Reitz (2002) found a decrease in F. occidentalis in May,

although the temperatures never rose above F. occidentalis' optimal temperature range









(29-310C) throughout the experiment (van Rijn et al. 1995, Florida Automated Weather

Network UF/IFAS 2003).

One possible reason for the summer decline in thrips populations is the presence of

natural enemies. Orius insidiosus (Say), a natural predator ofF. occidentalis, is most

abundant in May and June and may decrease thrips densities in summer months

(Ramachandran et al. 2001, Reitz et al. 2003).

Increased host resistance may also influence thrips summer decline. DeKogel et al.

(1997a) found a negative correlation between thrips damage and solar radiation. The

authors suggested that summer plant hosts might be more resistant to thrips, increasing

thrips population decline.

Populations increase slightly in the fall, possibly due to additional host bloom

and/or reduced predation by natural enemies (Cho et al. 1995b, Reitz 2002, Groves et al.

2003). Although 0. insidiosus is present during the fall, it is less abundant than during

early summer months, and this decline may enable thrips populations to increase again

(Ramachandran et al. 2001).

Biotic Factors

Predation by 0. insidiosus affects thrips population size, and may affect spatial and

temporal dynamics (Funderburk et al. 2000). Thrips populations may migrate to

alternative hosts in the presence of 0. insidiosus (Funderburk 2002) causing less

preferred hosts to gain higher populations in the presence of predators. Predation from O.

insidiosus may also affect thrips species ratios by predating one thrips species

preferentially (Baez et al. 2004).









Thrips spatial and temporal dynamics are also affected by parasitism and possibly

competition. Nematodes (Thripinema spp.) infect thrips, leaving females sterile and

reducing population growth (Funderburk et al. 2002). In addition, competition may

affect population dynamics. No research has been published on thrips competition, but

distribution, fecundity and mortality appear density-dependent in greenhouse thrips

(Puche and Funderburk 1992, Kirk 1994 as cited by Kirk 1997a). Competition may be a

cause of the density dependent distribution and decreased fecundity and survivorship.

North Florida Thrips

Three native thrips species Frankliniella bispinosa, F. fusca and F. tritici, and an

introduced species, F. occidentalis damage crops in North Florida. Feeding and

reproduction of these pests causes chlorosis, deformation of leaves and leaflets, stunting

of plants, reduction of photosynthesis, and induction of air pockets in cells, causing fruit

malformation and scarring (Chamberlin et al. 1992, Funderburk et al. 1998, Fung et al.

2002, Hao et al. 2002). In addition, F. bispinosa causes premature cellular evacuation,

cellular collapse, necrosis and plasmolysis, leading to premature fruit drop (Childers et al.

1994).

Species that vector tospoviruses such as Tomato spotted wilt virus can also

indirectly reduce crop yields. Tomato spotted wilt causes plant wilt and fruit

malformation, greatly reducing crop yield (see Prins and Goldbach 1998 for review).

Vectors of tomato spotted wilt in North Florida include F. occidentalis, F. fusca, and F.

bispinosa (Sakimura 1962, 1963, Webb et al. 1997). Thrips tabaci, and Frankliniella

schultzei also vector tomato spotted wilt (Sakimura 1963, Cho et al. 1988), but are not

common in the area. Although F. tritici is a common crop pest, this species does not

vector tomato spotted wilt (Sakimura 1953, 1962, de Assis Filho et al. 2004).









Only larvae acquire transferable virus, when the salivary glands are adjacent to the

midgut and movement of the virus from the midgut to the salivary glands is possible

(Moritz et al. 2004). The virus replicates in the salivary glands as the thrips matures and

is eventually transferred to other plants via the saliva (Ullman et al. 1993). Because only

larvae acquire the virus, only those plant species that are reproductive hosts can serve as

sources for the virus.

Reproductive Hosts

Little is known of the range of thrips' reproductive hosts. Although reproductive

hosts can be identified as those plants with larvae present, there is no key for

Frankliniella larvae, so identifying the larvae is difficult. Most publications list adult

feeding hosts, or include reproductive hosts, but do not specify the reproducing species,

due to the inability to identify larvae (though see Childers et al. 1990, Chamberlin et al.

1992, Childers et al. 1994, Cho et al. 1995b, Groves et al. 2002). Rearing thrips larvae to

determine species is often difficult due to high mortality in lab rearing, especially when

thrips are reared on an alternative host. However, a molecular technique has been

developed to determine Frankliniella larval species, aiding in future experiments (Moritz

et al. 2002).

Frankliniella fusca, F. occidentalis, F. tritici, and F. bispinosa reproduce on a

range of dicots and some monocots. Although knowledge is limited, there are some

known taxonomic groups that thrips prefer. For example, F. occidentalis reproductive

hosts include three or more host species each in Asteraceae, Fabaceae, Rosaceae, and

Solanaceae (Table 1-1), and F. fusca reproduces mostly on Asteraceae, Fabaceae and

Poaceae plants (Table 1-2). Little research has been focused on F. tritici and F. bispinosa

due to F. tritici's inability to vector TSWV, and the geographically limited range off.









bispinosa (Childers et al. 1990, Childers et al. 1994, Tsai et al. 1996). At present, there

are no apparent trends in F. tritici and F. bispinosa reproductive hosts (Tables 1-3 and 1-

4).

Conclusion

There are several density dependent and independent factors affecting thrips

population dynamics. However, most of these factors are poorly understood. More

research is needed to understand aspects of thrips ecology such as competition, predation,

parasitism, and utilization of uncultivated hosts. Understanding these factors will allow a

more complete knowledge of thrips population dynamics, enabling development of better

pest management programs. This thesis will present research conducted on utilization of

uncultivated plant hosts and competition between thrips species.












Table 1-1. F. occidentalis Reproductive Hosts.
F. occidentalis Reproductive
Host Common Name
Arctium lappa Burdock
Chi in/lllw'aniUn morifolium Florist's Daisy
Lactuca serriola Prickly Lettuce
Lactuca sativa var. longifoliaRomaine Lettuce


Verbesina encelioides
Impatiens walleriana
Raphanus raphanistrum *
Cucumis sativus
Trifolium velki lhoin'"
Medicago polymorpha
Medicago \ aiv'a"
Trifolium repens
Trifolium velki lhoin'"
Vicia villosa
Saintpaulia ionantha
Alstroemeria sp.
Rosa sp.
Prunus persica
Prunus Persica var.
nucipersica
Prunus .Se ,l i h'
Capsicum annuum
Datura stramonium
Lycopersicon esculentum
Solanum niagrum


Crownbeard
Buzzy Lizzy
Wild Radish
Cucumber
Arrowleaf Clover
Bur Clover
Alfalfa
White Clover
Arrowleaf Clover
Hairy Vetch
African Violet
Allstroemeria
Rose
Peach

Nectarines
Black Cherry
Pepper
Jimson Weed
Tomato
Amer. Black Nightshade


Family
Asteraceae
Asteraceae
Asteraceae
Asteraceae
Asteraceae
Balsaminaceae
Brassicaceae
Cucurbitaceae
Fabaceae
Fabaceae
Fabaceae
Fabaceae
Fabaceae
Fabaceae
Gesneriaceae
Liliaceae
Rosaceae
Rosaceae

Rosaceae
Rosaceae
Solanaceae
Solanaceae
Solanaceae
Solanaceae


*Not confirmed, assumed due to high abundance of adults in the presence of larvae


Source
Bautista et al. 1995
Monteiro 2002
Stewart et al. 1989
Bautista et al. 1995
Stewart, et al. 1989, Mitchell and Smith 1996
Chen et al. 2004
Buntin and Beshear 1995
deKogel et al. 1997b
Chamberlin et al. 1992
Stewart et al. 1989
Monteiro 2002
Heagle 2003
Chamberlin et al. 1992
Toapanta et al.. 1996
Monteiro 2002
Monteiro 2002
Chamberlin et al. 1992, Monteiro 2002
Monteiro 2002

Monteiro 2002
Chamberlin et al. 1992
Scott Brown et al. 2002, Reitz et al. 2003
Bautista et al. 1995
Navas et al. 1994
Stewart et al. 1989












Table 1-2. F.fusca Reproductive Hosts.
F. fusca Reproductive Host Common Name
Emilia sonchifolia Lilac Tasselflower
Gnaphalium obtusifolium* Rabbit Tobacco
Gnaphalium purpureum Spoonleaf Purple Ever.
Hypochaeris radicata Hairy Catsear
Lactuca scariola Prickly Lettcuce
Lactucafloridana Woodland lettuce
Sonchus asper Spiny Sowthistle
Taraxacum officinale Dandelion
Verbesina encelioides Crownbeard
Raphanus raphanistrum Wild Radish
Cerastium vulgatum* Mouseear Chickweed
'lel /t///inh annuus German knotgrass
Stellaria media Common Chickweed
Arachis hypogaea Florunner Peanut
Arachis hypogaea Volunteer Peanut
Trifolium campestre Field Clover
Geranium carolinianum* Carolina Geranium
Allium vineale Wild Garlic
Lamium amplexicaule Henbit Deadnettle
Plantago rugeli Blackseed Plantain
Plantago lanceolate* Buckhorn Plantain
Agropyron I7le,\' Quackgrass
Secale cereale Winter Rye
Triticum aestivum Winter Wheat
Ranunculus sardous Hairy Buttercup
Capsicum annuum var. camelot Cheyenne Pepper
Datura stramonium Jimsonweed


Family
Asteraceae
Astaraceae
Asteraceae
Asteraceae
Asteraceae
Asteraceae
Asteraceae
Asteraceae
Astaraceae
Brassicaceae
Caryophyllaceae
Caryophyllaceae
Caryophyllaceae
Fabaceae
Fabaceae
Fabaceae
Geraniaceae
Lilliaceae
Lamiaceae
Plantaginaceae
Plantaginaceae
Poaceae
Poaceae
Poaceae
Ranunculaceae
Solanaceae
Solanaceae


* Not confirmed, assumed due to high abundance of adults in the presence of larvae


Source
Stumpf and Kennedy 2005
Cho et al. 1995b
Groves et al. 2002
Groves et al. 2002
Groves et al. 2002
Johnson et al. 1995
Johnson et al. 1995, Groves et al. 2002
Cho et al. 1995b, Groves et al. 2002
Mitchell and Smith 1996
Cho et al. 1995b, Groves et al. 2002
Cho et al. 1995b
Groves et al. 2002
Groves et al. 2002
Funderburk et al. 1998, Tipping et al. 1998
Chamberlin et al. 1992
Groves et al. 2002
Cho et al. 1995b
Cho et al. 1995b
Groves et al. 2002
Groves et al. 2002
Cho et al. 1995b
Cho et al. 1995b
Buntin and Beshear 1995
Buntin and Beshear 1995
Johnson et al. 1995, Groves et al. 2002
Toapanta et al. 1996, Reitz et al. 2003
Stumpf and Kennedy 2005














Table 1-3. F. tritici Reproductive Hosts.
F. tritici Reproductive Host Common Name Family Source
Raphanus raphanistrum* Wild Radish Brassicaceae Buntin and Beshear 1995
Trifolium vesiculosum* Arrowleaf CloverFabaceae Chamberlin et al. 1992
Vicia villosa Hairy Vetch Fabaceae Toapanta et al. 1996
Ranunculus sardous Hairy Buttercup Ranunculaceae Johnson 1995
Capsicum annuum var. camelot Cheyenne Pepper Solanaceae Reitz et al. 2003
Not confirmed, assumed due to high abundance of adults in the presence of larvae














Table 1-4. A list of plants F. bispinosa Reproductive Hosts.
F. bispinosa Reproductive Host Common Name
Alocasia cucullataA Chinese taro
Bidens pinosa/ Spanish needle
Phoenix roebelenii^ Pygmy date palm
Raphanus raphanistrum* Wild Radish
Capsicum annuum Cheyenne Pepper
Pinus elliottii*
var. densa/A


Pinus taeda*

Prunus Caroliniana*
Citrus paradise *
Citrus sinensis var. navel*
Citrus sinensis var. valencia*

Salix caroliniana*
Tyvha dominzensis/


Loblolly Pine
Carolina
Laurelcherry
Grapefruit
Navel Oranges
Valencia Oranges
Coastal Plain
Willow
Cattail


Family
Araceae
Asteraceae
Arecaceae
Brassicaceae
Solanaceae
Pinaceae
Pinaceae
Pinaceae

Rosaceae
Rutaceae
Rutaceae
Rutaceae

Salicaceae
Typhaceae


* Not confirmed, assumed due to high abundance of adults in the presence of larvae
AReproduced on lab Pollen


Source
Tsai et al. 1996
Tsai et al. 1996
Tsai et al. 1996
Eger et al. 1998
Childers et al. 1994, Reitz et al. 2003
Childers et al. 1994
Tsai et al. 1996
Childers et al. 1994

Childers et al. 1994
Childers et al. 1990
Childers et al. 1990
Childers et al. 1990

Childers et al. 1994
Tsai et al. 1996














CHAPTER 2
SPATIOTEMPORAL DYNAMICS ON REPRODUCTIVE HOSTS

Introduction

Frankliniella spp. thrips cause extensive economic damage to many types of

crops through feeding and oviposition (Kirk 2002). Damage is either direct, from feeding

that causes a silvering of plant tissue, or indirect, via the transmission of tospoviruses,

including Tomato spotted wilt virus, one of the most damaging worldwide plant viruses

(Prins and Goldbach 1998). The most researched of the Frankliniella thrips is

Frankliniella occidentalis (Pergande) (see Kirk and Terry 2003 for review). This

worldwide crop pest, native to California (Kirk and Terry 2003), has a broad host range

and feeds on many types of crops, including many present in North Florida (Buntin and

Beshear 1995, Puche et al. 1995, Funderburk et al. 2000, Funderburk et al. 2002). Other

Frankliniella species occurring in North Florida are F. fusca (Hinds), F. bispinosa

(Morgan) and F. tritici (Fitch), all three of which are native to the southeastern United

States. Tomato spotted wilt virus vectors include F. occidentalis, F. fusca, and F.

bispinosa, but not F. tritici (Sakimura 1953 as cited by Sakimura 1962, Sakimura 1962,

1963, Webb et al. 1997, de Assis Filho et al. 2004).

In order to make better predictions of thrips population abundance, it is necessary

to study cycles of thrips abundance (Funderburk 2002). These cycles have been studied

extensively in crops such as tomatoes (Reitz 2002, Nault et al. 2003), citrus (Childers et

al. 1990), nectarines (Felland et al. 1995, Pearsall and Myers 2000), and small grains

(Buntin and Beshear 1995), but little research has been conducted in uncultivated plant









hosts (though see Chamberlin et al. 1992, Chellemi et al. 1994, Cho et al. 1995, Toapanta

et al. 1996). Furthermore, past studies either did not monitor thrips populations over the

entire year (Chamberlin et al. 1992, Cho et al. 1995, Toapanta et al. 1996), or did not

present thrips numbers for each plant host (Chellemi et al. 1994). Thrips often migrate

from uncultivated hosts into cropping systems (Pearsall and Myers 2001), so cycles of

abundance on uncultivated hosts must be understood to locate potential sources of thrips

populations and Tomato spotted wilt virus.

Often adult thrips feed on a host, but do not reproduce on the plant (Chamberlin et

al. 1992), so a distinction must be made between feeding hosts and reproductive hosts.

Reproductive hosts have a more direct connection to population growth than feeding

hosts, and are the only sources of Tomato spotted wilt virus, since a transferable virus can

only be acquired by a larva (Ullman et al. 1993, Wijkamp et al. 1993). Therefore, it is

important to focus on reproductive hosts rather than plants where only adults occur (i.e.

feeding hosts). The objective of this study was to determine the cycles of abundance of

Frankliniella species on several species of potential reproductive plant hosts growing in

field margins. The leaves, fruits and flowers of each plant were sampled to compare

plant parts inhabited by the larvae and adults.

Materials and Methods

Sampling Procedure

The study was conducted at the North Florida Research and Education Center in

Quincy, Gadsden County. The plants sampled were Solidago canadensis L.,

Chenopodium ambrosioides L., Rubus trivialis Michx., R. cuneifolius Pursh., Raphanus

raphanistrum L., Trifolium repens L., and Vicia sativa L. These species were selected

based on the work of Chamberlin et al. (1992), Cho et al. (1995), Groves et al. (2002),









Heagle (2003) and Dean Paini (Personal Communication). Each species was sampled

biweekly, when available, between November 19, 2003 and November 5, 2004

On each sample date, 10 different sites were selected, and one plant was sampled

from each site. For each plant, 20 leaves, 20 flowers, 20 fruits, and 20 racemes were

placed, as appropriate for each plant species, in vials containing 70-95% ethanol. For

clover, only four racemes were sampled per plant due to the low number and large size of

the racemes. For V sativa, which has prominent terminal buds, four buds were sampled

per plant, in addition to the flowers, fruits and leaves. The total numbers of flowers,

fruits and leaves per plant were also estimated. Because thrips were highly aggregated in

the flowers, the number of each thrips species per flower and the number of flowers per

plant were used to estimate the total number of each thrips species per plant on dates

when thrips were common on the plant host. In the laboratory, the contents of each vial

were placed in a Petri dish, and the plant parts were dissected to extract thrips. Adult

thrips were identified under a microscope using 6.5-40x magnification. Larvae were

counted, but not identified, because no morphological keys were available.

Data Analysis

Repeated measures ANOVA analyses and Tukey's tests were used to determine

the effect of plant part and date on combined thrips densities for data collected when all

plant parts were present. A one-way ANOVA was conducted to analyze plant part means

of R. cuneifolius, as all three parts were only present on one sample date.

Separate repeated measures ANOVA analyses were conducted on the number of

Frankliniella spp. thrips per flower on each sample date, and the interaction of

Frankliniella species by date was used to compare the patterns of abundance of the

different thrips species (Littell et al. 1996). An unidentified non-Frankliniella species









was abundant on C. ambrosioides, so it was included in the analysis. Effects were

considered significant when p < 0.05. In order to compare the patterns of abundance of

the two most abundant species present, contrast procedures were conducted on the

interaction between date and species. Specifically, contrast procedures were conducted

on the interaction between date and the means ofF. tritici and F. bispinosa on each host:

R. raphanistrum, R. cuneifolius and T. repens. Contrast procedures were conducted on

the interaction between date and the means ofF. tritici and F. fusca on each host: R.

trivialis, and V. sativa. For S. canadensis, no contrast procedure was conducted due to

the low numbers ofF. fusca, F. bispinosa, and F. occidentalis. For C. ambrosioides, a

contrast procedure was conducted on the interaction between date and the means ofF.

tritici and the non-Frankliniella species. Data were only analyzed when thrips were

present. For R. raphanistrum these dates were April 14 through July 20, 2004. For R.

trivialis, these dates were March 29 through April 29, 2004. For R. cuneifolius, these

dates were March 29 through April 29, 2004. For V. sativa, these dates were March 29 to

April 14, 2004. For T repens, these dates were April 14 to May 24, 2004. For S.

canadensis, these dates were September 9 to October 21, 2004. For C. ambrosioides,

these dates were August 19 to October 21, 2004.

All analyses were conducted using SAS (SAS Institute 2000). A Cochran's test

for homogeneity showed there was significant heterogeneity in the data, so the data were

log-transformed using the formula log(x+l) to increase homogeneity. There was still

significant heterogeneity in the data for comparing abundances of different thrips species,

but the F- tests were considered robust enough to remain unaffected due to the number of

treatments and sample size (Underwood 1999).









Results

There were 8,112 thrips extracted from 2,068 samples of the seven hosts, and

62% were adults. The adult thrips collected were 75.9% F. tritici, 14.7% F. bispinosa,

3.5% F. fusca, 1.1% F. occidentalis, and 4.8% non-Frankliniella spp. thrips.

Raphanus raphanistrum flowered from December 5, 2003 to July 20, 2004.

Thrips were most abundant from April 14 to July 20 (Tables 2-1 through 2-3). There was

a significant interaction between plant part and date (F = 4.44; df = 18, 184; p<0.0001).

There was a significant difference between thrips densities on plant parts (F = 183.01; df

= 2, 34; p<0.0001), with more thrips on flowers than on leaves or fruits (Tukey's

p<0.0001). The most abundant thrips species were F. tritici and F. bispinosa, comprising

74.5% and 19.9% of adults, respectively. There was a significant difference between the

densities of thrips species (F = 65.97; df = 3, 36; p<0.0001), and a significant date effect

(F = 25.76; df = 6, 184; p<0.0001) and interaction between species and date (F = 7.29; df

= 18, 184; p<0.0001). There was a significant interaction between date and the means of

F. tritici and F. bispinosa (F = 7.17; df = 6, 184; p<0.0001). Both species were abundant

from April 14 to July 20, and there were more F. tritici than F. bispinosa collected on all

dates. However, there were low abundances of both species on June 9, and since F. tritici

was more abundant on the previous date than F. bispinosa, there was a greater relative

decrease in F. tritici than in F. bispinosa, causing a difference in the patterns of

abundance. Larvae consisted of 34.6% of total thrips present and were most abundant

from April 14 to July 20.

Rubus trivialis flowered from March 2 to April 29, 2004 and thrips were most

abundant March 29 through April 14 (Tables 2-4 through 2-6). There was a significant

interaction between plant part and date (F = 3.13; df = 4, 35; p<0.05). There was a









significant difference in thrips densities on plant parts (F = 16.41; df = 2, 38; p<0.0001)

from March 29 to April 29, with significantly more thrips on flowers than fruits or leaves

(Tukey's p<0.0001). The most abundant thrips species were F. tritici and F. fusca,

comprising 54.0% and 14.7% of adults, respectively. There was a significant difference

in densities of thrips species (F = 2.70; df = 3, 36; p<0.005), and a significant interaction

between species and date (F = 2.70; df = 6, 48; p<0.05), but no significant date effect (F

= 2.60; df = 2, 95). There was a significant interaction between date and means ofF.

tritici and F. fusca (F = 5.58; df = 2, 48; p<0.01). F. tritici was abundant on R. trivialis

from March 17 through April 29, but F. fusca was not abundant until April 29. Larvae

consisted of 33.9% of thrips collected and were most abundant from March 29 to April

29.

Rubus cuneifolius flowered from March 29 to May 12, 2004, and thrips densities

were most abundant on April 29 (Tables 2-7 through 2-9). There was a significant

difference in thrips densities on plant parts (F = 67.64; df = 2, 14; p<0.0001), with

significantly more thrips on flowers than on fruits or leaves (Tukey's p<0.05) on April

29. The most abundant species were F. tritici and F. bispinosa, comprising 87.5% and

7.3% of adults, respectively. There was a significant difference in densities of thrips

species (F = 33.52; df = 3, 36; p<0.0001), and a significant date effect (F = 19.06; df= 2,

68; p<0.0001) and interaction between date and species (F = 6.91; df = 6, 68, p<0.0001).

There was a significant interaction between date and the means ofF. tritici and F.

bispinosa (F = 5.78; df = 2, 68; p<0.005). F. tritici were abundant from April 14 to April

29, but F. bispinosa were not present until April 29. Larvae were most abundant on R.

cuneifolius on April 29, and consisted of 27.5% of thrips collected.









Vicia sativa flowered from December 5 to April 14, and thrips were collected

from March 29 to April 14 (Tables 2-10 through 2-13). There was a significant

interaction between date and plant part (F = 4.01; df = 6, 52; p<0.005), and a significant

difference in thrips densities on plant parts (F = 6.43, df = 3, 40; p<0.005). There were

significantly more thrips on flowers than on the leaves or fruits (Tukey's p<0.05), but not

buds (Tukey's p>0.05). The most abundant species were F. tritici, and F. fusca,

comprising 81.9% and 15.3% of adults, respectively. There was no significant difference

in densities of thrips species (F = 1.77; df = 3, 36), or significant difference in thrips

densities on March 29 and April 14 (F = 0.00; df = 1, 8). There was no significant

interaction between date and species (F = 0.00; df = 1, 8), indicating that there was no

difference in the patterns of abundance of any thrips species. Larvae consisted of 47.1%

of thrips collected and were present from March 29 to April 14.

Trifolium repens flowered from December 12, 2003 to July 7, 2004, and thrips

were most abundant April 29 through May 24 (Tables 2-14 and 2-17). There was a

significant interaction between date and plant part (F = 6.32; df= 5, 88; p<0.0001), and

there were significantly more thrips on racemes than leaves (F = 337.44; df = 1, 18;

p<0.0001). The most abundant thrips species were F. tritici and F. bispinosa, comprising

79.4% and 12.0% of adults, respectively. There was a significant difference in densities

of thrips species (F = 52.49; df = 3, 36; p<0.0001), and a significant date effect (F = 9.36;

df = 3, 108; p<0.0001) and interaction between date and species (F = 4.61; df = 9, 108;

p<0.0001). There was no significant interaction between date and the means ofF. tritici

and F. bispinosa (F = 1.97; df = 3, 108), indicating that there was no difference in the









patterns of abundance of the two species. Larvae consisted of 27.4% of thrips collected,

and were abundant from April 14 to May 12.

Solidago canadensis flowered from August 19 to November 5, 2004. Thrips were

present on S. canadensis from September 24 to October 21, although 82% of thrips were

observed on October 21 (Tables 2-16 and 2-17). There was a significant interaction

between plant part and date (F = 9.16; df= 2, 22; p<0.005), and significantly more thrips

on racemes than leaves (F = 84.63; df = 1, 27; p<0.0001). Of the adults collected, 81.5%

were F. tritici and 17.7% were a combination of non-Frankliniella species. There was a

significant difference in thrips species (F = 19.72; df= 3, 36; p<0.0001), and a significant

date effect (F = 6.50; df = 3, 72; p<0.001) and interaction between date and species (F =

6.23; df = 9, 72; p<0.0001). This interaction was due to the absence ofF. bispinosa, F.

fusca, and F. occidentalis during F. tritici abundance on October 21, 2004. Larvae

consisted of 68.5% of thrips collected, and were most abundant on October 21.

Chenopodium ambrosioides flowered from November 19 to December 5, 2003 and

from June 24 to November 5, 2004. Thrips were collected on November 19, 2003 and

from August 2 to October 21, 2004 (Tables 2-18 and 2-19). There was a significant

interaction between date and plant part (F = 3.27, df = 5, 66; p<0.05), and there were

significantly more thrips on racemes than leaves (F = 79.45; df= 1, 20; p<0.0001). The

most abundant species were an unidentified non-Frankliniella sp. and F. tritici,

comprising 68.0% and 14.6% of adults, respectively. There was a significant difference

in densities of thrips species (F = 13.92; df = 4, 45; p<0.0001), and a significant

interaction between date and species (F = 1.97; df= 16, 170; p<0.05), but no significant

difference in the means of different sample dates (F = 0.60; df = 4, 170). There was a









significant interaction between date and the means ofF. tritici and non-Frankliniella sp.

(F = 6.98, df = 4, 170; p<0.0001). F. tritici were only abundant on August 19, while non-

Frankliniella sp. were abundant from September 2 through November 5, 2004. Larvae

consisted of 67.3% of thrips collected, and were abundant from August 19 to November

5.

There were 332 adult thrips and 116 larvae per R. raphanistrum plant from May 12

to June 24, 2004 (Table 2-20). There were 7 adult thrips and 8 larvae per R. trivialis

plant from March 29 to April 14, 2004 (Table 2-20). There were 43 adult thrips and 11

larvae per R. cuneifolius plant on April 29, 2004 (Table 2-20). There were 3 adult thrips

and 3 larvae per V. sativa plant on April 14, 2004 (Table 2-20). There were 142 adult

thrips and 49 larvae per T repens plant from April 29 to May 24, 2004 (Table 2-20).

There were 574 adult thrips and 2,142 larvae per S. canadensis plant on October 21, 2004

(Table 2-20). There were 378 adult thrips and 765 larvae per C. ambrosioides plant from

August 19 to October 21, 2004.

Discussion

More thrips were found on the flowers than leaves or fruits of all sampled plants,

suggesting there is a nutritional or morphological preference for flowers. Thrips may

prefer flowers to leaves because of the higher nitrogen content of pollen (Brodbeck et al.

2002), or for the microclimates flowers provide, that reduce desiccation, freezing, and

access by predators (see Kirk 1997 for review). The statistically significant interaction

between date and plant part on all plants was not considered biologically significant,

because thrips were so highly aggregated in the flowers when flowers were present. The

abundance of larvae on R. cuneifolius fruits when fruits were first present was probably









due to the inability of larvae to move to new flowers quickly, as there were very few

adults collected from fruits on the same date.

There were 2.2 larvae per female collected on S. canadensis, and F. tritici was the

only Frankliniella sp. collected, suggesting that S. canadensis is a good reproductive host

for F. tritici. The high number of larvae per plant, and the abundance of plants

throughout the country (USDA, NRCS 2004) suggest that S. canadensis may be an

important source ofF. tritici larvae that migrate into fall crops as adults. In addition, S.

canadensis could be a source of larvae that overwinter as pupae in the soil. When

temperatures rise, these developing adults may initiate the build up in thrips population

numbers in early spring. If S. canadensis were not available to thrips, there may be a

reduction in fall thrips populations and a delay in the spring population growth of thrips.

There were many F. tritici and F. bispinosa collected from R. raphanistrum,

suggesting that R. raphanistrum is a good feeding host for both species. There were only

0.53 larvae per female collected, suggesting that it was not as good a reproductive host as

S. canadensis. However, the mean numbers ofF. tritici and F. bispinosa per plant were

high for R. raphanistrum, indicating that utilization ofR. raphanistrum as a feeding and

reproductive host may still be an important part of each species' ecology. Furthermore,

R. raphanistrum is a host for Tomato spotted wilt virus and may therefore be a source of

virus infection in thrips populations (Parrella et al. 2003). R. raphanistrum is common in

most of the United States (USDA, NRCS 2004) and may be an important factor in thrips

ecology throughout the country.

The most common thrips species on R. cuneifolius and T. repens were F. tritici

and F. bispinosa. There were 0.39 larvae per female collected from each plant species,









suggesting that neither species is a preferred host. Fewer thrips per plant were collected

from T repens and R. cuneifolius than from S. canadensis and R. raphanistrum. Reduced

abundance per plant may be partially due to a lower number of flowers per plant during

thrips abundance and the frequent mowing of T. repens during spring and summer.

There were high numbers of thrips per plant on C. ambrosioides, but a majority of

adults were not Frankliniella species, indicating that it may not support as many crop

pests as other plants sampled. Low numbers of thrips per plant were collected from R.

trivialis and V sativa, suggesting that neither is a preferred feeding or reproductive host.

Reasons for high Frankliniella species abundance on S. canadensis, R.

raphanistrum, R. cuneifolius, and T. repens are unclear, since they are all from different

taxonomic families (Asteraceae, Brassicaceae, Fabaceae and Rosaceae). Flowering time

does not appear to be a major factor, since all plants were flowering when thrips were

abundant. Nutritional differences among the plant species may cause thrips to prefer R.

raphanistrum, R. cuneifolius, T. repens and S. canadensis. Thrips are known to prefer

aromatic amino acids, which enhance cuticle production and hardening, and these amino

acids may be more common in the more suitable hosts (Mollema and Cole 1996,

Brodbeck et al. 2002). Furthermore, there may be a difference in chemical or

morphological defenses, such as primary or secondary metabolites and trichomes that

cause some plant species to be more attractive than others (Felland et al. 1995, deJager et

al. 1996). More research must be conducted on the physical and chemical characteristics

of plant hosts to understand why there were more Frankliniella spp. thrips on R.

raphanistrum, R. cuneifolius, and S. canadensis than other plants surveyed.









In addition to plant host charachterisitics, there may be competitive interactions

between thrips species on R. trivialis, R. cuneifolius, and C. ambrosioides that affect

species abundance. In R. trivialis, F. fusca was only abundant on April 29, when F.

tritici was reduced. However, only one plant flowered on April 29, so this difference in

patterns of abundance may be due to sampling error and the low abundance ofF. fusca

on most sample dates. In R. cuneifolius, F. tritici may benefit from an early abundance,

allowing a competitive advantage over F. bispinosa. If interspecific competition is

occurring, F. bispinosa may find the plant host less desirable if there are established

interspecific competitors present.

There may be climatic or plant physiological changes altering the species ratios

on C. ambrosioides. For example, the cooler fall temperatures may benefit non-

Frankliniella sp. populations more than F. tritici, or the plant may accumulate defenses to

which only non-Frankliniella sp. have adapted. In addition, competitive displacement

may have occurred locally, limiting abundance ofF. tritici in the presence of non-

Frankliniella sp.

Reasons for the low abundance of thrips on R. raphanistrum on June 9, that

caused the interaction between F. tritici and F. bispinosa, are unclear. Populations ofF.

tritici on T. repens were also reduced on that date, indicating that reduced thrips

abundances were not limited to R. raphanistrum. Climatic effects, such as heavy rain or

wind, prior to the sample date may have reduced thrips densities on R. raphanistrum.

Although R. raphanistrum flowered from December 5 to August 2, there were no

thrips collected from the flowers until April 14. The reasons for the delay in abundance

of thrips on R. raphanistrum are unclear. Highest numbers of thrips were not found in R.









raphanistrum until after R. cuneifolius finished flowering on May 24 and larvae to adult

ratios were low, suggesting that R. raphanistrum may not be a preferred host, and is only

utilized when alternative hosts are not available. Raphanus raphanistrum could also be

utilized as an enemy-free niche as there was only one Orius insidiosus, an important

thrips predator, collected from this host and thrips presence on this host corresponds to

the seasonal increase of 0. insidiosus in crops, as presented in past studies. For example,

Reitz et al. (2003) collected most 0. insidiosus during May and June in peppers, reaching

abundances of one individual per 1.4 flowers during the same season as peak thrips

densities on R. raphanistrum in my study.

Reasons for the delay in abundance of larvae on R. raphanistrum are also unclear.

The slight delay in abundance from April 29 to May 24 could be due to naturally

occurring oviposition and incubation times. However, it is unclear why there were much

higher larvae per female numbers from June 24 to July 20 than from April 29 to May 24.

There may be a physiological change occurring in the plant during this time that makes

the host more suitable for reproduction. Conversely, thrips may choose to oviposit in R.

raphanistrum during these dates due to an abundance of 0. insidiosus in cropping

systems (Reitz et al. 2003). 0. insidiosus preferentially feeds on larvae (Baez et al.

2004), and there may be selective pressure to reproduce in areas with less predation

during this time of high predator abundance.

Fewer F. occidentalis were collected than were collected previously in North

Florida tomatoes and peppers (Funderburk et al. 2000, Reitz et al. 2003), and fewer F.

fusca than were collected from North Florida peanuts (Funderburk et al. 2002).

Frankliniellafusca and F. occidentalis may have different nutritional requirements to









those ofF. tritici, and this may partially explain their general absence from these hosts.

Furthermore, there may be competitive interactions occurring on uncultivated hosts that

are different from those in crops. Although F. occidentalis are excellent competitors on

fertilized pepper plants (see chapter 3), they may not be able to compete as well on

unfertilized wild hosts.

More F. occidentalis per flower were found in R. raphanistrum in Georgia than in

Florida (Buntin and Beshear 1995). Reasons for the reduced densities ofF. occidentalis

in Florida are unclear. Because of their abundance on cultivated crops in past studies,

climate does not appear to be the only factor limiting F. occidentalis densities on R.

raphanistrum. However, there may be an interaction between climate and availability of

other reproductive hosts. An increase in alternative host species in Georgia would

increase the overall population and may increase the number of thrips migrating into R.

raphanistrum. The difference in densities may also be due to an interaction between

climate and interspecific competition. Although conditions in Georgia and Florida are

similar, the slight change may affect competition in F. occidentalis, decreasing their

competitive ability on R. raphanistrum under Florida conditions compared with those in

Georgia.



Conclusion

Seven uncultivated reproductive hosts were sampled to determine the seasonal

abundance of Frankliniella tritici, F. bispinosa, F. fusca, and F. occidentalis populations

on each plant host. The abundant thrips species on Raphanus raphanistrum, Rubus

cuneifolius, and Trifolium repens were F. tritici and F. bispinosa. The abundant thrips

species on Rubus trivialis and Vicia sativa were F. tritici and F. fusca. The abundant









thrips species on Solidago canadensis was F. tritici. The abundant thrips species on

Chenopodium ambrosioides were F. tritici and an unidentified non-Frankliniella sp.

Thrips were highly aggregated in the flowers, rather than leaves or fruits, of every plant

species. The spring hosts that supported the largest Frankliniella spp. thrips populations

included R. raphanistrum, T repens, and R. cuneifolius, and the fall host that supported

the largest Frankliniella spp. populations was S. canadensis. Reducing the occurrence of

these uncultivated hosts in areas surrounding crops may decrease the number of thrips

migrating into cropping systems, leading to a reduction in crop damage.











Table 2-1. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F.
bispinosa, adult F. occidentalis and Frankliniella species larvae from samples
of 20 flowers of Raphanus raphanistrum collected biweekly on 17 dates from


December 12,


2003 to August 2, 2004 in Gadsden County, Florida.


Adult Adult Adult Adult Frankliniella
Date F. tritici F. fusca F. bispinosa F. occidentalis spp. Larvae
2003
12-Dec 0 0 0 0 0
5-Dec 0.7(0.4) 0 0 0 0
2004
2-Jan 0.9(0.3) 0 0 0 0
15-Jan 0.6(0.3) 0 0 0 0
29-Jan 0.1(0.1) 0 0 0 0
17-Feb 0.3(0.2) 0.1(0.1) 0 0 0
2-Mar 0.3(0.2) 0 0 0 0
17-Mar 0.3(0.2) 0 0 0 0
29-Mar 0.2(0.1) 0.1(0.1) 0 0 0
14-Apr 5.3(2.0) 0.6(0.3) 0.2(0.2) 0 0.1(0.1)
29-Apr 21.3(11.2) 1.7(1.6) 4.1(2.1) 0.3(0.2) 6.3(4.0)
12-May 100.3(19.9) 6.1(3.1) 25.6(13.5) 2.7(1.1) 26.9(11.5)
24-May 60.8(14.9) 0.5(0.4) 15.3(2.8) 0.7(0.3) 37.4(11.4)
9-Jun 11.4(3.1) 0.9(0.3) 5(1.4) 0.2(0.1) 7.1(1.5)
7-Jul 19.0(4.9) 0.2(0.2) 10.2(3.0) 1.0(0.6) 72(0.2)
20-Jul 13.8(2.7) 0.3(0.2) 4.6(1.9) 0 27.4(5.6)
2-Aug 2.3(0.9) 0 0.5(0.5) 0 8.5(5.3)









Table 2-2. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F.
bispinosa, adult F. occidentalis and Frankliniella species larvae from samples
of 20 leaves ofRaphanus raphanistrum collected biweekly on 17 dates from
December 12, 2003 to August 2, 2004 in Gadsden County, Florida.
Adult Adult Adult Adult Frankliniella
Date F. tritici F. fusca F. bispinosa F. occidentalis spp. Larvae
2003
12-Dec 0 0 0 0 0
5-Dec 0 0 0 0 0
2004
2-Jan 0 0 0 0 0
15-Jan 0 0 0 0 0
29-Jan 0 0 0 0 0
17-Feb 0 0 0 0 0
2-Mar 0 0 0 0 0
17-Mar 0 0 0 0 0
29-Mar 0 0 0 0 0
14-Apr 0 0 0 0 0.2(0.2)
29-Apr 0.3(0.3) 0.3(0.3) 0.4(0.4) 0 0.4(0.3)
12-May 1.8(1.0) 0.4(0.2) 0 0.1(0.1) 3.5(1.2)
24-May 1.7(0.7) 0.3(0.2) 0.9(0.4) 0.1(0.1) 4.6(1.5)
9-Jun 0.1(0.1) 0 0 0 1.3(1.7)
7-Jul 0.2(0.2) 0 0 0 0.2(0.2)
20-Jul 0.3(0.2) 0.2(0.1) 0 0 0.5(0.2)
2-Aug 0 0 0 0 0











Table 2-3. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F.
bispinosa, adult F. occidentalis and Frankliniella species larvae from samples
of 20 fruits of Raphanus raphanistrum collected biweekly on 17 dates from
December 12, 2003 to August 2, 2004 in Gadsden County, Florida.
Adult Adult Adult Adult Frankliniella
Date F. tritici F. fusca F. bispinosa F. occidentalis spp. Larvae


2003
12-Dec
5-Dec
2004
2-Jan
15-Jan
29-Jan
17-Feb
2-Mar
17-Mar
29-Mar
14-Apr
29-Apr
12-May
24-May
9-Jun
7-Jul
20-Jul
2-Aug


0
0.1(0.1)

0
0
0
0
0
0
0
0.1(0.1)
0
0.6(0.3)
0.8(0.4)
0.3(0.2)
0
0.3(0.2)
0


0
0
0
0
0
0
0
0
0.1(0.1)
0
0
0
0
0
0


0
0
0
0
0
0
0
0
0.1(0.1)
0
0.2(0.1)
0
0
0
0


0
0
0
0
0
0
0
0
0.1(0.1)
3.9(1.7)
3.3(1.2)
0.1(0.1)
0
0.1(0.1)
0


Table 2-4. Mean number (SEM) of adult Franklniella tritici, adult F. fusca, adult F.
bispinosa, adult F. occidentalis and Frankliniella species larvae from samples
of 20 flowers of Rubus trivialis collected biweekly on 5 dates from March 2 to
April 29, 2004 in Gadsden County, Florida.

Adult Adult Adult Adult Frankliniella
Date F. tritici F. fusca F. bispinosa F. occidentalis spp. Larvae
2-Mar 0 0 0 0 0.3(0.6)
17-Mar 0.5(0.3) 0.2(0.1) 0 0 0.1(0.1)
29-Mar 2.3(1.0) 0.3(0.2) 0 0.2(0.1) 1.1(0.4)
14-Apr 10.0(0.5) 0 0 0 3.5(0.1)
29-Apr* 8 5 0 2 2
*n=l (Only one plant flowering)










Table 2-5. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F.
bispinosa, adult F. occidentalis and Frankliniella species larvae from samples
of 20 leaves of Rubus trivialis collected biweekly on 5 dates from March 2 to
April 29, 2004 in Gadsden County, Florida.
Adult Adult Adult Adult Frankliniella
Date F. tritici F. fusca F. bispinosa F. occidentalis spp. Larvae
2-Mar 0 0 0 0 0.1(0.1)
17-Mar 0 0 0 0 0
29-Mar 0 0 0 0 0
14-Apr 0 0 0 0 0
29-Apr 0 0.1(0.1) 0 0 0

Table 2-6. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F.
bispinosa, adult F. occidentalis and Frankliniella species larvae from samples
of 20 fruits of Rubus trivialis collected biweekly on 3 dates from March 29 to
April 29, 2004 in Gadsden County, Florida.
Adult Adult Adult Adult Frankliniella
Date F. tritici F. fusca F. bispinosa F. occidentalis spp. Larvae
29-Mar 0 0 0 0 0.2(0.2)
14-Apr 0 0.1(0.1) 0 0.1(0.1) 1.4(0.5)
29-Apr 0 0.1(0.1) 0 0 0.2(0.1)










Table 2-7. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F.
bispinosa, adult F. occidentalis and Frankliniella species larvae from samples
of 20 flowers of Rubus cuneifolius collected biweekly on 4 dates from March
29 to May 12, 2004 in Gadsden County, Florida.
Adult Adult Adult Adult Frankliniella
Date F. tritici F. fusca F. bispinosa F. occidentalis spp. Larvae
29-Mar 0.2(0.1) 0.1(0.1) 0 0 0
14-Apr 23.0(17.3) 0.1(0.1) 0.4(0.3) 0.1(0.1) 0.8(0.3)
29-Apr 49.6(18.0) 0.2(0.1) 5.5(2.0) 0.3(0.2) 11.0(3.9)
12-May 0 0 0 0 0


Table 2-8. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F.
bispinosa, adult F. occidentalis and Frankliniella species larvae from samples
of 20 leaves of Rubus cuneifolius collected biweekly on 4 dates from March
29 to May 12, 2004 in Gadsden County, Florida.
Adult Adult Adult Adult Frankliniella
Date F. tritici F. fusca F. bispinosa F. occidentalis spp. Larvae
29-Mar 0 0 0 0 0
14-Apr 0 0 0 0 0.4(0.3)
29-Apr 0.1(0.1) 0.1(0.1) 0 0 0
12-May 0 0.1(0.1) 0 0 0.2(0.1)

Table 2-9. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F.
bispinosa, adult F. occidentalis and Frankliniella species larvae from samples
of 20 fruits of Rubus cuneifolius collected biweekly on 2 dates from April 29
to May 12, 2004 in Gadsden County, Florida.
Adult Adult Adult Adult Frankliniella
Date F. tritici F. fusca F. bispinosa F. occidentalis spp. Larvae
29-Apr 1.7(0.7) 0.2(0.2) 0.2(0.2) 0 20.8(4.3)
12-May 0 0.1(0.1) 0 0 0.7(0.4)









Table 2-10. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F.
bispinosa, adult F. occidentalis and Frankliniella species larvae from samples
of 20 flowers of Vicia sativa collected biweekly on 6 dates from January 29 to
April 14, 2004 in Gadsden County, Florida.
Adult Adult Adult Adult Frankliniella
Date F. tritici F. fusca F. bispinosa F. occidentalis spp. Larvae
29-Jan 0 0 0 0 0
17-Feb 0.1(0.1) 0 0 0 0.1(0.1)
2-Mar 0 0 0 0 0
17-Mar 0 0 0 0 0
29-Mar 0.1(0.1) 0 0 0 0.7(0.6)
14-Apr 3.7(2.7) 0.5(0.3) 0.2(0.2) 0 3.2(1.4)


Table 2-11. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F.
bispinosa, adult F. occidentalis and Frankliniella species larvae from samples
of 20 leaves of Vicia sativa collected biweekly on 6 dates from January 29 to
April 14, 2004 in Gadsden County, Florida.
Adult Adult Adult Adult Frankliniella
Date F. tritici F. fusca F. bispinosa F. occidentalis spp. Larvae
29-Jan 0 0 0 0 0.1(0.1)
17-Feb 0 0 0 0 0
2-Mar 0 0 0 0 0
17-Mar 0 0 0 0 0
29-Mar 0 0 0 0 0
14-Apr 0.1(0.1) 0 0 0 0

Table 2-12. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F.
bispinosa, adult F. occidentalis and Frankliniella species larvae from samples
of 20 leaves of Vicia sativa collected biweekly on 3 dates from March 17 to
April 14, 2004 in Gadsden County, Florida.
Adult Adult Adult Adult Frankliniella
Date F. tritici F. fusca F. bispinosa F. occidentalis spp. Larvae
17-Mar 0 0 0 0 0
29-Mar 0 0.2(0.2) 0 0 0.1(0.1)
14-Apr 0.1(0.1) 0.1(0.1) 0 0 0.3(0.2)










Table 2-13. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F.
bispinosa, adult F. occidentalis and Frankliniella species larvae from samples
of 4 buds of Vicia sativa collected biweekly on 6 dates from January 29 to
April 14, 2004 in Gadsden County, Florida.
Adult Adult Adult Adult Frankliniella
Date F. tritici F. fusca F. bispinosa F. occidentalis spp. Larvae
29-Jan 0 0 0 0 0.2(0.2)
17-Feb 0 0 0 0 0
2-Mar 0 0 0 0 0.2(0.2)
17-Mar 0 0 0 0 0
29-Mar 0 0 0 0 0
14-Apr 1.4(1.0) 0.1(0.1) 0 0 0.9(0.4)


Table 2-14. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F.
bispinosa, adult F. occidentalis and Frankliniella species larvae from samples
of 4 racemes of Trifolium repens collected biweekly on 14 dates from


December 12,


2003 to July 7, 2004 in Gadsden County, Florida.


Adult Adult Adult Adult Frankliniella
Date F. tritici F. fusca F. bispinosa F. occidentalis spp. Larvae
2003
12-Dec 0.2(0.2) 0 0 0 0.1(0.1)
2004
2-Jan 0 0 0 0 0
15-Jan 0.8(0.6) 0 0 0 0
29-Jan 0.1(0.1) 0 0 0 0.2(0.1)
17-Feb 0.1(0.1) 0 0 0 0.2(0.1)
2-Mar 0 0 0 0 0.1(0.1)
17-Mar 0.1(0.1) 0 0 0 0
29-Mar 0.2(0.1) 0 0.2(0.1) 0 0.2(0.1)
14-Apr 1.7(0.7) 0.1(0.1) 0.2(0.1) 0 3.1(1.4)
29-Apr 9.0(4.1) 0.6(0.3) 1.5(0.7) 0.2(0.1) 4.4(0.9)
12-May 5.9(2.2) 1.5(0.7) 0.8(0.4) 0.1(0.1) 10.8(6.1)
24-May 21.5(7.6) 0.1(0.1) 3.5(1.3) 0 0.4(0.2)
9-Jun 1.0(0.4) 0.0(0.0) 0.1(0.1) 0 0.6(0.3)
7-Jul 1.9(0.5) 0.1(0.1) 0.1(0.1) 0.1(0.1) 0.1(0.1)









Table 2-15. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F.
bispinosa, adult F. occidentalis and Frankliniella species larvae from samples
of 20 leaves of Trifolium repens collected biweekly on 14 dates from
December 12, 2003 to July 7, 2004 in Gadsden County, Florida.
Adult Adult Adult Adult Frankliniella
Date F. tritici F.fusca F. bispinosa F. occidentalis spp. Larvae
2003
12-Dec 0 0 0 0 0
2004
2-Jan 0 0 0 0 0
15-Jan 0 0 0 0 0.1(0.1)
29-Jan 0 0 0 0 0
17-Feb 0 0 0 0 0
2-Mar 0 0 0 0 0
17-Mar 0 0 0 0 0
29-Mar 0.1(0.1) 0 0 0 0
14-Apr 0 0.3(0.2) 0 0 0
29-Apr 0.1(0.1) 0 0.1(0.1) 0 0.2(0.1)
12-May 0 0.1(0.1) 0.1(0.1) 0 0.1(0.1)
24-May 0 0.1(0.1) 0 0 0.1(0.1)
9-Jun 0.1(0.1) 0 0 0 0
7-Jul 0 0 0 0 0










Table 2-16. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F.
bispinosa, adult F. occidentalis and Frankliniella species larvae from samples
of 20 racemes of Solidago canadensis collected biweekly on 5 dates from
September 2 to November 5, 2004 in Gadsden County, Florida.
Adult Adult Adult Adult Frankliniella
Date F. tritici F. fusca F. bispinosa F. occidentalis spp. Larvae
2-Sep 0 0 0 0 0.5(0.3)
24-Sep 5.7(2.7) 0 0.1(0.1) 0 0.4(0.4)
7-Oct 3.2(0.9) 0 0 0 3.2(1.2)
21-Oct 23.8(8.9) 0 0 0.2(0.2) 89.5(44.1)
5-Nov 0 0 0 0 0

Table 2-17. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F.
bispinosa, adult F. occidentalis and Frankliniella species larvae from samples
of 20 leaves of Solidago canadensis collected biweekly on 5 dates from
September 2 to November 5, 2004 in Gadsden County, Florida.
Adult Adult Adult Adult Frankliniella
Date F. tritici F. fusca F. bispinosa F. occidentalis spp. Larvae
2-Sep 0 0 0 0 0
24-Sep 0 0 0 0 0
7-Oct 0 0 0 0 0
21-Oct 0.2(0.2) 0 0 0 0
5-Nov 0 0 0 0 0










Table 2-18. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F.
bispinosa, adult F. occidentalis, adult non-Frankliniella species, and larvae
from samples of 20 racemes of Chenopodium ambrosioides collected
biweekly on 2 dates from November 19 to December 5, 2003 and 6 dates from
August 19 to November 5, 2004 in Gadsden County, Florida.
Adult
Adult Adult Adult Adult non-Frankliniella
Date F. tritici F. fusca F. bispinosa F. occidentalis sp. Larvae
2003
19-Nov 0 0 0 0 0 0.4(0.3)
5-Dec 0 0 0 0 0 0
2004
19-Aug 1.1(0.5) 0 0 0 0.1(0.1) 4.4(0.1)
2-Sep 0.1(0.1) 0 0 0 1.7(1.1) 2.2(0.9)
24-Sep 0.2(0.2) 0 0 0 1.6(0.7) 5.1(2.0)
7-Oct 0.1(0.1) 0 0 0 1.5(0.6) 1.8(0.6)
21-Oct 0.3(0.3) 0 0 0 2.0(1.6) 5.3(1.7)
5-Nov 0 0 0 0 1.6(0.7) 7.4(2.8)

Table 2-19. Mean number (SEM) of adult Frankliniella tritici, adult F. fusca, adult F.
bispinosa, adult F. occidentalis, adult non-Frankliniella species, and larvae
from samples of 20 leaves of Chenopodium ambrosioides collected biweekly
on 2 dates from November 19 to December 5, 2003 and 6 dates from August
19 to November 5, 2004 in Gadsden County, Florida.
Adult
Adult Adult Adult Adult non-
Date F. tritici F. fusca F. bispinosa F. occidentalis Frankliniella sp. Larvae
2003
19-Nov 0.1(0.1) 0 0 0 0 0.1(0.1)
5-Dec 0 0 0 0 0 0
2004
19-Aug 0 0 0 0 0 0.1(0.1)
2-Sep 0 0 0 0 0 0.1(0.1)
24-Sep 0 0 0 0 0 0
7-Oct 0 0 0 0 0 0
21-Oct 0 0 0 0 0 0
5-Nov 0 0 0 0 0 0













Table 2-20. Mean number of adult Frankliniella tritici, F. fusca,
plant for seven plant species on selected dates.


F. bispinosa, F. occidentalis, larvae and non-Frankliniella sp. per


Adult Adult Adult Adult Larvae Adult
F. tritici F. fusca per F. bispinosa F. occidentalis per non-Frankliniella
Plant Species Dates per Plant Plant per Plant per plant Plant spp. per plant


R. raphanistrum

R. trivialis
R. cuneifolius
V sativa

T repens
S. canadensis

C. ambrosioides


12-May to
24-Jun
29-Mar to
14-Apr
29-Apr
14-Apr
29-Apr to
24-May
21-Oct
19-Aug to
21-Oct


244.30

6.66
40.79
2.62

115.17
570.41

70.22


11.37

0.09
0.08
0.57

6.96
0


71.51

0
1.97
0.12

18.35
0


5.04

0.47
0.12
0.00

0.95
3.99


115.80

8.13
11.41
2.74


49.36
2142.03 0

764.60 307.90














CHAPTER 3
INTRASPECIFIC AND INTERSPECIFIC COMPETITION IN THRIPS ON
FLOWERING PEPPER PLANTS

Introduction

Competition can be an important factor in determining population size, structure

and interactions (Inouye 1999a, b, Hansen et al. 2003, Young 2004). Interspecific

competition may be particularly important when assessing the impact of invasive species,

which are often good competitors (Mooney and Cleland 2001). Competitive ability

enhances invasive species' capabilities to increase rapidly and become pests in new

environments (Gurnell 1996, Petren and Case 1996, Holway et al. 1998, Callaway and

Aschehoug 2000).

There have been several examples of invasive species out-competing native

species and becoming pests. The Argentine ant Linepithema humile has outcompeted

several native species, and become a pest in the southern United States, and many other

areas of the world (Holway et al. 1998, Holway and Suarez 2004). Competitive

superiority of the invasive fire ant Solenopsis invicta over the native ant Forelius mccooki

has enhanced the spread of S. invicta, allowing the species to reach pest status in the

southeastern United States (Mehdiabadi et al. 2004). Competitive superiority of the

invasive mosquito Aedes albopictus over several native species also appears to have

aided in the spread of this invasive pest (Griswold and Lounibos 2005, Juliano and

Lounibos 2005). Competition has therefore been an important factor assisting the spread

of invasive species and should be considered when assessing any invasive species'









success. Once the factors affecting the spread of the invasive organism are understood,

invasions can be predicted, and pest management programs can be improved (Strong and

Pemberton 2000, Yasuda et al. 2004).

Interspecific competition studies on animals have often used an additive or

substitutive design (e.g. Connell 1961, Moran and Whitham 1990, Forseth et al. 2003).

Additive design experiments maintain one species at a constant density, while varying the

density of the other (Figure 3-1). Using this design does not distinguish the effect of

interspecific competition from intraspecific competition due to the varying number of

overall individuals (Damgaard 1998, Inouye 2001, Young 2004). Substitutive designs

vary the frequency of the two competing species while maintaining a constant combined

density (Figure 3-2), and are therefore useful in comparing interspecific and intraspecific

competition. Because the treatments are conducted at the same overall density,

interspecific and intraspecific competition can only be measured in relation to each other.

The statistical significance of either form of competition can not be determined (Snaydon

1991, Inouye 2001).

A third type of design, the response surface design, varies the densities of each

species independently and competition models can be used to generate a quantitative

value of competition (Inouye 2001). By so doing, empirical data can be fit to a

theoretical model, providing a connection between empirical and theoretical approaches

that is not possible using an additive or substitutive design experiment (Damgaard 1998,

Inouye 2001). Response surface designs have often been used in plant intserspecific

competition studies (Law and Watkinson 1987, Rees et al. 1996, Damgaard 1998), but









have been rarely used in animal interspecific competition experiments (though see Inouye

1999a, Young 2004).

Frankliniella occidentalis is an invasive crop pest that causes damage to flowers

and developing fruits through feeding and ovipositing, as well as by spreading Tomato

spotted wilt virus (Sakimura 1962), one of the most damaging worldwide plant viruses

(Prins and Goldbach 1998). This species has spread from its native western North

America to every continent except Antarctica in the last 30 years (Kirk and Terry 2003).

Research has been conducted on some of the factors contributing to the spread ofF.

occidentalis, including host range (Chellemi et al. 1994), climate (Br0dsgaard 1993,

Wang and Shipp 2001) and predation (Baez et al. 2004), but no research has been

conducted on the population effects of thrips competition. Determining whether or not

competition between F. occidentalis and native species occurs may partially explain the

spread of this worldwide crop pest. I used a response surface design to test for

competition between F. occidentalis (Pergande) and F. bispinosa (Morgan), a native

Florida species. A competition model was fit to the data to generate quantitative values

measuring the effects of intraspecific and interspecific competition, and make qualitative

(presence or absence) determinations of each type of competition.

Materials and Methods

Experimental Design

Female F. bispinosa were collected from perennial peanuts (Arachis glabrata) in

Gainesville, FL. Female F. occidentalis were taken from a colony maintained at 21-23C

and 50-80% relative humidity, with a 14:10 photophase: scotophase, and regularly

supplemented with wild individuals. The experiment was conducted on flowering pepper

plants (Capsicum annuum), a known reproductive host for both species (Funderburk et al.









2000, Ramachandran et al. 2001, Hansen et al. 2003). Pepper plants were grown in a

greenhouse with no insecticides and were checked regularly for insects, which were

killed manually. For the experiment, each pepper plant was enclosed in a plexiglass

cylindrical cage 15.5-cm in diameter and 36.5-cm in height. The top of each cage was

covered with thrips screen (Green-Tek, Inc., WI), and the bottom was inserted into the

soil to prevent thrips escape. Each cage had two 2-cm diameter holes covered with thrips

screen to increase ventilation. The experiment was conducted in a climate-controlled

room set at 23 C with greater than 95% relative humidity within the cages.

The densities of female F. bispinosa and F. occidentalis were arranged in a

bivariate factorial arrangement from 0 to 30 per plant in increments of ten, with

additional single species treatments of 60 (Figure 3-3). These densities reflect those

previously recorded in the field (Ramachandran et al. 2001). The single species

treatments of 60 were added to increase the chance of including the population carrying

capacity in the treatment range, since the carrying capacity was unknown. Each

treatment was replicated five times.

Female thrips were introduced to the pepper plants and allowed to feed and

oviposit. After ten days, plants were destructively sampled, and all larvae were removed.

The larvae from each treatment were placed in 30-ml containers with green beans and bee

pollen, and the species of each was determined after development to adult. The species

ratio of emerged adults was assumed to be the species ratio of larvae produced for the

treatment. The larval species ratio and the overall number of larvae produced were

multiplied together to estimate the number of larvae produced by each species. Then the

number of larvae produced per species for each treatment was divided by the number of









adult females in the treatment to estimate the number of larvae per female of each

species. The number of larvae produced per female of each species at the various

densities was used to evaluate the effect of competition on female oviposition.

Model Fitting

Larvae per female of each species were used as the response variable to obtain a

measurement of oviposition. The model was fit using maximum log-likelihood

estimation, assuming a Poisson error distribution. This method uses the data to estimate

the probability of occurrence for each possible value of each parameter. The log of each

likelihood (probability) value is then calculated. Then the value for each parameter that

had the highest log-likelihood (probability of occurring) was selected. Confidence

intervals were determined using log-likelihood ratios. This technique used the X

distribution of the log-likelihood of each parameter to determine the confidence intervals

with the other parameters fixed at the best fit values. All calculations were completed

using R (R Development Core Team 2005). Several models were tested, and the best-fit

model was the following:

Rx = 2 (Law and Watkinson 1987)
1+ c(X+ flY Y)

Where Rx is the number of larvae produced per female of species X after ten days,

and 2 is generated by the model to predict the larvae per female of species Xproduced at

low densities, in the absence of competition. The parameter c measures intraspecific

competition. The parameter fxy is the competition coefficient, which measures the

relative effect of species Y on the reproduction of species X. This competition coefficient

compares the effects of interspecific competition with that of intraspecific competition,

which is set at one. For example, if the competition coefficient were estimated as 3, it









would indicate that each interspecific competitor would have 3 times the effect of a

conspecific on reproduction. The competition model was fit for both focal species.

Model simulation of intraspecific competition was conducted by graphing the models for

each species with the number of interspecific competitors set at zero.

Results

The mean number of larvae per female from the different treatment densities of

adult female F. occidentalis and F. bispinosa are presented in Table 3-1. In treatments

with only one species present, there were more F. occidentalis larvae produced per

female than F. bispinosa. The larvae per F. bispinosa female averaged over all

treatments was 0.58 (SE 0.13), and the larvae per F. occidentalis female averaged over

all treatments was 2.6 (SE 0.28).

The model parameters and confidence intervals measuring effects of competition

on the number of larvae produced per F. bispinosa female are presented in Table 3-2.

Statistically significant intraspecific competition affected F. bispinosa, as indicated by

the confidence interval for c, which did not include zero. The maximum likelihood

estimation for f/xy indicated that the effect of interspecific competition from F.

occidentalis was 4.62 times greater than intraspecific competition on F. bispinosa

reproduction. The 95% confidence intervals for fxy did not include one, proving that

interspecific competition from F. occidentalis had a significantly greater effect on the

number ofF. bispinosa larvae produced per female than intraspecific competition.

The model parameters and confidence intervals measuring effects of competition

on the number of larvae produced per F. occidentalis female are presented in Table 3-3.

Statistically significant intraspecific competition affected F. occidentalis, as indicated by

the confidence interval for c, which did not include zero. The estimated value for f/xy for









F. occidentalis was negative, suggesting that F. occidentalis benefited slightly from the

presence ofF. bispinosa.

Intraspecific competition had a greater effect on F. bispinosa than on F.

occidentalis, as indicated by the 95% confidence intervals for c (Tables 3-3 and 3-4),

which did not overlap. Model simulation ofintraspecific competition also indicated that

F. bispinosa were more affected by intraspecific competition than F. occidentalis as the

number of larvae per female decreased more rapidly with increased intraspecific

competition for F. bispinosa than for F. occidentalis (Figure 3-4). However, the values

for 2 were similar, indicating that in the absence of competition, both species would

produce similar numbers of larvae per female.

Discussion

These results indicate that F. occidentalis is competitively superior to F.

bispinosa on pepper plants. Being a superior competitor may enhance the spread and

abundance ofF. occidentalis.

The competitive mechanism that occurred between the two species is not clear,

although the behavior of these two species indicate that interference occurred. F.

bispinosa are more mobile than F. occidentalis in pepper flowers and are more likely to

flee in the presence of a predator (Reitz et al. 2002). F. bispinosa may also be more

inclined than F. occidentalis to move to another feeding or oviposition site when in the

presence of a competitor. This extra time spent locating feeding or oviposition sites

would decrease time allotted for feeding and reproducing, reducing fecundity in the

presence of intraspecific and interspecific competition.

The negative competition coefficient measuring the effect ofF. bispinosa on F.

occidentalis suggests that F. occidentalis benefited from the presence ofF. bispinosa.









This benefit may be due to two effects. First, there may have been intraguild predation,

with F. occidentalis increasing fecundity by feeding on F. bispinosa, as F. occidentalis

are facultative predators (Faraji et al. 2002). If intraguild predation is occurring, it may

increase the spread ofF. occidentalis. In addition, there was a higher mortality rate for F.

bispinosa than F. occidentalis in the single species larval growth chambers. This

differential mortality may have slightly altered the species ratios in interspecific

treatments, causing an apparent increase ofF. occidentalis due to the misidentification of

F. bispinosa larvae. However, if superior larval survivorship is occurring, there may be

further characteristics in the relationship between the two species that would increase the

spread ofF. occidentalis.

Effects of Competition on F. occidentalis Population Abundance in Florida

Currently F. occidentalis populations are abundant in North America. However,

F. occidentalis are much more abundant in North Florida than Central and Southern

Florida, while F. bispinosa is the most abundant thrips species in Central and Southern

Florida and its range extends to North Florida (Childers et al. 1990, Kirk 2002, Hansen et

al. 2003). High effects of interspecific competition on F. bispinosa, and no effects of

interspecific competition on F. occidentalis should influence the abundance off.

occidentalis in Central and Southern Florida. If there were no extrinsic factors, F.

occidentalis would be able to out-compete F. bispinosa in pepper flowers throughout

Florida. Furthermore, there is an abundance of pepper plants available to support

populations off. occidentalis in southern Florida (Kokalis-Burelle et al. 2002, Hansen et

al. 2003), and the species has existed in North Florida and Georgia long enough to invade

the southern portions of Florida (Beshear 1983). However, F. occidentalis is not

common in central and southern pepper plants, indicating some additional factors affect









the abundance ofF. occidentalis in Florida. Differential predation may be one of the

factors maintaining higher F. bispinosa abundance in central and southern Florida.

Differential predation benefits native mosquitoes and ants by limiting population

densities of invasive species (Mehdiabadi et al. 2004, Griswold and Lounibos 2005).

Similarly, F. bispinosa may be the most abundant species in southern Florida due to a

reduction in F. occidentalis from differential predation by Orius insidiosus, a predator of

both species (Reitz et al. 2002). Although 0. insidiosus is abundant in the eastern United

States, it is only able to actively overwinter in central and southern Florida (Bottenberg et

al. 1999 as cited by Hansen et al. 2003). This winter predation may limit F. occidentalis

numbers in central and southern Florida. Climate may also limit F. occidentalis

populations in central and southern Florida, as the species is considered a temperate to

subtropical pest (Kirk and Terry 2003). In addition, F. occidentalis may be limited by

alternative host availability.

Effects of Competition on World-Wide F. occidentalis Spread

F. occidentalis has spread to six continents, all of which have native thrips

inhabiting the local flora (Moritz et al. 2001, Kirk and Terry 2003). IfF. occidentalis is

capable of out-competing the native species, as was shown in this study, it will increase

the invasive threat of this economically-damaging crop pest. A better understanding of

the competitive interactions between F. occidentalis and native species may lead to new

methods of controlling the world-wide spread ofF. occidentalis by adjusting

environmental or ecological conditions to decrease the competitive advantage ofF.

occidentalis when it is competing against less damaging species.













Table 3-1 The mean number (SEM) of larvae per female 10 days after different
treatment densities of adult female F. occidentalis and F. bispinosa were introduced in


cages containing a pepper plant.
Number of Adult Females
per Pepper Plant
F. occidentalis F. bispi
0
0
0
0
10
10
10
10
20
20
20
20
30
30
30
30
60


Mean number of larvae per
female (SEM)


nosa
10
20
30
60
0
10
20
30
0
10
20
30
0
10
20
30
0


F. occidentalis


3.18(1.41)
3.15(0.29)
5.10(1.41)
4.48(1.28)
0.72(0.12)
3.57(1.39)
1.33(0.56)
3.11(1.23)
1.93(0.54)
2.60(0.66)
2.32(0.41)
1.69(0.76)
1.03(0.18)


F. bispinosa
1.48(0.51)
2.16(1.26)
0.81(0.28)
0.42(0.09)

0.03(0.03)
0.37(0.20)
0.25(0.14)

0.44(0.18)
0.68(0.36)
0.03(0.03)

0.17(0.11)
0.13(0.08)
0.55(0.27)












Table 3-2. Model parameters and confidence intervals measuring effects of competition
on the number of larvae produced per F. bispinosa female after 10 days on a
pepper plant (see text for model). Where c is the value of intraspecific
competition, f is the competition coefficient, and 2 is the larvae per female
produced in the absence of competition.
Parameter Value 95% Confidence Interval
c 0.266 0.111 to 4.86
f 4.62 3.58 to 5.97
2 8.46 4.11 to 134.82


Table 3-3. Model parameters and confidence intervals measuring effects of competition
on the number of larvae produced per F. occidentalis female after 10 days on
a pepper plant (see text for model). Where c is the value of intraspecific
competition, f is the competition coefficient, and 2 is the larvae per female
produced in the absence of competition.
Parameter Value 95% Confidence Interval
c 0.0741 0.0543 to 0.104
f -0.161 -0.274 to -0.0574
2 6.16 4.98 to 7.93


(n 25


15
--
0

10
= 5
0


Density of Species A


Figure 3-1. Example of an additive design. Each point represents a density treatment,
which includes a combination of densities of species A and B per unit area.














'/ 25 -
25

0 20

0
' O


S5
Q


0$








o2

0 5 10 15 20 25 30


Density of Species A
Figure 3-2. Example of a substitutive design. Each point represents a density treatment,
which includes a combination of densities of species A and B per unit area.



































Density of F. bispinosa


Figure 3-3. Treatments of varying F. bispinosa and F. occidentalis densities to measure
the larvae produced per female at different levels of competition.


0 10 20 30
Density


40 50 60 70


Figure 3-4. Simulation of intraspecific competition ofF. bispinosa and F. occidentalis
based on the competition model (refer text) predicting the number of larvae
per female produced after 10 days on pepper plants.


*000






0 0 0


F. bispinosa
- F. occidentalis


N
-S

















CHAPTER 4
CONCLUSION

Developing a better understanding of the factors affecting thrips abundances in

cropping systems may lead to new methods of limiting damage from thrips feeding and

the vectoring of Tomato Spotted Wilt Virus (TSWV). Known elements affecting

population dynamics include host suitability, migrations from alternative hosts, predation,

parasitism, and competition.

Migration from uncultivated hosts into crops is known to occur in thrips

populations (Pearsall and Myers 2001), but little research has been conducted on the

sources of thrips migration (though see Chamberlin et al. 1992, Chellemi et al. 1994, Cho

et al. 1995, and Toapanta et al. 1996). My research documents several sources, from

which thrips may migrate into cropping systems. The most important of these thrips

hosts included R. raphanistrum in the spring and S. canadensis in the fall. Raphanus

raphanistrum may serve as a predator free niche, as well as a source of TSWV (Parrella

et al. 2003). Solidago canadensis may be an important source of thrips feeding on fall

crops. Furthermore, larvae developing on S. canadensis may overwinter as pupae that

initiate the establishment of thrips populations in the early spring. Solidago canadensis

may also be a TSWV host (Parrella et al. 2003), indicating that it could also be a source

of viral infection in fall and spring crops. Plant nutrition and defense influence thrips

dynamics, and may influence thrips abundance on these and other reproductive hosts.

Furthermore, effects such as predation, parasitism, and competition may be affecting the

abundance and distribution on reproductive hosts.









Past research has demonstrated effects from predation (Funderberk et al. 2000,

Reitz et al. 2003, Baez et al. 2004), and parasitism (Funderberk et al. 2000, 2002), but

my study is the first to give evidence of competition occurring in thrips populations.

Frankliniella occidentalis is a better competitor than F. bispinosa on peppers in Florida

conditions, demonstrating that competitive superiority may be a reason for the invasive

ability of this worldwide pest.

More research must be conducted on the interactions between the host quality,

migration, predation, parasitism and competition. For example, predation by Orius

insidiosus may preferentially be feeding on F. occidentalis, limiting the abundance ofF.

occidentalis in Florida. In addition, research must be conducted on the effect of host

plant variation on competition, as host plant quality may influence competition if the

thrips species have different nutritional requirements. Conversely, niche displacement

and host utilization may be caused by competition or predation. Research on the

complex interactions between host quality, migration, predation, parasitism and

competition will enable a better understanding of thrips population dynamics, enabling

the development of more efficient pest management programs.
















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BIOGRAPHICAL SKETCH

Tobin Northfield was born and raised in Enumclaw, WA, where he attended school

at Enumclaw High School. He then attended Pacific Lutheran University in Tacoma,

WA, where he first decided to pursue a career in entomology during a general

entomology course. He earned a Bachelor of Science degree in biology at Pacific

Lutheran University. Tobin plans to continue working on insects, and eventually earn a

Ph.D. in a related field.