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DIFFERENTIAL PREDATION BY Orius insidiosus (Say) ON Frankliniella
occidentalis (Pergande) AND Frankliniella bispinosa (Morgan) IN SWEET PEPPER
SCOT MICHAEL WARING
A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENT FOR THE DEGREE OF
MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
I thank my Mom for getting me interested in what nature has to offer: birds, rats,
snakes, bugs and fishing; she influenced me far more than anyone else to get me where I
am today. I thank my Dad for his relentless support and concern. I thank my son,
Sequoya, for his constant inspiration and patience uncommon for a boy his age. I thank
my wife, Anna, for her endless supply of energy and love. I thank my grandmother,
Mimi, for all of her love, support and encouragement. I thank Joe Funderburk and Stuart
Reitz for continuing to support and encourage me in my most difficult times. I thank
Debbie Hall for guiding me and watching over me during my effort to bring this thesis to
life. I thank Heather McAuslane for her generous lab support, use of her greenhouse and
superior editing abilities. I thank Shane Hill for sharing his love of entomology and for
being such a good friend. I thank Tim Forrest for introducing me to entomology. I thank
Jim Nation and Grover Smart for their help navigating graduate school and the academics
therein. I thank Byron Adams for generous use of his greenhouse and camera.
I also thank (in no particular order) Aaron Weed, Jim Dunford, Katie Barbara,
Erin Britton, Erin Gentry, Aissa Doumboya, Alison Neeley, Matthew Brightman, Scotty
Long, Wade Davidson, Kelly Sims (Latsha), Jodi Avila, Matt Aubuchon, Emily
Heffernan, Heather Smith, David Serrano, Susana Carrasco, Alejandro Arevalo and all of
the other graduate students that kept me going and inspired about the work we have been
doing. Finally, I thank The University of Florida and IFAS for making it possible for me
to earn my M.S.
TABLE OF CONTENTS
ACKNOWLEDGMENTS ............................................................... ii
LIST OF FIGURES................................................................ ....... iv
ABSTRACT.................................. ................................. v
IN TR OD U CTION .................. ................. ......................... ...... 1
L ife H history ................... ........ ....... ................. ..... .. ...... 1
Thrips Ecology and Population Dynamics ................................ .......... 4
Host Plant......................................... ......................... 5
Abiotic Factors............ ...... ........... ....................... .......... 8
Natural Enemies..................................................... ........ 10
Flow er T hrips................... ................................. ......... 13
Tomato Spotted Wilt .................. ............................................. 14
Thrips M anagem ent........................................................ ........ 17
Orius insidiosus (Say).................................................... ......... 18
Differential Predation ............ .................................... ............ 20
MATERIALS AND METHODS............ ............ ... .......................... 22
Predators ...................................... ................................. 23
Objective 1.1: Single Species Arena ................................................ 24
Objective 1.2: Mixed Species Arena.................................................. 26
Objective 1.3: Whole Plant Arena ...................................................... 26
RESULTS ........... .... ............. ........................... ........ 29
Objective 1.1: Single Species Arena.............. ..... ............................. 29
Objective 1.2: Mixed Species Arena.................................................. 31
Objective 1.3: Whole Plant Arena.................................................... 33
DISCUSSION...................................... .................................... 36
REFERENCE LIST................... ................... ............................... 41
BIOGRAPHICAL SKETCH ................. ...... ..................... ............... 56
LIST OF FIGURES
1 Number of encounters................................. ......... ... .......... 29
2 Number of captures................................... ...........................30
3 Total handling time................. ................. ......................... 31
4 Number of encounters, captures, and handling time............ ............. 32
5 Mean differences in number surviving....................................... 34
6 N um bers of surviving................................. ............ ............. 35
Abstract of Thesis Presented to The Graduate School
of The University of Florida in Partial Fulfillment of the
Requirement for the Degree of Master of Science
DIFFERENTIAL PREDATION BY Orius insidiosus (Say) ON Frankliniella
occidentalis (Pergande) AND Frankliniella bispinosa (Morgan) IN SWEET PEPPER
Chair: Joseph E. Funderburk
Major Department: Entomology and Nematology
Generalist predators can show preferences for certain prey. These preferences can
either result from active choice by predators or by certain prey being more vulnerable to
predation than others. Consequently, differential predation can alter the population
dynamics of closely related prey species and ultimately community structure. I
investigated interactions between the generalist predator Orius insidiosus (Say) and
adults of two thrips species, Frankliniella bispinosa (Morgan) and F. occidentalis
(Pergande). Specifically I investigated interspecific differences between these prey
species that could affect predation by 0. insidiosus. 0. insidiosus were offered either F.
bispinosa or F. occidentalis as prey in single species trials, but there were no significant
differences in the number of prey captured. However, 0. insidiosus encountered more F.
bispinosa than F. occidentalis. In arenas with equal numbers of both thrips species, 0.
insidiosus had more encounters with and captures ofF. occidentalis than ofF. bispinosa.
In large arenas with two pepper plants, 0. insidiosus preyed more on F. occidentalis
than on F. bispinosa. These results indicate that 0. insidiosus can prey on adults of both
thrips species, but that in mixed species groups, this predator preferentially captures
F. occidentalis. F. occidentalis are much larger than F. bispinosa, the but it is
the greater locomotion and movement ofF. bispinosa that allow it to avoid interactions
with 0. insidiosus and evade predation better than F. occidentalis. Consequently, the
observed preference of 0. insidiosus for F. occidentalis is not exclusively a function of
active selection by the predator but arises from inherent differences among prey.
Because F. occidentalis is more vulnerable than F. bispinosa to predation by O.
insidiosus, this differential predation can affect the temporal dynamics of these species,
and allow populations ofF. bispinosa to persist longer into the growing season than
populations ofF. occidentalis.
Thrips are minute insects (between 1 to 4 mm) classified in the insect order
Thysanoptera (fringed-wing bearers). They were first described in 1744 by De Geer who
called them Physapus, however the genus was renamed Thrips in 1758 by Linnaeus. In
1836, the English entomologist Haliday raised the status of the thrips taxon to order and
renamed them Thysanoptera, although the members of the order are still commonly
referred to as thrips. A common vernacular name for thrips is "thunder flies," which
comes from association of dispersal after springtime storms.
The order Thysanoptera is divided into two suborders: Tubulifera and Terebrantia
and is further subdivided into eight families containing approximately 5000 described
species (Mound 1997). Tubulifera and Terebrantia differ in respect to body, specifically
abdominal shape, wing structure and number of larval instars (Tubulifera have an extra
pupal stage). The seven families that make up Terebrantia are Uzelothripidae,
Merothripidae, Aeolothripidae, Adiheterothripidae, Fauriellidae, Heterothripidae and
Thripidae. Combined, these families account for about 1900 of the approximately 5000
known species. Phlaeothripidae, the sole family in the suborder Tubulifera, contains
close to 3100 described species.
All thrips are believed to have evolved from a common ancestor along with
Heteroptera, Psocoptera, and Pthiraptera (Kristensen 1995). This early form led a
detritivorous lifestyle and fed primarily on fluids of fungal hyphae and decaying organic
matter. As a testament to the success of thrips in securing that niche, most thrips today
are still saprophytic; although it is actually a reverted behavior in some species (Mound
Merothripidae, all 15 known species of which are fungus feeders, are thought to
best resemble the ancestral form (Mound 1997). The only species found in
Uzelothripidae is a fungus feeder as well. Many thrips species evolved in their food
preferences and moved from feeding on senescing leaves to live leaves and eventually to
other plant tissues. Some went beyond this to feeding on other arthropods found in those
environments as well (Mound 1997). Although the majority of Heterothripidae and
Adiheterothripidae are associated with flowers (the latter specifically with those of date
palms [Pheonix dactylifera L.]) (Mound 1997). A new species of Heterothripidae
(Aulacothrips dictyotus Hood) is the first reported parasitic thrips (Izzo et al. 2002).
Members of Aeolothripidae contain trophic lifestyles that span from phytophagous to
predaceous (Mound 1997). At least 45% of the species constituting Phlaeothripidae are
fungal feeders with the remaining portion made up of predacious and phytophagous
groups (including several specifically associated with flowers, trees, cereals or mosses).
Members of Thripidae also run the gamut from plant feeding to predatory. The plants
feeders show diversity in association with all types and parts of plants. Little is known
about the natural history of Fauriellidae.
Within the phytophagous groups, there is wide variation in ecologies and diets
can consist of several types of plant tissues (leaves, flowers, fruits, stems, pollen, etc.)
Several of these species are also omnivorous, feeding on both plants and arthropod prey
(Wilson et al. 1996) and many species have been reported as crop pests (Mound 1997).
As the name suggests, the distinguishing characteristics of this insect order are the
fringed wings. The cilia that make up the fringes laterally surround a chitinous rod
(Moritz 1997). There are also numerous species that exhibit intraspecific variation in
wingform and other species that have secondarily lost the wings altogether. Both
macropterous and brachypterous thrips jump as a means of motility. This jumping is
accomplished with specially evolved meso- and metacoxae, which are generally and
advantageously folded under the body.
Most thrips reproduce sexually, but a majority of the sexual species are facultative
and capable of asexual reproduction as well (Moritz 1997). Thrips exhibit all variations
of parthenogenesis including arrhenotoky (most common), thelytoky and deuterotoky.
Recent investigations reveal that the bacterial symbiont Wolbachia may play a significant
role in reproductive rates (Heliothrips haemorrhoidalis (Bouche), Hercinothrips
femoralis (Reuter) (Pintureau et al. 1999) and Franklinothrips vespiformis (Crawford)
(Arakaki et al. 2001)), and in conjunction with temperature, determining sex ratios
(discussed in Moritz 2002). Facultative parthenogenesis is a key adaptations associated
with their successful r-strategy based life history. Although there is limited research on
chemical mediation of sexual activity in Thysanoptera, within the species Frankliniella
occidentalis (Pergande) a chemical substance produced by males has been shown to
attract male and female conspecifics (Kirk and Hamilton 2004). But the most likely and
well-studied aggregation cues for phytophagous thrips are visual and are theorized to be
associated with particular plant parts (Matteson et al. 1992).
The Thysanopteran lifecycle is divided into six (or seven) stages: ovum, 1st and
2nd instars (larvae), prepupa, 1st pupa, 2nd pupa (in Tubulifera only) and adult.
Depending on temperature, the development from egg to adult lasts 10 d to 30 d on
average, with the adult stage lasting about the same amount of time (Lewis 1997a).
The piercing-sucking feeding action of all thrips is accomplished with a uniquely
evolved stylet consisting of three modified mouthparts (one mandible and two maxillae)
that originate from the left side of the head (the mouthparts on the right side of the head
have become vestigial or disappeared all together) (Moritz 1997). In some species, the
hypopharynx may serve as a fourth stylet (Borden 1915). Feeding Thysanoptera
generally accomplish their task by piercing the plant or pollen wall or the prey
exoskeleton while releasing saliva (sometimes containing virus particles in infected
individuals) into the puncture site (Hemming 1978). They then suck up the fluid puddles
using the cibarial pump. In addition to potential severe dehydration of the plants, thrips
feeding can result in "silvering" or "bronzing" of fruits and leaves (Boumier 1983).
Further damage to plants by Terebrantain thips to a host plant occurs when females
oviposit on the fruits, flowers, stems and leaves (Salguero Navas et al. 1991). At high
densities this type of damage will generally cause plants to grow malformed.
Thrips Ecology and Population Dynamics
Patches of decomposing vegetation where developing fungi can be found, and
where thrips origins are commonly believed to lie, are characterized by specific
conditions that include brief existence of optimal conditions, rapid change and wide
variation of environment from patch to patch (Kristensen 1995). These origins impart
this group with several fitness-enhancing characteristics that were selected for.
The successful traits of the primitive thrips are now believed to be possessed by
and give an advantage to the extant descendants. These traits include high vagility, a
broad food range, high fecundity and regeneration rates, parthenogenesis, as well as
gregarious and competitive breeding behavior (Mound 1997). These phenomena are
better understood in phytophagous thrips, especially those associated with agricultural
For thrips, there are several factors affecting the four standard categories (births,
deaths, immigration and emigration (Turchin 1991, Thomas and Kunin 1999) used for
measuring population trends for species. Included among the factors that most
significantly control the dynamics of phytophagous thrips are type and condition of host
plant, climate and weather, predation, parasitism, disease, ability to overwinter and
human interactions (Kirk 1997).
Interactions between thrips and their host plants can be influenced by the host's
cultivar type (Fery and Schalk 1991, de Kogel et al. 1998), food quality and availability
(Shibao 1997, Agrawal and Klein 2000), the host's age and phenotypic state (including
flower color) (Ram and Mathur 1984, Stoddard 1986, Baqui and Kershaw 1993, de Kogel
et al. 1997) and the ability of the host plant to recruit predators (Venzon et al. 1999,
Agrawal et al. 2000, Norton et al. 2001, van Rijn et al. 2002).
Lab and field studies have shown differential suitability of crop species, varieties
and non-crop plants (weeds) as host plants for many arthropods (Mansour et al. 1982,
Appel and Martin 1992, Mound 1997, Beard and Walter 2001, Fogleman and Danielson
2001). This holds true for thrips. The differential spatial and temporal presence,
availability and quality of nutrients between different species of host plants affect thrips
fitness. It is important to point out that while thrips may feed on a specific plant or group
of plants, those plants may not be used or found suitable for oviposition and development
of the larvae (Mound and Marullo 1996). In addition, many plants are used as a site
where thrips can seek shelter until other florae appear that are more suitable for feeding,
mating and/or development. Each species of phytophagous thrips seems to prefer a
specific plant part for feeding, oviposition and/or congregation for mating (Palmer et al.
1991, Mound and Marullo 1996, Hansen et al. 2000), although they are usually able to
derive nutrition from multiple tissue types on a plant. Studies on most Frankliniella
species have shown that over 95% of the thrips on pepper plants in the field are located in
flowers (Gerin et al. 1999, Hansen et al. 2000, Ramachandran 2001).
The use of multiple hosts contributes to predisposing flower thrips as crop pests,
specifically in regards to generation, survival and persistence as well as dispersal and
movement. Subsequently these behaviors enhance movement of thrips-vectored plant
diseases. In agricultural ecosystems, this is relevant in that flower thrips, with their broad
host range, are often able to find suitable hosts on the periphery of agricultural fields
(Puche et al. 1995, Hobbs et al. 1996, Chatzivassiliou et al. 2001, Groves et al. 2001). In
modern agricultural practice, a lack of attention to or understanding of how communities
outside the designated cultivation area affect population movement and dynamics within
the field has generally resulted in approaches to controlling pest populations that are
myopic. In these cases, the portion of the environment considered when planning a
control strategy is the cropping area, while the heavy impact from influences outside the
tillage is ignored. In order to have a truly successful impact on lowering thrips numbers
in the field, one must consider the impact of the surrounding biotic systems. The use of
alternative hosts by thrips coupled with their high rate of movement and good dispersal
capabilities allow "weeds" to act as a reservoir of immigrating populations and vectored
plant diseases (Mound 1997, Cossentine et al. 1999, Chatzivassiliou et al. 2001, Groves
et al. 2001, Pearsall and Myers 2001). However timing is important as well; Weed
control measures in the middle of a crop growth cycle can actually increase disease
incidence in a cultivated crop (Eberwine 1995, 1998).
On a smaller level, the architecture of a host plant is a major factor in the ecology
of a plant-dwelling arthropod species. Data reveal that many smaller organisms acquire a
significant amount of protection from predation and adverse conditions by seeking refuge
in small pockets of a more favorable environment, termed domatia (O'Dowd and Willson
1991, Agrawal 1997, Agrawal et al. 2000, Roda et al. 2000, Norton et al. 2001). Thrips
are a characteristic example. They are shown to persist longer in sites where there are
microclimates that enhance fitness. These domatia optimize one or a number of
environmental factors, such as humidity and temperatures. Domatia include structures on
plants such as small invaginations, crevices, tufts of hairs and arthropod-created
structures on plants that exclude or repel predators. Localized sources of food, nutrient or
water also fall into this grouping.
Experimentally, domatia have been shown to have significant effects on
predation. They appear to serve primarily in harboring populations of smaller predators
thereby increasing overall predation rates (Grostal and O'Dowd 1994, Agrawal 1997,
Agrawal et al. 2000), although the size and trophic level of the organism(s) involved in
the interaction appear to be relevant.
In some cases the presence of domatia can have the opposite effect (O'Dowd and
Willson 1991, Roda et al. 2000, Norton et al. 2001). These pockets can serve to protect
several arthropods (including thrips) from predation (Venzon 2000, Norton et al. 2001).
Although there are few published studies involving thrips, Orius insidiosus (Say)
(Heteroptera: Anthocoridae) and the effect of domatia, Agrawal et al. (2000) found that
plants showed a positive correlation between domatia and numbers of predatory bugs
(including 0. insidiosus) and a negative correlation between domatia and phytophagous
thrips. Conversely, Norton et al. (2001) demonstrated that 0. insidiosus had less success
preying on mites in crops with a greater level of domatia.
One obvious benefit of these shelters to thrips relates to their relatively small size:
desiccation is a serious factor for smaller organisms. The high surface area to volume
ratio of thrips results in a high rate of desiccation. This principle dictates that thrips are
only able to make relatively short flights, require regular feedings of fluid and must seek
protection from exposure when possible. These objectives can be accomplished by
spending non-foraging time in low airflow/high humidity spots like domatia (O'Dowd
and Willson 1991).
From 1932 to 1946, data for a population study were collected on Thrips imaginis
Bagnall on roses in Adelaide, South Australia. The information gathered by Davidson
and Andrewartha (1948) showed dramatic fluctuations in thrips numbers based on
climate. This paper generated much study and debate on (leading to further study and
illumination of) the complex ecology of thysanopteran population dynamics (Smith 1961,
Varley et al. 1973).
Thrips populations are highly susceptible to changing climate. As indicated, arid
conditions can have a significant impact on fitness, since thrips size makes them highly
vulnerable to desiccation. Therefore drought could have a serious effect on thrips
populations directly by decreasing humidity. While the small size of flower thrips can
predispose them to rapid dehydration and desiccation in locations with arid ambient
humidity, use of more humid microclimates found within 5 mm of leaf surfaces allows
them persist (Shipp et al. 1996). The optimal relative humidity for thrips is between 70%
and 90% (Kirk 1997). In phytophagous species one of the drought-related phenomena of
greatest influence on thrips populations is a decrease of the nutritional value of the host
plant through plant wilting. Withering of flowers (which reduces availability of sites for
mating) and drying out of the soil (whereby decreasing success in pupation) are also
impacts of drought that can affect thrips on a population level (Moritz 1997, Mound
1997). One strategy used by Terebrantians to combat desiccation of ova is to lay the eggs
inside plant tissues (Mound 1997). Although Tubuliferans deposit ova on plant surfaces,
this strategy is still beneficial because of the higher humidity of the microclimates in
close proximity to the host plant surface. Of the post-ova ecolsion stages, the pupae have
the highest tolerance in low humidity environments, due to their lower rate of respiration
Rain can impact thrips populations as well (Harris et al. 1936, North and Shelton
1986). Buhl (1937) reported as many as 95% of Kakothripspisivorus (Westwood) were
lost during a heavy rain event. In such cases, population densities generally rebound
slowly. Many of the losses seem to be from drowning, but even more so from the
ensuing rain-induced soil conditions that can trap larvae and adults or that prevent
successful eclosion by pupae (Moritz 1997). Those that are lost generally end up caked
in the mud and are unable to escape.
Just as important as humidity is temperature. Frankliniella occidentalis is able to
develop more rapidly with increasing temperatures until approximately 30C, after which
the developmental rate is hindered. Beyond a given temperature threshold, all thrips
become impaired in their feeding, movement, development and behavior. Kirk (1997)
recommends that degree-day models for thrips do not include temperatures above 35C.
Winter with its associated cold temperatures is an event that needs to be endured
by all organisms outside (and sometimes inside) the tropics. Several thrips species,
including F. occidentalis, Limothrips cerealium (Haliday) and Thrips palmi Karny are
able to survive air temperatures below freezing in all stages of development (Kirk 1997).
In outdoor sites where leaf litter, soil and sometimes snow are available, many species
can survive full winters and emerge to repopulate areas again when favorable
temperatures return. Thrips may overwinter in either the pupal stage or the adult stage.
Another measurably strong influence (though sometimes it is quite difficult to
measure) is the pressure exerted on thrips populations by natural enemies. Predators,
parasites and diseases have been shown to play a significant role in controlling thrips
numbers in many trophic systems for several species.
Members of the predator guilds with the greatest impact include Heteroptera
(primarily members of the Anthocoridae in the genus Orius, but also Lygaeidae,
Pentatomidae, Reduviidae, Nabidae and Miridae), Acari (primarily the Phytoseiidae in
the genus Amblyseius, but also Trombidiidae, Erythaeidae, Laelapidae, Pyemotidae and
Acarophenacidae), Chrysopidae, Coccinellidae, Diptera (primarily Syrphidae, but also
Cecidomyiidae, Dolichopodidae, Chloropididae and Hypotidae), several Aranae and
other Thysanoptera (Aeolothripidae, Phlaeothripidae and Thripidae) (Sabelis and van
Rijn 1997). Other thrips predators that are known are some solitary wasps (Spilomena
spp.) (Danks 1971), mantids (Mohandaniel et al. 1983), crickets (Ghabn 1948), some
birds (Buhl 1937) and toads (Hamilton 1930).
Thrips parasitoids include mostly endoparasitic wasps (Loomans and van
Lenteren 1995). All of these wasps are classified in the superfamily Chalcidoidea and
most are in the family Eulophidae (larval parasites), Trichogrammatidae and Mymaridae
(egg parasites). Many non-insect parasites can be found among the Nematoda. As in the
case of Thripinema nicklewoodii (Siddiqi) and Thripinema reniroai (Reddy) and their
relationship with thrips in the genus Frankliniella, the parasites decrease fecundity in the
female thrips by damaging reproductive tissues (Nickle and Wood 1964, Funderburk et
al. 2002, Stavisky et al. 2002).
Diseases too play a significant role in regulating thrips populations and, in some
cases, can have the highest impact on thrips numbers (Dyadechko 1964, Butt and
Brownbridge 1997). Prominent fungal pathogens include Aspergillus, Entomopthora,
Verticillium and Beauveria. The last of these was recorded to have caused 100%
mortality in cereal thrips (Dyadechko 1964). The same is true for some protozoan
parasites in other thrips species (Raizada 1976).
Impacts on populations due to interspecific competition with other thrips species
and/or arthropods undoubtedly occur. This is often most dramatic in cases of the
introduction of non-native species. Several Acari and Heteroptera that share the same
niche as a thrips have been able to efficiently compete for limited resources and, in turn,
have had a significant effect on thrips populations (Karban 1987, 1989; Janssen et al.
1998, Pallini et al. 1999). For example, flower dwelling thrips must compete with other
sympatric species for the limited space provided in flowers (Kirk 1997). As expected,
when Reitz and Trumble (2002) analyzed a number of studies they determined that
interactions between more closely related species are characterized by a higher level of
competitive displacement than those of less related organisms.
Intraspecific competition in thrips results in some unique adaptive behaviors,
including fighting (for territory, mates and food resources) (Immaraju and Crespi 1986,
Terry 1997), caste development (Kranz et al. 1999) and sexual dimorphism (Crespi
1986). Other phenomena in thrips increasing competitive ability include high levels of
fecundity (Nugaliyadde and Heinrichs 1984, Malchau 1991, Kirk 1994), high levels of
mating (Kawai 1987, Kawai and Kitamura 1987, 1990) and increased precision in
dispersal timing and rates (Gopinathan et al. 1981, Crespi and Taylor 1990, Puche et al.
While larvae and pupae are limited in their movement, adult thrips are quite adept
at dispersal. In macropterous thrips, cues such as increasing humidity, changes or
disturbance to habitat and increased competition can be linked to flights of thrips and
invasion of new habitats (Kirk 1997). Movements may be made by individuals at any
time, but generally certain conditions (such as the aforementioned cues) result in a
massive exodus. Species (Vierbergen 1996), sex (van de Wetering et al. 1998), climate
(Pearsall and Myers 2001), density (Crespi and Taylor 1990) and host plant changes
(Peters et al. 1996) are influential factors in dispersal behavior in thrips. Flying thrips
generally fly while remaining just above the vegetation and their movement is strongly
influenced by prevailing wind currents (Lewis 1997b, Pearsall and Myers 2001).
Thrips have a propensity for enormous increase in population. A small number of
colonists can multiply to reach massive numbers in days (Wang and Shipp 2001).
Appanah and Chan (1981) found lab-reared thrips to be at 30x their original numbers
after 10 d. Basic exponential and logistic growth curves are used in modeling various
populations, but exclude extenuating aspects, such as the above-mentioned impacts of
predation and parasitism, competition, climatic phenomena, microhabitats and human
interactions (including control methods). Two other significant attributes of flower thrips
population growth are initial conditions (Schaffer and Kot 1985, Camilo and Willig
1995) and mate availability (a significant density-dependent factor) (Wang and Shipp
2001). Successfully incorporating all of these factors into a model of thrips population
dynsmics is still difficult, mainly due to the limited knowledge of their biology.
The highest-evolved Thysanoptera, the flower thrips (represented mainly by the
genera Frankliniella and Thrips), are most often found in flowers as adults and larvae
(Pearsall 2000, Hansen et al. 2003). They inhabit tropical and temperate areas throughout
the world (Mound 1997). These species feed primarily on the contents of plants cells
including fruits, leaves, inflorescence tissues and pollen. Several flower thrips are also
facultative predators of other arthropods (Wilson et al. 1996, Agrawal and Klein 2000,
Venzon et al. 2000)
Two major flower thrips species represented in Florida are the western flower
thrips, Frankliniella occidentalis and the Florida flower thrips, F. bispinosa (Morgan). F.
occidentalis is an invasive species from the western US and is found throughout Florida.
F. bispinosa is a native species found throughout Florida, with the greatest abundance in
the south of the state (Childers and Brecht 1996, Hansen 2002). They share similar
morphology and ecology and both are identified as pests of agriculture (Lewis 1997a).
They inflict direct feeding damage to plants as well as being transmitters of plant disease.
The characteristic movement by flower thrips, coupled with their feeding
behavior and a relatively broad host range, make them an effective and efficient vector
for plant diseases. A number of thrips-vectored viruses have been reported. Some thrips-
vectored viruses that have surfaced as notable problems in agriculture are impatiens
necrotic spot virus (Deangelis et al. 1994), peanut yellow spot virus (Satyanarayana et al.
1998), groundnut ringspot virus (Wijkamp et al. 1995), tomato chlorotic spot virus and
multiple strains of tomato spotted wilt virus (Palmer et. al 1991, Wijkamp et al. 1995,
Latham and Jones 1998, Nagata 2002).
Tomato Spotted Wilt
Tomato spotted wilt, first described in Australia in 1915 and discovered to have
viral origins in 1930 (tomato spotted wilt virus), impacts a very large range of plants and
has a tremendous impact on agriculture throughout the world. Tomato spotted wilt virus
is the type species for the genus Tospovirus (family Bunyaviridae) and is known to infect
over 1090 plant species in 85 families worldwide (Parrella et al. 2003). Susceptible
vegetation includes peanuts, tomato, lettuce, potato, pepper, tobacco, peas, ornamentals
and several other cultivated and non-cultivated plants (Murphy et al. 1995). Tomato
spotted wilt is characterized by top distortion, stunted growth, necrotic and chlorotic
spots on leaves as well as mosaic, necrosis and chlorosis on stems and fruit. Infection in
younger plants by the virus is often fatal (Nagata 2002).
The virus is solely transmitted by phytophagous thrips, is generally acquired only
by larvae due to a barrier in the midgut of adult flower thrips. Although the larvae are
physiologically capable of transmission, adults almost exclusively transmit the virus due
to the fact that larval thrips generally have a lower titer of the virus and limited dispersal
capabilities (Sherwood et al. 2000). The shear number of susceptible hosts coupled with
the innate dispersal characteristics of the thrips vector consequently results in potentially
epidemic virus transmission rates (Ullman et al. 1997). Unchecked non-crop plants or
"weeds" near or adjacent to the agricultural field can function as a virus and vector
reservoir thereby increasing incidence of disease transmitted into that plot (Puche et al.
1995, Peters et al. 1996, Groves et al. 2001, Pearsall and Myers 2001).
For the complete inoculation/transmission cycle of the virus to occur in thrips,
first instars must feed on an infected plant and ingest a sufficient amount of virus
(Wijkamp et al. 1996). The mean acquisition period for F. occidentalis larvae was
determined to be 67 min (Peters et al. 1996, Wijkamp et al. 1996). After the virus is
ingested it passes through the midgut walls and begins replication in the larval midgut
cells. A barrier that develops in the midgut of adult thrips prevents passage of viral
particles and effective inoculation at this late stage (Ullman et al. 1997, Ohnishi et al.
Tomato spotted wilt virus generally survives development of the thrips through
the 2nd instar, prepupal and pupal stages and comes to infect several organs throughout
the thrips, including the salivary glands (Wijkamp et al. 1996). Additional evidence
suggests that larvae reared at a higher temperature are less effective transmitters than
larvae reared at lower temps, due to the developmental rate of the thrips host outstripping
the replication rate of the virus (Wijkamp and Peters 1993). Confirmation of tomato
spotted wilt virus infection in thrips can be achieved using enzyme-linked
immunosorbent assay (ELISA) (Ullman et al. 1997) or reverse transcriptase-polymerase
chain reaction (RT-PCR) (Bandla et al. 1994).
As adults, infected thrips disseminate virus through their saliva to plants that they
feed upon. Factors influencing the rate of tomato spotted wilt virus transmission include:
concentration of virus in host plants and host plant suitability (Peters et al. 1996), the sex
of the thrips vector (van de Wetering et al. 1998), vector movement and dispersal
characteristics (Wijkamp and Peters 1993), amount of time spent probing by hosts on
plants and transmission time (Wijkamp et al. 1996), and the presence or absence of virus
in a host plant (F. occidentalis has been shown to prefer feeding on plants infected with
tomato spotted wilt virus (Bautista et al. 1995)).
In Florida, four species in the genus Frankliniella have been established as
vectors of TSWV, with F. occidentalis recognized as the most efficient vector. The other
three are F. bispinosa, F. fusca (Hinds) and F. schultzei (Trybom). F. schultzei is found
in northern Florida while F. occidentalis, F. bispinosa and F. fusca are found throughout
Traditional controls to prevent thrips infestation and virus epidemics have focused
on regular applications of broad-spectrum, chemical insecticides. In retrospect, we have
learned that this tactic decreases the number of natural enemies while exacerbating thrips
numbers and ultimately virus transmission rates (Etienne et al. 1990, Izawa et al. 2000).
The failure in control by chemical insecticides is due in part to physiological
characteristics in thrips that allow for resistance (Br0dsgaard 1994). Other population
characteristics that make chemical control unrealistic include thrips' high reproductive
capacity, a relatively short transmission time and constant inundation by thrips colonizers
from peripheral metapopulations (Diekmann et al. 1988). F. occidentalis, especially,
shows enormous capabilities in developing resistance against several common classes of
insecticides, including organophosphates, carbamates and pyrethroids (Immaraju et al.
1992, Br0dsgaard 1994, Robb et al. 1995, Jensen 1998).
In addition to this evidence, knowledge that pesticides are innately toxic and have
been recorded to bioaccumulate in several phyla of organisms (including vertebrates)
(Honrubia et al. 1993, Nebeker et al. 1994, Albanis et al. 1996) is even further motivation
to select a solution that is more effective, affordable, more target-specific while being as
ecologically and temporally sustainable as possible. These revelations lead to The Food
Quality Protection Act of 1996 (FQPA 1996).
With awareness of the hazards of chemical pesticides and the subsequent
legislation, emphasis has been placed on developing sustainable agricultural practices and
practical, holistic approaches to dealing with pest problems. The concept of "integrated
pest management" and biological control had been introduced several decades previously,
but only reached conventionality in the United States after being mandated by the FQPA
(Wellings 1996). Out of concern for ecologically sustainable practices and in an effort to
comply with the law, more growers turned to natural enemies and cultural control
practices to suppress or prevent pest outbreaks while maintaining, and even improving,
profitability in some systems (Hara et al. 1990, Hoffmann et al. 1995).
Orius insidiosus (Say)
Nymphs and adults of the insidious flower bug, Orius insidiosus (Heteroptera:
Anthocoridae), prey on several species ofthrips, whiteflies and mites, as well as the ova
and young larvae of Lepidoptera (Sterling et al. 1989). Although natural enemies were
not thought to have a significant impact on thrips numbers (Davidson and Andrewartha
1948, Butt and Brownbridge 1997, Loomans et al. 1997, Parker and Skinner 1997,
Parrella and Lewis 1997), Orius species have proven and been accepted as effective
controls of thrips populations in multiple agricultural and ornamental crops (Van de Viere
and Degheele 1992, Coll and Ridgway 1995, Dissevelt et al. 1995, Glenister 1998,
Funderburk et al. 2000, Ramachandran et al. 2001). 0. insidiosus is currently being
mass-reared for control of multiple arthropod pests in several agricultural systems
Orius species are generally associated with flowers, which coincides with their
major prey items: flower thrips. They use specific visual and volatile cues to locate
suitable habitats and prey items (Reid and Lampman 1989, Teerling et al. 1993, Henaut
et al. 1999). Female 0. insidiosus require 12.5 thrips per day in order to maintain
sufficient levels of protein for egg production (Tommasini and Nicoli 1993).
Ramachandran et al. (2001) show that 0. insidiosus was capable of high levels of
movement between plants and flowers meeting requirements to access sufficient numbers
ofthrips to support this demand. Funderburk et al. (2000) determined that
unsupplemented populations of 0. insidiosus were responsible for rapid suppression of
three key species of tomato spotted wilt virus -vectoring thrips in field peppers once
predator:prey ratios reached 1:212. Eventually F. occidentalis populations in northern
Florida were driven to virtual extinction within days of reaching a predator:prey ratio of
1:40. Persistence of 0. insidiosus (Say) in the field in the absence of high numbers of
prey prevents buildup and recovery of thrips populations. The survival of 0. insidiosus
populations with little prey is facilitated by the fact that this predator is able to survive in
fields in the absence of thrips prey and complete a full lifecycle by feeding on pollen and
plant fluids (Dissevelt et al. 1995, Richards and Schmidt 1996).
Domatia have a positive impact on populations of Orius. Agrawal (1997) found
that numbers of Orius were positively correlated with abundance of domatia on plants.
Eggs and nymphs were able to endure using these structures and were more likely to
survive to adulthood on plants with these small sanctuaries. However, even in the
presence of persistent plants, 0. insidiosus diapauses in northern and central Florida from
winter through early spring, which results in a subsequent, primaveral eruption of thrips
populations during this time (Toapanta et al. 1996, Ruberson et al. 2000). Another
limitation to predation on thrips by 0. insidiosus is the host plant. While predation
occurs at higher levels on bean, pepper and corn, the physical and chemical properties of
tomato plants limit movement of 0. insidiosus and its efficacy as a predator of thrips
(Coll and Ridgway 1995). The dense collection of glandular trichomes on the surface of
tomato leaves significantly hinders the searching ability of 0. insidiosus (Say).
Behavioral differences in related species of arthropods can cause an overall
predation pressure to bear more heavily on one or a few certain species (Chesson 1983,
McPeek 1990). Two main factors in this interaction influence predation: predator choice
and passive selection (Pastorok 1981). These two aspects of the predator-prey
relationship shape the overall dynamics of the outcomes. Active choice occurs when
predators choose prey based on nutritional or energy versus payoff bases (Williams 1987,
Lang and Gsodl 2001). Conversely, passive selection is a result of the prey's behaviors
and characteristics. Potential prey may select habitat based on a lower probability of
encountering a predator (Hanna and Wilson 1991; Cloutier and Johnson 1993) or may
change the outcome by way of altered behavior during or after an encounter (Lang and
Gsodl 2001). These differences in otherwise morphologically similar species can have
broad implications for community structure and ecology.
Interspecific variation of prey can affect predation levels in any given trophic
system. Honda and Luck (1995) found that Aonidiella aurantii (Maskell) was one of
several species of scale insects that were preyed upon by the coccinellid Rhyzobius
lophanthae (Blaisdell). It was determined that A. aurantii was preyed upon at a
significantly lower level than the other sympatric scales due to behavioral differences that
made it harder to capture and subdue. Likewise, Fritsche and Tamo (2000) found that
Orius albipennis (Rueter) captured and consumed fewer Megalurothrips sjostedti
(Trybom) than two other thrips species (CcuIil,,lI iptaie'\ cameroni (Priesner) and
Frankliniella schultzei). The reason for the lighter predation on M. sjostedti was
determined to be its high level of activity and movement.
The innate vagility of a species is a balance between factors affecting fitness of an
insect. Slower rates of movement allow some species to more closely examine host
plants and thereby determine the most favorable site (Byers 1996). Other, more actively
moving species may be less competitive in the absence of a predator guild, but are better
able to escape from predators when they are present (Baez et al. 2004, Northfield et al.
unpublished data). If host plant suitability is not significantly different across the habitat
(as in the case of agricultural fields) and predators are present, increased activity and
movement by a particular species may give it a competitive edge in colonization and
In general, my research will attempt to illuminate the predator-prey dynamics
between 0. insidiosus and two of its thrips prey, F. occidentalis and F. bispinosa.
Specifically, I will attempt to determine if 0. insidiosus preys on the two thrips species at
different levels in single-species populations as well as in mixed-species populations and
at the level of a single pepper flower habitat. I will also use a larger arena to determine if
there is any differential predation between the two thrips prey when presented together on
the whole-plant level.
MATERIALS AND METHODS
When possible, wild, field collected F. occidentalis and F. bispinosa were used in
the assays. Verification of thrips species was acquired from Dr. Joe Funderburk from the
University of Florida's Department of Entomology. Voucher specimens were also
deposited in the University of Florida Department of Entomology's insect collection.
The thrips were collected from wild mustard (Brassica kaber DC.) and crepe myrtle
(Lagerstroemia indicia L.) in Alachua and Marion Counties, Florida. Field collected
thrips were housed in 100 cm3 plastic, Gladware containers with a 5x8 cm ventilation
hole in the lid covered with thrips screen (0.15 x 0.15 mm holes) until use in the assays.
A 20cm2 square of paper towel was used to line the bottom of the container. Pole beans
(Phaseolus vulgaris L.) purchased from the local Winn Dixie supermarket were used as a
food, a fluid source. The beans were scrubbed with Fit Fruit and Vegetable Wash and
rinsed in order to remove any residual insecticide. They were then lightly streaked with
honey. Bee pollen was also offered as an alternative source of protein.
For those times when thrips were not available in the field, colonies were set up
and maintained using field-captured individuals. Pole beans (Phaseolus vulgaris L.) of
the same origin and that were handled the same way as for the field-collected thrips were
used as a food, a fluid source and oviposition material. All lab-reared thrips used in these
experiments were kept in the same 100 cm3 plastic, Gladware
containers with a 5x8 cm ventilation hole in the lid covered with thrips screen (0.15 x0.15
mm holes) with a 20cm2 square of paper towel lining the bottom.
All lab colonies were maintained in a Percival plant growth chamber at the
University of Florida's Department of Entomology at a photoperiod of 14L: 10D, 70-
80%RH. All field-collected and lab-reared thrips used in the experiments were adults
selected at random.
When possible, 0. insidiosus to be used in the assays were collected from crepe
myrtle (Lagerstroemia indicia L.) blossoms in Alachua and Manatee Counties Florida.
Verification of specimen identification was acquired from Dr. Joe Funderburk from the
University of Florida's Department of Entomology. Voucher specimens were also
deposited in the University of Florida Department of Entomology's insect collection. For
those times when 0. insidiosus were not available in the field, colonies were set up and
maintained using field-captured individuals or were acquired from Entomos, Gainesville,
Florida. Colonies maintained at the Entomology Department at University of Florida
were given pole beans as oviposition material that were scrubbed with Fit Fruit and
Vegetable Wash and rinsed in order to remove any residual insecticide. The colonies
were maintained at 14L: 10D and 70-80%RH in the same type containers as the thrips
were reared in.
In order to provide nutrition and precondition them to recognizing thrips as a prey
item, thrips from both Frankliniella species were added to all 0. insidiosus colonies on a
regular basis (every 5 days) when available. In order to maintain sufficient thrips
numbers for experimentation, Helicoverpa zea (Boddie) ova and bee pollen were often
offered as a supplement in addition to thrips or as a substitute source of protein when
thrips numbers were not high enough to use as food.
All 0. insidiosus used in the tests were adults selected at random. Specimens
used from lab colonies were all 3-5 d post ecolsion.
Objective 1.1: Single Species Arena
The arena was constructed from a polystyrene petri dish 15 cm in diameter and 3
cm in depth and given three ventilation holes and one entrance hole that were punched in
the lid of the dish using a hot, metal cylinder 3.5 cm in diameter. The ventilation holes
had thrips screen glued over them to prevent escape of the thrips and predator. The cap
and upper 1 cm of a 0.2 mL flip-top Eppendorf vial was sliced off and glued into place
using a hot glue gun so that the lower portion would sit in the entrance hole and the cap
could be opened and closed as to allow for the introduction of thrips and predators. A
screw cap from a 0.2 mL Eppendorf vial (to be used as a miniature floral pic) was glued
approximately 5 cm off-center onto the base of the dish. A straight pin was poked into
the mini-vase approximately 1 cm off the floor of the arena and a flip-top Eppendorf vial
cap was glued to the apical end of the pin and deemed to be a landing for the introduced
predators. The floral pic and the landing site were at the same distance from the center so
that the single entrance could service both by simply rotating the arena lid.
A single, clean pepper flower (Capsicum annuum L. (cultivar Camelot)) was
placed in the floral pic. The floral pic was filled with water to maintain turgidity in the
flower. The requisite number and species of thrips were collected and counted from the
colonies using a mouth aspirator. They were then chilled in the collecting container for
1.5 minutes (to facilitate easier handling) and then emptied into the entrance hole over the
flower. The arena containing the flower and the thrips was placed under dissecting
microscope while lighted by an adjustable 150W fiber optic illuminator. The thrips were
allowed to acclimate for 1 h before introduction of a predator.
A single species of thrips was used for each trial. Densities of 5, 10 and 20 thrips
were added to the arena and a single predator was added after 1 h. The trial was carried
out upon introduction of a predator. The lid of the arena was rotated so that the entrance
was directly over the landing and the predator was dropped in. The recording time began
once the predator left the landing and began walking on the pin towards the flower.
Behavior was recorded for exactly Ih.
Observations were made with the aid of a dissecting microscope at 30x
magnification. Occasionally observations had to be made without aid of the scope, when
a predator disappeared from view the field of vision (i.e. under a petal). Five activities
were recorded for each predator (Observer v 2.1, Noldus Information Technology,
Sterling, VA) and included the number of encounters (directed, physical contact with a
thrips), number of captures (time spent subduing a thrips), amount of time spent moving,
the duration of feeding activity and the amount of time spent resting.
For each trial inflorescences were removed from greenhouse reared plants just
before use. All inflorescences were cleaned of any existing insects and mites, using an
aspirator and a fine-bristle paintbrush.
Fifteen replicates of each predator sex/prey species/prey density combination
were recorded. Data were analyzed by a three-factor analysis of variance (ANOVA), sex,
species of prey and density of prey being the three factors. Means for each species at all
treatments and treatment combinations were separated by least squares means t-tests.
Untransformed means and their standard errors are presented.
Objective 1.2: Mixed Species Arena
For Objective 1.2, a mixture ofF. occidentalis and F. bispinosa were used at a
fixed density of 10 thrips (5 F. bispinosa : 5 F. occidentalis), and similar experimental
procedures were followed as in Objective 1.1. Likewise, the variables recorded in
Objective 1.2 were the same as in Objective 1.1, but in addition, the species of thrips
captured and/or encountered was recorded as well.
For each encounter I determined if the differences between prey species and the
dependent variables were significantly different from zero and if there was predation
variability between the two sexes of 0. insidiosus. This was accomplished by fitting the
data to an ANOVA with sex as the main effect and the intercept term and testing if the
mean differences were significantly different from zero (PROC GLM, intercept option,
Differences for dependent variables were calculated as F. bispinosa minus F.
occidentalis in each replicate arena; therefore, negative values reflect greater values for
F. occidentalis than for F. bispinosa. Data were checked for normality and
homoscedasticity, and did not need transformation.
Objective 1.3: Whole Plant Arena
Plexiglas cylinders, 15.5 cm in diameter and 36 cm in height, were used as arenas
and four holes 10 cm in diameter were cut into the cylinders as ventilation. Two holes
were placed at 1/3 distance from the top and two were placed at 2/3 distance from the top
and covered with thrips screen that was secured with silicone caulk. A fifth hole, 4 cm in
diameter, was drilled 5 cm from the top and used to introduce predators. It was covered
with a rubber stopper to prevent escape after the predator entered. The top of the cylinder
was covered with thrips screen.
A pot containing two specimens of Capsicum annuum L. (cultivar Camelot) of
roughly the same size, each with a single open flower at approximately the same height,
was used as an experimental setting. Another pot with plants similar in height, size and
placement of flower to each other and those in the experimental pot was used as a control
(no predator). The same number of thrips was placed in the experimental and control
arenas. The plants in all of the arenas were manicured as to prevent direct contact
between the leaves and other plant parts.
Thrips were tested at four densities: 10, 20, 40 and 80 total thrips per arena in a
1:1 species ratio. A vial of the thrips used for each trial was placed directly under and
touching one of the plants allowing the thrips to crawl out of the vial and onto the plant.
The initial plant that the thrips were introduced to was deemed Plant A.
The cylinder was placed over both of the plants and the base was pushed
approximately 5 cm into the soil as to prevent escape of the thrips and the predator by
that direction. The thrips were allowed to acclimate for 4 h, after which a predator was
introduced into the experimental arena.
Twenty-four hours after the introduction of the thrips, the flowers were sampled
by placing them in vials containing 70 % isopropyl alcohol and labeled corresponding to
its designation (Plant A Control, Plant A Experimental, Plant B Control or Plant B
Experimental). The thrips were counted and recorded to species.
The remainder of each plant was lopped off at the base just above the soil line
then chopped into small parts, and placed in a Ziploc bag with a corresponding label
(Plant A Control, Plant A Experimental, Plant B Control and Plant B Experimental) and
immediately doused with 70% isopropyl alcohol. Each plant was thoroughly agitated in
the alcohol to remove thrips before the plant parts were taken out and carefully rinsed one
more time using a wash bottle containing 70% isopropyl. The thrips present in the
alcohol were then counted and recorded to species. Any live thrips remaining in the
arenas were aspirated, identified to species and counted.
The control arenas were used to represent background mortality in the absence of
any predatory influence. The difference in the number of thrips surviving between
corresponding control and experimental arenas was presumed to be a result of predation
by 0. insidiosus. Differences between experimental and control arenas for numbers of
each species were analyzed and compared.
Two factor ANOVAs using the number of surviving F. occidentalis, F. bispinosa
and the difference between the two were carried out with density and predator treatments
as factors. Specific comparisons were made using least squares means t-tests. Because
there was no expectation of one species surviving better than the other species, two-tailed
tests to test for interspecific differences in survivorship in the respective treatment were
conducted. The hypothesis was that numbers of surviving thrips would be lower in the
presence of the predator 0. insidiosus than in its absence, so one-tailed tests to compare
numbers of each species surviving in the control and experimental treatments at each
density were used.
Objective 1.1: Single Species Arenas
Both female and male 0. insidiosus had significantly more encounters with F.
bispinosa than with F. occidentalis (F = 10.59, df= 1, 168, P = 0.0014; Fig. 1). There
was also a significant difference between the sexes of 0. insidiosus as females were
significantly more likely to encounter prey than males (F = 12.42, df= 1, 168, P =
0.0005). The interaction between sex and density was significant (F = 3.46, df= 2, 168,
P = 0.0337).
A MM F. bispinosa B
10 -10 F. occidentalis
M 6 6
5 10 20 5 10 20
Prey Density Prey Density
Female 0. insidiosus Male 0. insidiosus
Figure 1 Number of Encounters
Number of encounters (mean + SEM) by female (A) and male (B) 0. insidiosus
with either F. bispinosa or F. occidentalis adults at three different densities of
thrips in single prey species trials.
The level of encounters as related to density was not significant for female O.
insidiosus (P > 0.05, least squares means t-tests; Fig. 1A); however, males had
significantly more encounters at the highest density (20 thrips per arena) than at the lower
densities (P < 0.05, least squares means t-tests; Fig. 1B).
While 0. insidiosus had more encounters with F. bispinosa than with F.
occidentalis, there was no difference in the numbers of the two prey species captured (F
= 0.03, df= 1, 168, P = 0.86, Fig. 2). The capture rate was significantly higher with 20
thrips per arena than at the two lower densities (F = 5.32, df= 2, 168, P = 0.0057).
There was a significant difference in the number of captures and encounters between the
predator sexes. Females caught significantly more thrips than males did (F= 35.07, df=
1, 168, P < 0.0001).
A F. bispinosa B
1.0 1.0 -
5 10 20 5 10 20
Prey Density Prey Density
Female 0. insidiosus Male 0. insidiosus
Figure 2 Number of Captures
Number of captures (mean + SEM) by female (A) and male (B) 0. insidiosus of
either F. bispinosa or F. occidentalis adults at three different densities of thrips
in single prey species trials.
The mean time for a capture to occur was 1.5 + 0.07 sec. Although F.
occidentalis are larger than F. bispinosa, 0. insidiosus spent significantly more time
subduing and feeding on F. bispinosa (722 + 29.7 sec) than on F. occidentalis (618 +
35.5 sec; F = 6.24, df= 1, 78, P = 0.0146: for trials in which feeding was completed
before the end of the observation session; Fig. 3).
Objective 1.2: Mixed Species Arenas
Both male and female predators encountered significantly more F. occidentalis
than F. bispinosa in mixed arenas (test for intercept: F = 103.19, df= 1, 38, P < 0.0001;
Fig. 4A). 0. insidiosus attacked both species ofthrips when encountered, but individuals
ofF. bispinosa exhibited behavior of deliberately evading predators as well as more
consistent movement. In contrast, F. occidentalis appeared more inactive, which left
S 0 F.bisnosa A B
. 800 800
- 600 600
" 400 400
5 10 20 5 10 20
Prey Density Prey Density
Female O. insidious Male 0. insidiosus
Figure 3 Total Handling Time
Total handling time, in seconds (mean + SEM), by female (A) and male (B) O.
insidiosus when successfully preying upon either F. bispinosa or F. occidentalis
adults in single species prey trials at three different densities ofthrips. Total
handling time consists of time to capture and subdue prey and feeding time.
them more prone to attack from 0. insidiosus. My conclusion is that the prey of O.
insidiosus (in regards to flower thrips) is based on the vulnerability of the prey.
Female 0. insidiosus are better hunters than males with a higher number of
captures (F= 5.02, df= 1, 38, P = 0.031). Overall, both male and female 0. insidiosus
Figure 4 -Numbers of Encounters, Captures, and Handling Time
Mean (+ SEM) numbers of encounters (A), captures (B), and total
handling time for successful captures (C) that female and male O.
insidiosus had with F. bispinosa or F. occidentalis adults in mixed prey
species trials. Total handling time consists of time to capture and subdue
prey and feeding time. Mean (+ SEM) differences between prey species
for each variable are also shown. Differences are calculated as F.
bispinosa F. occidentalis. Therefore, negative values for the differences
indicate that quantities for F. occidentalis are greater than for F. bispinosa.
Asterisks (*) indicate mean differences that are significantly different
from zero (P < 0.05).
S F bispinosa
O F occidentalis
captured significantly more F. occidentalis than F. bispinosa (test for intercept: F =
40.97, df= 1, 38, P < 0.0001; Fig. 4B). Both female and male 0. insidiosus spent
significantly more time feeding on the smaller F. bispinosa than on the larger F.
occidentalis (F= 23.63, df= 1, 23, P < 0.0001). This difference in feeding times
between the two prey species was similar for both male and female 0. insidiosus
(predator sex by prey species interaction: F= 0.10, df= 1, 23, P = 0.758; Fig 4C).
Objective 1.3: Whole Plant Arenas
The effect of predation in the whole plant assays was determined by comparing
the number of thrips surviving in the experimental arenas (with predator) with the
number surviving in the corresponding control arenas (without predator). There was a
significant difference in the number of thrips surviving between the experimental and
control arenas. Although the impact was dependent on prey density, significantly more
F. bispinosa survived than F. occidentalis in the predator treatments (F = 4.28, df= 3, 96,
P = 0.0070; Fig. 5). Due to the significant interaction between predation and prey
density, I analyzed prey species separately for each density x predator treatment.
In treatments without 0. insidiosus, there were no significant differences in
survival ofF. bispinosa and F. occidentalis at the two lowest densities of 10 and 20 total
thrips per arena (P > 0.05 for least squares means t-tests that mean differences between
species = 0); yet, at the higher densities of 40 and 80 thrips per arena, significantly more
F. occidentalis survived than F. bispinosa (P < 0.05). However, at every density when
0. insidiosus was present, significantly more F. bispinosa than F. occidentalis survived
(P < 0.05 for least squares means t-tests that mean differences between species = 0; Fig.
0 With Orius
-10 -8 -6 -4 -2 0 2 4 6 8 10
More F. occidentalis More F. bispinosa
Mean Difference in Survival
(F. bispinosa -- F. occidentalis)
Figure 5 Mean Differences in Number Surviving
Mean differences (+ SEM) in numbers of surviving F. bispinosa and F.
occidentalis in whole plant arenas at each of four densities, with or
without 0. insidiosus present. Differences are calculated as F. bispinosa
F. occidentalis. Therefore, negative values for the differences indicate
that quantities for F. occidentalis are greater than for F. bispinosa.
Asterisks (*) indicate mean differences that are significantly different
from zero (P < 0.05).
To determine if predation was a significant factor for either prey species, we
compared numbers of survivors between the control and experimental treatments. At
each density, there were significantly fewer F. occidentalis that survived with O.
insidiosus present than without (P < 0.05 for least squares means t-tests that mean
differences between treatments for each prey species = 0; Fig. 6). In contrast, it was
determined that there was no significant difference in the numbers ofF. bispinosa that
survived with predators present and without (P > 0.05; Fig. 6). These results suggest that
F. bispinosa are able to survive better in mixed species environments than F.
F. bispinosa F. occidentalis
F. bispinosa F. occidentalis
S With Orius
F. bispinosa F. occidentalis
F. bispinosa F. occidentalis
Figure 6 Numbers of Surviving
Numbers of surviving F. bispinosa and F. occidentalis (mean + SEM) in
whole plant arenas at each of four densities, and with or without O.
insidiosus present. The initial densities of thrips are shown in the upper
right corner of each graph; note different scales for each. Equal
proportions ofF. bispinosa and F. occidentalis were used in all trials.
Previous research indicated that 0. insidiosus is a significant predator of flower
thrips. This study supports those assertions on a small arena and single-plant basis, but
also reveals that the species and numbers of prey available can influence short-term
results of predation. Specifically, my research indicates that there is a difference in the
level of predation by 0. insidiosus ofF. occidentalis over F. bispinosa when these two
species are presented together. A reason for this preference may be accounted for
through visual observations and previous movement/dispersal studies of the two prey
species (Ramachandran et al. 2001). Prior work with Heteroptera have shown that
species demonstrate preferential predation in mixed-species environments (Foglar et al.
1990, Hazzard and Ferro 1991, Cloutier and Johnson 1993, Cisneros and Rosenheim
1997, Eubanks and Denno 2000). Tests of Heteropteran predators have also shown that
there may often be a marked preference for one species out of a group of confamilial prey
(Fritsche and Tamo 2000, Meyling et al. 2003). The slightly different behaviors of even
the most closely related prey species can have far reaching consequences for the
ecological interactions and population dynamics in any given system.
Orius insidiosus is known to prey on both F. bispinosa and F. occidentalis in field
and laboratory conditions (Funderburk et al. 2000). Flower thrips are known to be
suitable for providing nutrient to active and gravid Orius spp. (Chyzik et al. 1995,
Wearing and Colhoun 1999, Fritsche and Tamo 2000, Venzon et al. 2002; Paik et al.
2003). While conducting my research, I did not observe 0. insidiosus rejecting either
species of thrips once it had been captured and subdued and failing to indicate any non-
preference for prey of a substandard quality (Meyling et al. 2003). Therefore, I
concluded that the lower levels of predation that I observed on F. bispinosa are likely a
result of some other difference than being a nutritionally less desirable food source.
Since F. bispinosa are significantly smaller than F. occidentalis, 0. insidiosus
needed to capture and consume more than one F. bispinosa to receive the same
nutritional gain as from one F. occidentalis. However when the different species of prey
were presented individually in the single species arenas, there was no significant
difference in the capture rate between F. bispinosa and F. occidentalis. Furthermore the
overall success rate (captures per encounter) was approximately 50% less for F.
bispinosa than it was for F. occidentalis in the single species trials.
While the smaller F. bispinosa took longer to subdue and consume than the larger
F. occidentalis, there were no observable interactions between F. bispinosa and 0.
insidiosus that would indicate that the predator was avoiding F. bispinosa during the
mixed-species trials. The behavior of 0. insidiosus was to attack any thrips that it was
aware of and that was nearby. The visual observations and data combined render a
scenario in which F. bispinosa were able to better avoid and escape predation by O.
insidiosus. As a result, the predator was able to capture more F. occidentalis than F.
bispinosa when they were presented together. It is important to note that in the mixed-
species arenas that, 0. insidiosus encountered both prey throughout the trials. This
indicated that predator satiation had no influence on its prey choice. This being said,
selection for F. occidentalis by 0. insidiosus was consistent during the entire series of
experiments. From this we can conclude that selective predation on F. occidentalis
appears to be a result of the behavioral characteristics of the prey as opposed to a
nutritional preference on the part of the predator (Lang and Gsodl 2001, Sukhanov and
In related research, Fritsche and Tamo (2000) found that the thrips predator Orius
albidipennis (Reuter) captured and consumed fewer Megalurothrips sjostedti (Trybom)
than Cci u'lilii ipi e, cameroni (Priesner) and Frankliniella schultzei (Trybom). The
difference was attributed to the behavior ofM. sjostedti (Trybom) in that it was more
active and vigorous in its movement while avoiding predators. Meyling et al. (2003)
hypothesize that Anthocoris nemorum (L.) and A. nemoralis (F.) forage more heavily on
Myzus persicae Sulzer than Macrosiphon euphorbiae (Thomas) because M. euphorbiae is
more prone to movement after an initial contact with a predator.
In the whole-plant arenas, where there was a greater expanse of space for the
thrips to occupy and utilize, the differences in predation were even more apparent. The
more vagile F. bispinosa incurred less mortality as a result of predation than did F.
occidentalis. Ramachandran et al. (2001) support these results in that they found that F.
bispinosa and F. tritici are more actively moving from plant to plant and colonize plant
resources faster than F. occidentalis Baez et al. (2004) showed that F. tritici disperses
at greater levels than does F. occidentalis ,and that the level of dispersal increases in the
presence of 0. insidiosus. However, they also showed that 0. insidiosus moves as
readily, enabling it to prey upon both species.
Orius insidiosus prefer to prey on Frankliniella larvae in field and lab conditions,
but as larvae become less abundant, predation on adult thrips increases (Baez et al. 2004).
My data and corresponding field observations suggest that 0. insidiosus would have a
similar impact on single-species populations ofF. occidentalis and F. bispinosa, but
suppression ofF. bispinosa populations would take longer than that of a F. occidentalis
population (Sabelis and Van Rijn 1997, Nomikou et al. 2002). As seen in field
observations in Florida, 0. insidiosus would first suppresses F. occidentalis populations
and then eventually those ofF. bispinosa (Funderburk 2002, Reitz et al. 2002).
Populations F. bispinosa and F. occidentalis increase during spring then decline
rapidly by early summer and remain at low levels until the following year (Chellemi et al.
1994, Funderburk et al. 2000, Ramachandran et al. 2001, Funderburk 2002, Reitz 2002).
Contrary to hypotheses by Parrella and Lewis (1997) that natural enemies are not a
significant factor in the regulation of thrips populations, Funderburk et al. (2000)
determined that there was a strong negative correlation between the level of thrips in the
field and the number of 0. insidiosus .
While 0. insidiosus are able to sustain populations on secondary sources of
nutrient (i.e. pollen and other species of prey) (Kiman and Yeargan 1985), female O.
insidiosus are unable to obtain sufficient protein for reproduction in the winter and until
prey once again becomes available in spring (Ruberson et al. 1998, 2000). Because of
this break in the reproduction cycle of 0. insidiosus, thrips populations are able to build
up in the temporary absence of recruitment by the predator (Sabelis and Van Rijn 1997;
Funderburk et al. 2000; Funderburk 2002).
The research I present here is a proposal that differential predation in this plant-
prey-predator trophic system is an important factor contributing to the fluctuating
population dynamics ofF. occidentalis and F. bispinosa, as well as other Frankliniella
species observed in Florida. These findings provide evidence that prey preference of a
generalist predator does not depend solely on the behavior and characteristics of the
predator, but can be affected by slight yet not insignificant differences in the behavior
and characteristics of closely related prey. In this case, these two closely related thrips
differ in the use of their environment and their response to the presence of a predator
enough to allows one species (F. bispinosa) to persist longer in the field than the other (F.
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I was born in Rochester, New York, in the Fall of 1972. Two weeks after my
birth my parents and I moved to Bradenton, Florida where I attended Palma Sola
Elementary School for kindergarten through fifth grade. I then rode my bike a little
further down 5th Avenue NW to King Middle School, where I attended sixth through
eighth grades. I started senior high at Manatee High School. It was in the middle of the
tenth grade that my family moved to Palatka, Florida, where I entered Palatka High
Upon graduating from Palatka High in 1990, I attended one year of classes at
Santa Fe Community College before returning to Palatka to attend St. Johns River
Community College, where I received my Associate of Arts Degree in 1993. I
immediately moved to Asheville, North Carolina, and took a couple of years off school
before entering The University of North Carolina at Asheville (UNCA). Shortly after
entering UNCA I got married and started a family and began to attend classes part time.
Eventually I earned my Bachelor of Science degree in biology in 2000.
I immediately moved to Gainesville, Florida, and began work in the lab of
Marjorie Hoy as a research assistant. After three months I started as a grad student under
Joe Funderburk. Just after finishing my classes I moved back to Asheville, North
Carolina, to deal with a family situation. In the time between finishing my classes and
defending my thesis I have taught as an adjunct professor at UNCA, functioned as Staff
Ecologist of the Southern Appalachian Biodiversity Project and currently am a Research
Technician and Lab Manager in the Biology Department of the University of North
Carolina at Chapel Hill.