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Copyright 2005, Board of Trustees, University
December 1985 Bulletin 853 (technical)
Biological Control of the
Twospotted Spider Mite
L. S. Osborne, L. E. Ehler, and J. R. Nechols
Agricultural Experiment Stations
Institute of Food and Agricultural Sciences
University of Florida, Gainesville
BIOLOGICAL CONTROL OF THE TWOSPOTTED SPIDER MITE
L. S. Osborne, L. E. Ehler and J. R. Nechols
L. S. Osborne is Assistant Professor of Entomology, Agricultural
Research and Education Center, University of Florida, IFAS, 2807
Binion Rd., Apopka, FL 32703; L. E. Ehler is Professor of Entomology
and an Entomologist in the Experiment Station, Department of Entomol-
ogy, University of California, Davis, CA 95616; and J. R. Nechols is
Assistant Professor of Entomology, Department of Entomology, Kansas
State University, Manhattan, KS 66506.
Dr. Lance S. Osborne
and Education Center
2807 Binion Road
Apopka, FL 32703
Introduction . . . . . . . . . 1
The Twospotted Spider Mite . . . . . . 3
Taxonomy . . . . . . . . . 3
Developmental Biology . . . . . . 4
Reproductive Biology . . . . . . 6
Dispersal and Diapause . . . . ... 10
Pest Status . . . . . . . . . 10
Biological Control . . . . . . . 12
Phytoseiulus persimilis . . . . . . . 13
Developmental and Reproductive Biology . . 13
Influence of Temperature and Relative Humidity 18
Feeding Habits . . . . . . . .. 21
Dispersal and Searching . . . . .... 21
Method of Release . . . . . . . 22
Common Problems . . . . . . . ... 24
Obtaining Natural Enemies . . . . .... 28
Acknowledgments . . . . . . . ... 30
References Cited . . . . . . . ... 31
There are over 140 species of insects and mites that are
known to be pests in greenhouses (Pritchard, 1949). In recent
years, the major pest species involved--such as aphids,
mealybugs, scales, and spider mites--have generally been
controlled by insecticides and acaricides. However, it is
becoming increasingly clear that the strategy of unilateral
reliance on chemical control will not be the final solution to
the problem. In this regard, there are four major problems
attendant to chemical control: (1) development of resistance to
chemicals in target pest species: (2) the dwindling supply of
useful, registered insecticides and acaricides; (3) the
damaging (or detrimental) effect of these chemicals on
nontarget species resulting in secondary pest outbreaks; and
(4) phytotoxic reactions by treated plants. On the other hand,
unilateral reliance on biological control should not be viewed
as a sound strategy because biological control alone does not
always give adequate protection.
A solution to this problem lies in the utilization of
integrated control--or what is now called integrated pest
management (IPM). This is a management system in which
ecologically suitable and economically rewarding control
tactics are employed to maintain pest populations at tolerable
levels. With respect to the current situation in many
commercial greenhouses, there is a critical need for
integrating biological control agents with existing cultural
and chemical control methods.
The purpose of this bulletin is to synthesize information
relevant to biological control of one of the major plant pests
found in greenhouses worldwide. This pest is the twospotted
spider mite, Tetranychus urticae Koch. Its most effective
control agent (at present) is the predatory mite, Phytoseiulus
persimilis Athias-Henriot. The twospotted spider mite is a
suitable subject for an IPM program in which biological control
plays a major role and may even be employed by itself for
extended periods of time. Biological control programs are
currently available for the control of greenhouse whitefly, but
many of the remaining pest species--particularly aphids,
leafminers, mealybugs, scales, and various species of
Lepidoptera--will require additional research. With adequate
support, the necessary information for these species and their
biological control agents should be forthcoming.
The emphasis in this bulletin will be on information--not
implementation. There are two reasons why we have chosen this
approach. First, during 12 years of research on biological
control of pests in greenhouses in Florida, California, and New
York, we have detected a strong interest, on the part of
growers and others, in the use of biological control. However,
information about the available biological control agents was
either lacking or too difficult for those interested in the
topic to obtain and eventually utilize. Second, because of the
great diversity of plant species (and cultivars) grown in
greenhouses, the wide variety of cultural practices used, and
the differing environmental conditions encountered, it is
virtually impossible to develop a set of guidelines for
implementing biological control which would cover every
conceivable situation. Instead, growers who are interested in
using biological control agents should be able to draw upon the
information contained herein an.d develop their own systems. In
fact, this approach is already being practiced.
Biological control can be defined as the action of natural
enemies which maintains a host's (or pest) population density
at a level lower than would occur in the absence of the
enemies. There are three classes of natural enemies of insects
and mites: predators, parasites (or parasitoids), and
pathogens. A predator is an insect (sometimes mite, spider,
etc.) whose immature form (larva or nymph) develops at the
expense of more than one host individual. The adult form can
be either predacious or free-living. Common examples of
predators include lady beetles, big-eyed bugs, lacewing larvae,
and ground beetles. Parasites, on the other hand, are insects
whose immature form (larva) develops in or on only one host
individual. The adult of the parasite is usually free-living,
feeding, for example, on nectar or insect body fluids.
Parasites are usually either certain flies (Diptera) or small
wasps (Hymenoptera) and can be considered "specialized
predators." Finally, pathogens are those micro-organisms whose
interactions with the host induce a disease (often lethal) in
the insect. Such pathogens include fungi, bacteria, protozoa
and viruses. Nematodes are often included in this category.
The fundamental premise for biological control of a
plant-feeding phytophagouss) insect can be summarized as
follows: in the native home (or region of origin) of a given
phytophagous insect, there should exist a natural enemy or
complex of natural enemies which maintains (or is capable of
maintaining) the population density of the insect at
comparatively low levels. Essentially, the same applies for
plant-feeding arthropods such as spider mites. This action is
called natural biological control and is the reason why so many
of our phytophagous insects, including several potential pest
species, are relatively rare and/or innocuous. Plant-feeding
insects which are introduced into a new country without their
effective natural enemies often reach outbreak proportions and
are thus subjects for classical biological control, that is,
importation of the appropriate natural enemies from the native
home of the pest. Also, natural enemies from areas other than
the native home of the pest species, but which are preadapted
to exploit the pest, can be of considerable value in classical
The effectiveness of a given natural enemy under
greenhouse conditions may not always coincide with its
performance outside of the greenhouse. Natural enemies which
are generally ineffective under field conditions may be
relatively effective in the greenhouse and vice versa. For
example, the parasite Encarsia formosa Gahan is effective
against greenhouse whitefly in the greenhouse; however, the
same species is generally ineffective in many field situations
especially where temperature extremes either favor the pest or
cause direct mortality of the parasites. The underlying
reasons for the differential performances of other natural
enemies are not always known. However, it is clear that
certain greenhouse conditions may be quite favorable for one
species of natural enemy, but not for another.
It is our view that, for biological control to be employed
successfully in the greenhouse, a proper attitude on the part
of grower is necessary. Indeed, colleagues in New York have
also noted the importance of this aspect (see Tauber, 1977;
Tauber and Helgesen, 1978). Some philosophical commitment to
biological control, including commitment to the integration of
biological and chemical control, is also necessary. A certain
amount of patience and a firm resolve for making biological
control work is generally needed. In our experience, one of
the foremost obstacles to the implementation of biological
control in the greenhouse is a negative attitude.
THE TWOSPOTTED SPIDER MITE
The twospotted spider mite, Tetranychus urticae Koch, is
the major spider mite pest of ornamental plants and vegetable
crops grown in greenhouses. Furthermore, this ubiquitous
spider mite is a serious pest of numerous ornamental plants in
home landscapes, and is of considerable importance as a pest of
food and fiber crops throughout the world. The literature on
spider mites in general, and the twospotted spider mite in
particular, is voluminous; however, much of the pertinent
information (including references) has been summarized by
Huffaker et al., (1969, 1970), McMurtry et al., (1970), van de
Vrie et al., (1972), Jeppson et al., T1975 and Hussey and
Contrary to popular belief, mites (including spider mites)
are not insects. Although both insects and mites belong to the
Phylum Arthropoda (because of their jointed appendages and
exoskeletons), there are major differences between the groups.
Adult mites characteristically possess 4 pairs of legs compared
to 3 pairs in insects (larval mites do however possess 3 pairs
of legs). Virtually all adult insects have 1 or 2 pairs of
wings. However, neither of these features is found in adult
mites. Whereas mites belong to the Class Arachnida which also
contains the spiders, insects belong to the Class Insecta. All
mites occupy the Subclass Acari (=Acarina).
The twospotted spider mite is a member of the family
Tetranychidae which contains many harmful species of
plant-feeding mites. There has been considerable confusion
concerning the nomenclature (i.e., scientific name) of the
twospotted spider mite. In the past, acarologists and applied
entomologists commonly referred to the spider mites in question
as Tetranychus bimaculatus Harvey or T. telarius (Linnaeus).
Boudreaux (1956) examined the so-called"twospotted spider mite
complex" and demonstrated that more than one species was
involved. In this case, the major species were the twospotted
spider mite, Tetranychus urticae Koch and the carmine spider
mite, T. cinnabarinus (Boisduval) (Boudreaux, 1956: Boudreaux
and Dosse, 1963; Jeppson et al., 1975). Common names such as
red mite, red spider mite, glasshouse spider mite, twospotted
spider mite, and common spinning mite generally referred to the
species complex: the same is true for the approximately 60
synonyms (i.e., scientific names) in the literature, the most
common ones being T. telarius (Linn.) and T. bimaculatus Harvey
(Boudreaux and Dosse, 1963; Jeppson et al., 1975).
In both male and female twospotted spider mites,
development proceeds through the following stages: egg, larva,
protonymph, deutonymph, and adult. The larval, protonymphal,
and deutonymphal stages are further divided into feeding
(active) and quiescent (resting) stages. The quiescent stages
are referred to as nymphochrysalis (=protochrysalis),
deutochrysalis, and teliochrysalis for larval, protonymphal,
and deutonymphal stages, respectively. Thus, development of
the twospotted spider mite can be summarized as follows: egg,
larva (including nymphochrysalis), protonymph (including
deutochrysalis), deutonymph (including teliochrysalis), and
adult (Laing, 1969: van de Vrie et al., 1972).
Females normally lay eggs (oviposit) on the undersides of
leaves. According to Cagle (1949), the spherical egg is about
0.14 mm in diameter (Figure 1). The newly deposited egg is
clear, but turns opaque and glassy as incubation progresses.
Just before hatching, the egg is strawcolored and the carmine
eyespotss" of the embryo become visible (Cagle, 1949). The
larva (Figure 2) has 3 pairs of legs (hexapod). At the time of
hatching, it is colorless, except for the carmine eye spots.
After feeding, its color changes to pale green, brownish green
or very dark green and two dark spots appear in the mid-portion
of the body (Cagle, 1949). At the end of the feeding stage,
the larva attaches to the substrate (i.e., leaf), becomes
quiescent (nymphochrysalis), and is later transformed into a
protonymph. The protonymph has 4 pairs of legs (octapod) and
is somewhat larger than the larva. Its color is usually pale
green to dark green and the two spots are larger and more
Figure 1. Eggs (lower portion) and adult male (upper) of the
twospotted spider mite.
Figure 2. Egg and larva of the twospotted spider mite (left)
and egg of P. persimilis (right).
pronounced than in the larva (Cagle, 1949). At the end of the
feeding stage, the protonymph attaches to the substrate, enters
the quiescent stage (deutochrysalis), and is later transformed
into a deutonymph (Figure 3). The octapod deutonymph is
generally larger than the protonymph (Figure 4), although
similar in color pattern. At this stage, the males can usually
be distinguished from the females because of the smaller size
and wedge-shaped posterior of the former (Cagle, 1949; Laing,
1969). Following cessation of feeding, the deutonymph attaches
to the substrate (Figure 5) and becomes quiescent
(teliochrysalis). The octapod adult (Figure 6) eventually
emerges from the teliochrysalis.
Developmental time of the twospotted spider mite will
generally vary with conditions such as temperature, humidity,
host plant, leaf age, etc. However, temperature is the most
important factor that influences the rate at which mites
develop. The lower threshold for development is about 12C
(53.6"F), whereas maximum upper limit to the development is
about 40*C (104"F) (Jeppson et al. 1975). Laing (1969)
maintained the mites on strawberry leaflets at an average
hourly temperature of 20.3"C (68.5"F) and relative humidity
fluctuating from 55% to 98%. Under these conditions, the mites
developed from egg to adult in an average of 16.5 days. Shih
et al. (1976) cultured the mites on lima beans at 27+1"C (81*F)
and 905% relative humidity. In this case, mites developed
from egg to adult in an average of 7.6 days. Sabelis (1981)
determined the developmental time required for an egg to
develop to a female capable of laying eggs. In his studies, he
reared the mites on detached rose leaves under two alternating
day/night temperature regimes. The regimens studied were
25-35'C (77-95'F) and 10-20*C (50-68F) for which he determined
the developmental times to be 8.3 and 28.2 days, respectively.
Additional aspects of developmental time are summarized in
In a given colony of twospotted spider mites, both adult
males and females can usually be found; however, females are
normally about three times more abundant than males. Cagle
(1949) provided an account of the characteristics of males and
females. The male is much smaller and is considerably more
active. The body is narrow and distinctly pointed posteriorly
(Figure 5). The color of the male varies from pale to dark
green, brownish, or at times, orange. The body of the female
is oval-shaped and rounded posteriorly (Figure 6). Color of
the females varies from light yellow or green to dark green,
straw color, brown, black and various shades of orange. There
are generally two large black spots one on either side of the
body, hence the common name. However, there can be
considerable variation in the expression of this particular
Figure 3. Quiescent nymphs of twospotted spider mite.
Figure 4. Deutonymph (left) and mature female (right) of
twospotted spider mite.
Figure 5. Male twospotted spider mite "guarding" quiescent
Figure 6. Mature female/twospotted spider mite.
Generally, adult males can be found in close association
with quiescent female deutonymphs (Figure 5). Evidence
indicates that the quiescent female deutonymph releases a sex
pheromone which attracts the male and keeps him in close
proximity (Cone et al., 1971a, 1971b; Penman and Cone, 1972,
1974). The male usually remains in the immediate vicinity of
the quiescent deutonymph and mates with the emergent female.
When more than one male attempts to "guard" a developing
female, fighting among the males often occurs; usually, larger
males win these encounters (Potter et al., 1976a, 1976b). Such
fights involve pushing and grappTn^g with the forelegs,
jousting with the mouth parts and entangling the opponent with
The life span of the adult female is divided into the
preovipositional period and the ovipositional period, the
former being the time between emergence from the teliochrysalis
to the deposition of the first egg. Apparently, the
preovipositional period (9% of the time required to develop
from egg to egg) can last less than 0.5 day and as long as 3
days depending on temperature. The period during which eggs
are deposited (ovipositional period) can last from 10 days at
35C (95'F) to 40 days at 15C (59"F) (Sabelis 1981). An
individual female can deposit over 100 eggs in her lifetime
(Shih et al., 1976; Carey and Bradley 1982). The total number
of eggs aTd/female and the eggs laid/female/day can, however,
vary with age, temperature, species of host plant, relative
humidity, nutrition of host plant, exposure to pesticides, etc.
(Watson, 1964; van de Vrie et al., 1972 Karban and Carey,
1984). Temperature and age of the female are especially
important determinants of egg production (fecundity). However,
Sabelis (1981) determined that fecundity was affected very
little at temperatures between 20-35C (68-95*F). In his
study, peak oviposition, (161 eggs/female) occurred at a
temperature of 25*C (77F), with the maximum rate (12
eggs/female/day at 25C (77'F)) occurring 2 days after the
first eggs are laid. The effect of temperature is particularly
evident in greenhouses, where spider mite populations often
develop rapidly soon after the onset of summer temperatures.
Sex determination in twospotted spider mites (as in many
other spider mites) is arrhenotokous. That is, females develop
from fertilized eggs and have the normal two sets of
chromosomes diploidd); on the other hand, males develop from
unfertilized eggs and have only one set of chromosomes
haploidd). Unmated females give rise only to males; mated
females can produce either female or male progeny. According
to Helle (1967), a single mating will suffice to provide a
female with enough sperm to produce diploid eggs for her entire
The phenomenon of arrhenotoky is of importance not only
from an academic standpoint, but also from a practical one.
Because the male has only one set of chromosomes, new genetic
features (arising from mutations) will be immediately
expressed. Through natural selection, these characteristics
can be added quickly to the population (Helle and Overmeer,
1973). Therefore, the potential for development of genetic
resistance to insecticides and miticides in the twospotted
spider mite is greatly enhanced by this method of reproduction.
Because of the high reproductive rate and fast generation time
and the intense selection pressure brought on by chemical
control of this pest in the greenhouse, resistance may develop
in a comparatively short time.
DISPERSAL AND DIAPAUSE
The dispersal ability of T. urticae in greenhouses is an
important factor to consider in te control of this pest.
Hussey and Parr (1963) indicated that twospotted spider mites
dispersed in the following ways: migration of newly emerged
(presumably mated) females to oviposition sites; dispersal from
infested plants, simply by dropping off; and movements over
soil surface in accordance with the plane of polarized light.
There is direct evidence that the mites are able to suspend
themselves on silken threads and thus be carried along by air
currents. Mites can also be dispersed on the clothing of
greenhouse personnel or through the movement of infested plant
material. Despite the dispersal ability of the mites, it is
not uncommon to find infestations in one portion of the
greenhouse throughout the season, and perhaps even from season
to season. Patchy infestations in the greenhouse are
characteristic of twospotted spider mites. In greenhouses
where ornamental plants are grown, patches are often found at
the ends of benches near the walls and away from center aisles.
The cause of these patches may be associated with poor spray
coverage in these hard to reach areas.
Under certain conditions, twospotted spider mites can
overwinter as diapausing (mated) females. The diapause is
presumably induced by photoperiod (i.e., shortened day length),
low temperatures, and unfavorable food supply (see Parr and
Hussey, 1966; Jeppson et al., 1975). These diapausing females
are yellowish-orange and hibernate in protected places (e.g.,
cracks, crevices). They neither feed nor reproduce while in
diapause. The diapause normally terminates in the spring when
favorable environmental conditions return. In Florida,
populations of twospotted spider mites cycle throughout the
year, although sometimes at reduced rates of development during
winter months. However, it is possible that a small portion of
the population enters diapause during the winter months.
Twospotted spider mites feed on many species of plants and
are a major pest of vegetables, ornamentals, fruit trees, hops,
cotton, and strawberries (van de Vrie et al., 1972). At
present, it is safe to assume that most oT the major spider
mite problems in greenhouses will involve twospotted spider
The larva, protonymph, deutonymph, and adult feed mainly
on the undersides of the leaves. When feeding, the body of the
mite is tipped upward such that the 3rd and 4th pairs of legs
are off the leaf surface and the mite is supported by the 1st
and 2nd pairs of legs (Jeppson et al., 1975). Feeding is
accomplished in the following manner: a pair of needle-like
stylets penetrates the plants' parenchyma cells, the contents
of which are then drawn into the body of the mite by a
pharyngeall pump". According to Laing (1969), protonymphs and
deutonymphs spend about half their developmental times feeding
and half in the resting or quiescent stage; the larvae spend
slightly more time feeding than resting.
Damage to the plants is effected in several ways. First,
feeding causes the destruction or disappearance of chloroplasts
which then leads to basic physiological changes in the plant.
Stomatal closure can be a primary host-plant response, and in
such cases, uptake of CO2 decreases resulting in a marked
reduction in transpiration and photosynthesis (Sances et al.,
1979a, 1979b). These effects can occur at spider miite
densities that are too low to cause visible damage. Reduction
of photosynthetic area by spider mite feeding is permanent and
can only be compensated for by production of new foliage.
Methods have been developed to quantify the amount of feeding
and therefore damage for both cucumber and tomato (Hussey and
Parr 1963, French et al. 1976, Anonymous 1976a, Anonymous
1976b). Both methodsui-tlize a leaf damage index (LDI) using
the following 0-5 scale: 0 = no damage; 1 = incipient damage
with one or two 1-5 mm diameter feeding patches or 5% of the
leaf area damaged; 2 = more and larger patches than 1 and with
15% of the area affected; 3 = denser speckling with 30% of the
area damaged; 4 = about 60% of the leaf area damaged; and 5 =
over 80% of the leaf area damaged with the leaf being chlorotic
(French et al. 1976). When the mean LDI reaches 2.0 on tomato,
33% of The leaf area is damaged and loss in yield can be
expected (Anonymous 1976b). Loss in yield for cucumbers occurs
when the mean LDI reaches 1.9 which corresponds to 30% of the
leaf area damaged (Hussey and Parr 1963). Secondly, it is
likely (but not firmly established) that the mites actually
inject phytotoxic substances into the plant when feeding (see
Avery and Briggs 1968; Jeppson et al. 1975; Liesering, 1960).
Finally, the stippling or specklTngof the upper leaf surface,
plus the webbing produced by protonymphs, deutonymphs, and
adults, leads to aesthetic injury, particularly in the case of
The factors which determine the abundance or density of
spider mites have been discussed in considerable detail by
Huffaker et al. (1969, 1970), McMurtry et al. (1970), and van
de Vrie et aT. (1972). With respect to outbreaks of spider
mites, particularly since World War II, there are two central
"hypotheses" or tentative explanations to account for these
events. The first is that the upsurge of spider mites is due
to improved cultural practices, such as pruning, fertilization,
and pesticide use. For example, outbreaks of spider mites can
be induced by certain fertilization practices or by certain
pesticides, regardless of natural enemies. Apparently, these
cultural practices increase the nutritive value of the plant
and thus enable greater reproductive activity on the part of
the spider mites. The second explanation is simply that
widespread use of broad-spectrum insecticides destroy or
greatly hamper natural enemies of spider mites and thereby
allow pest outbreaks to occur. There is reason to believe that
both mechanisms can act in concert in inducing spider mite
Most of the pertinent information in the literature
concerns the influence of pesticides on outbreaks of spider
mites under field conditions. According to van de Vrie et al.
(1972), increases in abundance of twospotted spider mites have
been observed following use of certain agricultural chemicals
in many different crops. Although the causes of such increases
in greenhouses have not been determined, it would be a sound
practice to minimize the use of insecticides and miticides in
the greenhouse since outbreaks of twospotted spider mites in
the field are often correlated with pesticide usage.
Probably the most common scenario for outbreaks of
twospotted spider mites in greenhouses is as follows: the
spider mites are accidentally introduced into the greenhouse
without any of their effective natural enemies; if host plants
and physical factors (e.g., temperature) are suitable, the
population "explodes". Common sources of inoculum include
infested plants carried into the greenhouse, spider mites
(especially mated females) which cling to the clothing of
greenhouse workers and weeds growing outside the greenhouse.
However, chemical controls used to control other pests
(e.g., mealybugs or greenhouse whitefly) can destroy natural
enemies which have been introduced into the greenhouses (see
below) and thus, engender serious outbreaks of twospotted
spider mites. Thus, insecticides, miticides, and fungicides
should be used judiciously when natural enemies are present in
order to minimize unnecessary problems with twospotted spider
Many different natural enemies are associated with spider
mites under field conditions. These enemies are either
predators or pathogens; there are no known parasites
(parasitoids) of spider mites (McMurtry et al., 1970).
In greenhouses, there are two categories of predacious
species that feed on twospotted spider mites: those which
occur naturally and those which are artificially introduced.
The predacious phytoseiid mite Phytoseiulus persimilis
Athias-Henriot, is the major species used to control twospotted
spider mites in greenhouses. However, Metaseiulus occidentalis
(Nesbitt), another predatory mite, has been evaluated for the
control of mites on greenhouse grown roses with some success
(Field and Hoy, 1984). Pathogens occur naturally under certain
field conditions and appear to be an important regulator of
spider mite populations. Hirsutella thompsonii Fisher has been
proposed as a possible microbial control for twospotted spider
mites in greenhouses but has only been effective in the
laboratory (Gardner et al., 1982). For this reason, P.
persimilis will be the only natural enemy treated in thTs
This predacious mite was accidentally introduced into
Germany from Chili in 1958 (Dosse, 1958). From Germany, it was
subsequently shipped to other parts of the world, including
California (McMurtry et al., 1978) and Florida (Hamlen, 1980).
P. persimilis has also become established in southern
California (Mcmurtry et al., 1978). It has also been collected
in the field in northern California. According to Kennett and
Caltagirone (1968), there are two synonyms for P. persimilis:
Phytoseiulus riegeli Dosse and Phytoseiulus tardi (Lombardini).
During the early 1960's, research on this species was
conducted in Great Britain, Holland, Canada, and the United
States. Since these initial studies, the ability of this
predator to control twospotted spider mites has been
demonstrated on many plants, including cucumber (Gould, 1970,
1971), tomato (French et al., 1976), ornamental ivy (Gould and
Light, 1971), rose (Simmonds, 1972; Boys and Burbutis, 1972),
lima bean (Force, 1967), dahlia (Harris, 1971), strawberry
(Laing and Huffaker, 1969), and dieffenbachia and schefflera
(Hamlen and Linquist, 1981). Although these studies were
conducted under greenhouse conditions (or in growth chambers),
there is evidence that P. persimilis can be an effective
natural enemy in commercial strawberry plantings (McMurtry et
al., 1978) and on ornamentals in commercial interior plantings
TEindquist, 1981). Whether the effectiveness of this predator
in these environments will be comparable to its performance
under greenhouse conditions remains to be seen.
DEVELOPMENT AND REPRODUCTIVE BIOLOGY
The developmental stage of P. persimilis is similar to
that of the twospotted spider mite, i.e., egg, larva,
protonymph, deutonymph and adult, and has been studied in
detail by many authors (Laing, 1968: Sabelis, 1981; Shaw,
1982). However, the three quiescent periods are absent. The
oval eggs are laid in close proximity to a food source. They
are light orange and translucent when first deposited, but with
age, they darken (Figure 7). The eggs of the predator can be
distinguished from those of the prey's by the color as well as
the shape (Figure 8).
The hexapod larva (Figure 9) apparently does not feed and
remains inactive unless disturbed. The first feeding
stage--the octapod protonymph--emerges from the larval
exoskeleton and begins to search for food almost immediately.
Feeding and searching continues, with intermittent periods of
inactivity presumably due to satiation. The next developmental
stage, the octapod deutonymph feeds throughout most of its
development. The deutonymph later molts, giving rise to the
adult; the adult is about the same size as the mature prey mite
and is red (Figure 10). Feeding begins soon after molting
(Figures 11, 12).
Mating usually occurs within a few hours after molting.
Multiple matings are common even though the sex ratio is
approximately 4 females to 1 male (Laing, 1968). A female that
has mated once can lay eggs throughout her life span; whereas
an unmated female will not reproduce (Amano and Chant 1978a,
1978b); Laing, 1968; Schulten et al., 1978). The system of
reproduction and sex determination is termed "parahaploidy"
(Helle et al.,1978; Hoy, 1982). In this system, both males and
females arise from diploid eggs having 8 chromosomes. However,
males retain only one complement of 4 chromosomes haploidd)
because of the loss or heterochromatization of one half of the
Laing (1968) studied the life history and developed life
tables for P. persimilis and T. urticae. His studies were
conducted 5n growth chamber Tin wh ch the temperature
fluctuated between 18-35C (65-95F). The time spent in each
developmental stage was recorded and various aspects of the
reproductive biology were studied.
Under these experimental conditions, Laing (1968)
determined that P. persimilis would develop from egg to adult
in an average of7.45 days; this is approximately half the time
required for development of the twospotted spider mites under
similar conditions. For a detailed comparison of its
developmental times, at three different temperatures, for
various life stages of T. urticae and P. persimilis, see
Table 1. Furthermore, the mean generation time (T) was 17.32
days (compared to 24 for the prey), during which the predator
population increased 44X (compared to 31X in the prey)
(Table 2). Finally, the maximum rate of increase (rmax) for
the predator was higher than that for the prey (Table 2).
Given these statistics, it is not surprising that P. persimilis
is one of the most effective natural enemies of twospotted
spider mites known. In fact, at times it can be too effective--
i.e., it can often eradicate the prey in the greenhouse (see
Figure 7. Eggs of P. persimilis.
Figure 8. Egg of twospotted spider mite (left) and egg of P.
Figure 9. Larva of P. persimilis.
Figure 10. Adult of P. persimilis.
Figure 11. Adult of P. persimilis feeding on egg of twospotted
Figure 12. Adult of P. persimilis feeding on immature
twospotted spider mite.
Table 1. Developmental times (days) for Tetranychus urticae
and its predator, Phytoseiulus persimilis.*
Temp. Developmental stage**
"C Egg Larva PN DN PO Total
15 14.3 6.7 5.3 6.6 3.5 36.3
20 6.7 2.8 2.3 3.1 1.7 16.6
30 2.8 1.3 1.2 1.4 0.6 7.3
15 8.6 3.0 3.9 4.1 5.6 25.2
20 3.1 1.1 1.4 1.6 1.9 9.1
30 1.7 0.6 0.8 0.8 1.1 5.0
Data obtained on roses in growth chambers (Sabelis, 1981).
Key to symbols: PN = protonymph; DN = deutonymph; and PO =
length of time before an adult female begins to oviposit.
The rate of oviposition does not depend on the age of the
female, but on the number of eggs previously laid. Eggs will
be laid at a rate dependent on conditions until the maximum
number is reached or until the female dies from "old age" at
about 50 days (Sabelis, 1981). The most important conditions
that influence the rate of oviposition are temperature,
humidity, and prey density.
INFLUENCE OF TEMPERATURE AND RELATIVE HUMIDITY
Temperature has been shown to affect prey consumption,
generation time, oviposition, and longevity of P. persimilis
(Pruszynski, 1976; Plotnikov and Sadkowskij, 1972; Sabelis,
1981; Shaw, 1982; Laing, 1968; McClanahan, 1968: Takafuji and
Chant, 1976). The ultimate outcome of the predator-prey
interaction is also influenced by temperature (Force, 1967).
The number of deutonymphs eaten by the most voracious stage
(the young ovipositioning female) generally increases as the
temperature increases. For example, at a relative humidity of
75%, the average consumption of spider mite deutonymphs by a
single female was 8.8 at 170C (62.6"F) compared to 13.5 at 26C
(78.8F) (Pruszynski, 1976). Pruszynski also demonstrated that
consumption of prey increased as the relative humidity
decreased and the temperature increased. The author also cited
Table 2. Reproductive biology of the twospotted spider mite
and its predator, Phytoseiulus persimilis.
Average value at 20.30C (68.5"F)
Parameter spider mite* persimilis**
Preoviposition period (days) 2.1 3.0
Oviposition period (days) 15.7 22.3
Longevity (days) 17.8 29.6
Eggs laid per female 37.9 53.5
Eggs per female per day 2.4 2.4
Sex ratio (M:&) 2.9:1 4.1:1
rmax 0.143 0.219
o 30.93 44.36
T 24.0 17.32
Data from Laing (1969).
Data from Laing (1968).
Key to symbols: Rmax = Intrinsic rate of increase or the
number of individuals produced per female per day. Ro = The
number of daughters that replace an average female in- the
course of one generation; and T = Mean generation time
a Russian study (Plotnikov and Sadkowskij, 1972) in which
spider mite eggs were offered as the prey item. The same trend
seemed to occur, i.e., as temperature increased, so did the
consumption of eggs. Pruszynski stated that P. persimilis is
more sensitive than the prey to temperatures a5ove 30C (86F)
and that the predator would stop feeding at about 35C (95'F).
The number of eggs and the rate at which they are consumed is
also affected by the predator's ability to effectively search
for food (see later section).
The rate at which P. persimilis develops is a function of
temperature and is described by a straight line over the range
of temperatures between 15-30*C (59-86'F) (Sabelis, 1981); as
temperature increases, the time needed to develop decreases
(Table 1). However, developmental times in the literature are
quite variable and are possibly dependent on the strain
Fecundity (number of eggs laid per female) is also
influenced by temperature. The temperature at which the
maximum reproduction (75 eggs) occurs is approximately 26C
(79*F); the optimum range for reproduction is 17-28C (63-82'F)
(McClanahan, 1968; Sabelis, 1981). At constant temperatures
outside this range, females lay fewer eggs. The rate of
oviposition, as stated earlier, does not depend on the age of
the female, but on the number of eggs previously laid until the
maximum number is reached.
The effect of temperature on the overall predator-prey
interaction was studied by Force (1967). He used constant
temperatures of 15*C (59"F), 200C (68F), 25C (770F), and 30C
(86F) and obtained excellent control of twospotted spider
mites at 20C; however, at 30C, P. persimilis was unable to
affect control. At 15C and 25"C, the prey was controlled, but
not as dramatically as at 20*C. One problem with this study
was the artificial condition of constant temperatures.
However, the important points to note are that an optimum
temperature apparently does exist and that extreme 'temperatures
can have detrimental effects on the ability of the predator to
control the pest population. Despite the temperature
limitations, some effort has been made toward developing "heat
tolerant" strains (Voroshilov, 1979).
Developmental time can also be affected by relative
humidity. A slight increase in the predator's developmental
time was observed when the humidity was increased from 40% to
70% (Begljarow, 1967; Ustchekow and Begljarow, 1968; and
Stenseth, 1979). Begljarow (1967) noted that development
almost stopped at humidities of 25% to 35%. Pralavorio and
Almaguel-Rojas (1980) reported that relative humidities below
70% resulted in a significant reduction in the ability of
immature predators to molt from one stage to another.
Humidity also exerted an influence on survival of predator
eggs at temperatures above 21C (70F). At 27C (81F) and 40%
relative humidity, only 7.5% of the eggs tested in one study
hatched compared to 99.7% at the same temperature, but at 80%
relative humidity. At 21C (70*F), there was only a 10%
reduction in hatching when eggs were held at the 40% RH
compared to those held at 80% (Stenseth 1979). Begljarow
(1967) showed that when eggs were held at a relative humidity
of 50%, they appeared to shrivel at all temperatures between
13C (55F) and 37C (99F); while at 60% RH, hatching was
successful at temperatures below 30C (86'F). Sabelis (1981)
suggests that the critical relative humidity is 50% and
therefore has little influence because the relative humidity in
greenhouses rarely falls to levels where predators would be
The searching behavior and activity of P. persimilis can
also vary in response to relative humidity. Mori and Chant
(1966a, 1966b) investigated the influence of relative humidity
on the behavior and activity of this mite and concluded that
relative humidity was an important factor limiting the number
of prey consumed per predator. In these studies, predator
activity and the number of prey consumed per predator increased
as relative humidity decreased (i.e., from 100% to 33%). This
activity response due to humidity combined with recent evidence
(see Sabelis and van der Baan, 1983) that phytoseiid mites,
including P. persimilis, are able to use odors (i.e.,
kairomones) associated with mite infested plants to locate
their prey at a distance, further increases the predator's
chances of finding and consuming twospotted spider mites.
All developmental stages of the twospotted spider mite are
eaten by the adult female P. persimilis. The predator's larval
stage does not feed, but the protonymph and deutonymph will
feed on the egg, larva, and protonymph stages of spider mites
(Takafuji and Chant, 1976). The number of each stage eaten
depends on the density of prey and predator, temperature,
humidity, stage of predator feeding and which prey stages are
available for it to feed upon (Shaw, 1983).
Phytoseiulus persimilis depends almost entirely on animals
as food (Ashihara et al. 1978, Chant 1961, and Dosse 1958).
Ashihara et al. TT97S- reported that this predator fed,
reproduced, and completed development only on mites in the
subfamily Tetranychinae. However, Chant (1961) observed P.
persimilis feeding on young thrips. P. persimilis is also
cannibalistic when no other food (i.e., spider mites) is
available (Dosse, 1958; Laing, 1968). Free-standing water (for
the predator to drink) will, in the absence of food, increase
survival by 23% (Mori and Chant, 1961b: Ashihara, et al.,
1978). Adult females, when fed on honey or a 10% sucrose
solution, can survive at least four times longer compared to
females being fed on water alone (Ashihara et al., 1978).
However, neither sucrose nor free water wouTd promote
reproduction. Ashihara et al. (1978) determined that females
would not reproduce ona diTet of honey, but if they were
removed from the honey diet after 35 days and fed spider mite
eggs they could achieve their normal reproductive potential.
DISPERSAL AND SEARCHING
When compared with five other predatory mites, P.
persimilis was rated as having high dispersal powers, and its
distribution and that of its prey were highly correlated
(McMurtry, 1982). The ability of P. persimilis to disperse and
find new colonies of prey depends on the physical
characteristics of the environment (Takafuji, 1977), prey
distribution and density, predator density, and the duration of
infestation or the amount of the spider mite webbing present.
One important environmental characteristic is the density
of plants within the greenhouse. For example, when infested
plants are dense enough for their leaves to touch, the predator
can disperse readily. When the plants have little physical
continuity, the predator's ability to disperse can be reduced
by about 70% (Takafuji, 1977).
The density of both predator and prey may play a part in
the rate at which predators leave an infested plant in search
of new sources of food. Young female predators increase the
rate at which they depart from a colony as their density
increases and that of the prey decreases (Sabelis, 1981;
Eveleigh and Chant, 1982). When prey density is low relative
to number of predators present, the adult predators begin to
disperse in search of new food sources. On the other hand,
nymphs of P. persimilis have a much lower capacity--and
tendency--to disperse than do the adults and, as a result, they
remain behind and feed on whatever food is left before they
begin to disperse (Takafuji, 1977). This behavioral
characteristic can be a contributing factor to the extinction
of prey. Also, the elimination or extinction of the prey in
the greenhouse is made possible because P. persimilis has a
much greater dispersal potential than its prey (Nachman, 1981).
In cases where little or no spider mite damage can be
tolerated, such as on ornamental plants, this is a desirable
situation. Because some damage can be tolerated in cucumber
and tomato crops, it would be desirable to have a stable
interaction between the predator and prey over an extended
period of time.
The webbing produced by twospotted spider mites aids the
searching predator in finding its prey. When webbing is
contacted, the predator intensifies its search in the immediate
area. The webbing appears to act as an arrestant for
dispersing predators. In one study, females were able to find
prey twice as fast when webbing was present compared to when
webbing was absent (Schmidt, 1976). Schmidt (1976) also
reported that spider mite eggs had a similar effect, but to a
lesser degree. Kairomones (chemical odors discussed earlier)
may be responsible for this nonrandom searching behavior.
METHOD OF RELEASE
The most critical phase in the implementation of any
biological control program is the release phase (French et al.,
1976; Gould, 1970; Markkula and Tiittanen, 1976). Thus proper
timing of predatory mite release is essential to achieve
adequate control of the twospotted spider mite. Many of the
past failures can be attributed to the detection of natural
spider mite infestations too late to utilize biological control
successfully (Stenseth, 1980). In these cases, insufficient
numbers of predatory mites were released to control an
established and rapidly increasing mite population.
Consequently, predators were unable to reduce mite numbers fast
enough to prevent economic injury.
Release methods have been developed in order to increase
the probability of successfully controlling T. urticae with P.
persimilis. One release method, termed "est-in-Firstr,
requires the prey to be released in a uniform pattern before
releasing the predator. At the appropriate time, the predator
is subsequently released. A specific predator-prey ratio can
therefore be established early in the season; the predator also
becomes established throughout the greenhouse before mites
naturally infest the crop (Markkula and Tiittanen, 1976). A
similar technique is the introduction of predator and prey at
the same time. This is accomplished by purchasing a mixture of
both species from a commercial insectary. In the Netherlands,
predators are packed in plastic bottles which contain wheat
bran and spider mites. These bottles are sealed with a screw
cap equipped with a gauze covered hole for ventilation. This
technique, like the "Pest-in-first" method, allows the
establishment of specific predator to prey ratios. Thus,
balanced control can be attained throughout the greenhouse
(Ravensberg et al., 1983).
The rational behind both of the above techniques is to
establish the predator evenly throughout the greenhouse early
in the season before the crop becomes naturally infested. This
is important where a large portion of the spider mite
population enters diapause. Experience has shown that spider
mites leave the sites where they diapause and infest the crop
as soon as environmental conditions become suitable. Because a
large and unpredictable number of mites may enter a crop over a
short period of time, severe damage can occur before the
problem is noted. These techniques will allow growers the
opportunity to establish predators early in the season which
will provide a buffer against the immigrating mite population,
and thus reduce the potential for damage.
In climates such as Florida, the mass influx of mites, as
a result of diapause termination, is seldom seen. Mite
infestations occur throughout the year and begin as small
isolated patches. Therefore, release techniques designed to
release predators when natural infestations are first found are
better suited to these conditions (French et al. 1976).
Sufficient numbers of predators are released to create a
desirable predator-prey ratio, e.g., 1:10 on cucumbers
(Markkula and Tiittanen 1976) or 1:6-1:25 on ornamental foliage
plants (Hamlen and Lindquist 1981). When distinct patches of
prey can be identified, predators should be released on every
plant within the patch. Predators should also be released on
plants around the outer edge of the infestation in order to
establish a barrier that should slow or prevent the spread of
the prey. If prey are found, but no distinct patches can be
identified, P. persimilis should be released on every fifth
plant (Anonymous 1976a). The number of predators to release is
dictated more by economics and their availability than any
other factor. In situations where the mite population has
reached high densities, the cost and logistics for releasing
adequate numbers of predatory mites is usually prohibitive. In
these cases, conventional chemical control should be employed
to reduce the possibility of economic damage.
Many acaricides disrupt the predator-prey interaction even
to the extent that acaricides must be applied during the
remainder of the growing season. However, studies have shown
that this problem can be avoided if either fenbutatin-oxide
(Lindquist et al., 1980) or insecticidal soap (Osborne and
Petitt, unpublTshed data) is used as the acaricide. Both
studies have demonstrated that a single application of either
material used in conjunction with an earlier release of
predators gave better control than when either method was used
Once the predators have been released in the greenhouse,
some additional conditions should be taken into account.
Insecticides, miticides and even certain fungicides can be
detrimental to P. persimilis and should be used judiciously.
Whenever possible, selective chemicals should be used (see
section on Common Problems). As stated earlier, the
performance of the predator is conditioned by temperature and
relative humidity and to consider this aspect could easily lead
to failure of the control program. Sometimes the predator may
simply exterminate (eradicate) the prey from the greenhouse;
thus, careful monitoring of the mites is required in order to
(1) detect such eradication and (2) determine when new
introductions of twospotted spider mite occur so that another
release of the predator can be made at the proper time.
In this section, we will summarize some of the most common
problems encountered during implementation of biological
control for twospotted spider mite in the greenhouses. These
are (1) improper timing of the release of P. persimilis; (2)
impatience on the part of the grower; (3) lack of adequate pest
suppression achieved by the control agent; and (4) cultural or
chemical practices which adversely affect the natural enemy.
The need for releasing predators at the proper time has
already been mentioned, but will be repeated because of its
overall importance. To effectively use biological control, the
grower must initiate the program when the pests are at
relatively low levels. In this regard, we have observed that
many growers opt for biological control when it is too late,
i.e., after a major pest problem has developed and is on the
verge of destroying the crop. Clearly, growers must use some
forethought and plan to initiate a control program when the
pests are first found. This requires careful monitoring of the
greenhouse crop(s). Growers must also realize that, unlike
effective chemical control, biological control does not produce
instant results. This may be true even when massive numbers of
natural enemies are released in the greenhouse. Nonetheless,
releasing predators at the proper time (and in adequate
numbers) will shorten the period before control is attained and
improve considerably the likelihood that biology control will
Impatience on the part of the grower can often lead to
failure of a biological control program. Such impatience
usually results in the resumption of the chemical control
program before there is any need to do so. The basic cause of
this is probably a lack of appropriate experience with
biological control. It must be remembered that mite
populations--regardless of whether predator or pest--require
time to develop and increase. On the other hand, "too much
patience" can also lead to problems. For example, in certain
instances P. persimilis may fail to establish, and long delays
before takTng corrective action may allow the mite population
to build up to very high densities. This problem can be
minimized by closely monitoring the pest population which is
critical to any pest control program. A monitoring program
should be designed to identify potential problems. It should
also be sensitive enough to provide the grower with enough
warning to allow time for remedial actions. This lead time is
important and is determined by grower-defined measure such as
the relative densities of predators and prey and the presence
of other pests. The level at which other control measures
should be initiated must be determined prior to implementing a
biological control program.
As an example, Hassen (1983) developed a program for
cucumbers. Forty cucumber plants in each of three different
greenhouses were chosen at random. These 40 plants were ca. 3,
6, and 20% of the total number of plants in each of the
greenhouses. Each plant was examined weekly for the presence
or absence of any stage of T. urticae or P. persimilis. At the
same time, these plants were observed for other potential
problems. If such a program is to be useful, the grower should
look for areas where damage is about to exceed a critical
damage threshold (Anonymous 1976a). Secondly, the grower
should look for "hot spots" where predators are scarce. The
remedial actions that could be taken would depend on what was
found. Hot spots could be controlled with a selective
pesticide such as soap, fenbutatin oxide, or oil. If the
potential for severe damage exists the grower would have to
consider treating the entire crop.
Another problem is that natural enemies may not control
their hosts at all times or under all conditions. Many
failures can be attributed to improper environmental conditions
within the greenhouse. As we have explained previously, P.
persimilis is adapted to certain temperature regimes, outside
of which their performance is reduced measurably. Certain
cultural practices can also be devastating to a biological
control program. One practice that can cause problems is the
movement of plants in and out of the greenhouse. Obviously,
when the plants one is trying to protect are held in the
greenhouse for only a short period of time, biological control
agents will often have a minor impact on the pest population.
Another common problem is the movement of plants from one
greenhouse to another, or when plants are brought in from
out-of-doors. These plants are seldom inspected for the
presence of pests and, invariably, there will be one plant that
is infested with some kind of pest. Furthermore, these pests
generally enter the greenhouse undetected and without their
effective natural enemies. Subsequently, they multiply
unchecked. There are methods to control a few of the
occasional pest species without the disruption of the
biological control program. However, in many cases pesticides
may have to be used, which may then kill or severely disrupt a
P. persimilis population. These disturbances can be minimized
througF sanitation and careful inspection of all plant
materials before they are brought into the greenhouse.
Plants contaminated with occasional pests may be present
in the greenhouse when the grower, in preparation for a
biological control program, discontinues the use of
broad-spectrum pesticides. These pests are then free to
multiply, unchecked, and usually require chemical control
which, again, jeopardizes the survival of existing biological
control agents. Perhaps the most effective way of eliminating
this problem is to treat the greenhouse with the appropriate
pesticide prior to releasing the predator.
This leads us to the quickest and most common way of
causing a biological control program to fail, that is, the
application of nonselective pesticides. As we have emphasized,
P. persimilis is very susceptible to many chemicals (Table 3).
However, there appears to be some discrepancy as to the
susceptibility of P. persimilis to specific pesticides. Some
of the confusion is probably due to differences between
strains. Efforts are being made toward identifying and/or
developing strains resistant to various pesticides (Schulten et
al., 1976; Schulten, 1980). Resistance or tolerance to
(Tazinon, demeton-S-methyl, malathion, mevinphos,
fenbutatin-oxide, tetradifon, cyhexatin, azinphosmethyl,
carbaryl, methidathion, pirimicarb, pyrazophos, and triforine
has been reported (Woets and van Lenteren, 1983). Because
important differences exist among strains, growers are advised
to consult with their suppliers as to the sensitivity of their
Table 3. Safety of commonly used pesticides to Phytoseiulus
persimilis eggs and adults*.
Chemical application Eggs Adults
Insecticides and Acaricides
Acephate HV H
Aldicarb GRANULES H
Bacillus thuringiensis HV S S
Carbaryl HV H
Cyhexatin HV H
Diazinon HV H
Diazinon DRENCH S
Diazinon ULV S
Dicofol HV H
Dienochlor HV I H
Dimethoate HV H H
Dimethoate Drench H
Endosulfan HV H
Fenbutatin-oxide HV S
Insecticidal Soap HV S H
Kinoprene HV S S
Lindane HV H H
Lindane DRENCH I
Lindane SMOKE S H
Malathion HV H H
Methomyl HV H
Nicotine HV I
Nicotine SMOKE S S
Oxamyl GRANULES S S
Oxamyl HV H
Oxydemeton-methyl HV H
Oxythioquinox HV H H
Oxythioquinox SMOKE H H
Parathion HV H H
Parathion DRENCH H H
Parathion SMOKE H H
Permethrin HV H
Permethrin FOG H
Pirimicarb HV S S
Pyrethrin HV H H
Propargite HV H
Resmethrin HV H H
Rotenone HV H
Tetradifon HV S S
Table 3. Safety of commonly used pesticides to Phytoseiulus
persimilis eggs and adults* (continued).
Chemical application Eggs Adults
Benomyl HV H H
Benomyl DRENCH I I
Captafol HV S S
Captan HV S S
Chlorothalonil HV S S
Dinocap HV S
Iprodione HV S S
Mancozeb HV S
Maneb HV S
Metalaxyl HV H
Thiophanate-Methyl HV H S
Thiram HV I S
Triforine HV S S
Vinclozolin HV S
Zineb HV S
These data were compiled from Hassen and Oomen 1985, Ledieu
1985, and Steiner and Elliott 1983.
HV = High volume and ULV = Ultra low volume.
Key to symbols: S = Safe; I = Intermediate; H = Harmful;
and = Unknown.
OBTAINING NATURAL ENEMIES
Natural enemies for use in the greenhouse can be obtained
from commercial insectaries. A number of such companies sell
biological control agents. The problem for the grower is
finding these companies. There are a few ways in which this
can be accomplished. The first would be to contact the Florida
Cooperative Extension Service. Offices are located within each
county, with the main entomology office located at the
University of Florida in Gainesville. There are a few
publications that list companies that sell biological control
agents (Hickman, 1984; Olkowski and Redmond, 1983). Interested
individuals might also consult recent books dealing with
organic methods of pest control as many of these will list
sources of biological control agents.
Once an individual obtains P. persimilis, it is also
possible to maintain a colony without much difficulty. There
are various advantages to the grower in doing so: (1) it would
provide a constant supply of the predators; (2) they would be
readily available; and (3) it might be less expensive to
propagate the predators than to periodically purchase them from
commercial sources, especially if growers form a cooperative to
maintain such a colony.
The efficient production of P. persimilis depends on the
availability of three isolated areas: (1) an area for growing
plants (usually Henderson bush lima beans) without any pests
present; (2) an area for maintaining pure colonies of the pest;
and (3) an area for raising the predators. In this manner,
noninfested plants can be placed in the pest colony. After a
sufficient infestation is obtained, they can then be transferred
to the final location where the infested plants are exposed to
the predators. Although it is possible to maintain pests and
predators in one place, this practice is not usually conducive
to mass production. If this method is necessary (e.g., because
of space shortage), the movement of mites among areas can be
reduced by placing the plants on platforms which rest over
containers of water. A surfactant or detergent should be added
to the water to reduce the surface tension. In this way, both
predator and prey mites which drop off the plants will fall
into the water and die. A second recommendation is to locate
the plants containing the predator colony "down wind" from the
prey colony to minimize contamination by air currents.
The method of rearing P. persimilis at the Agricultural
Research and Education Center in Apopka is quite simple. First
we sow bean seeds into a commercially prepared soil mix which
is contained within a 15x30x33 cm plastic dish washing tub.
These tubs are kept in a greenhouse until the first true leaves
are fully expanded. At this time they are moved into a room
maintained at 25C and lighted with four, 4-tube, 40 W Cool
White fluorescent light fixtures. Infested leaves from about
10 plants are placed on the beans in one tub. The beans in
this tub are grown until a heavy mite population develops.
Each tub is then placed in close contact to a tub that is
infested with predators. The appropriate time to harvest the
predators depends on when the two populations reach the
desirable ratio. This ratio (predator:prey) could be any where
from 1:0 (no prey) to 1:50 depending on need. Harvesting
predators is the next step. This is accomplished in a number
of ways. We place leaves cut from one tub into paper bags,
fold the top over about four times, and staple the bags shut.
This prevents the accumulation of excess moisture as the
foliage dries, and it allows the predators to feed on the
remaining spider mites. Leaves are then removed from the bag
as needed and placed throughout the crop. After most of the
leaves have been removed, the bag is then placed in close
proximity to the most heavily infested plants. Another method
of harvesting predators is to place leaves in a plastic bucket
which is sealed with a lid. A large hole should be cut into
the lid and covered with nylon organdy to allow ventilation.
After 24 to 48 hours predators can be found running around the
lid and upper portion of the bucket. These individuals can be
dislodged directly onto plants within the greenhouse or they
can be collected for later use with a simple suction device
such as an aspirator or with a soft camel-hair brush. Parr
(1968) has shown that predators and prey can be stored. He
placed 5 predators plus 15 prey in a tube and stored them at
8.3"C for 3 weeks to 4 months with ca. 60% survival regardless
of the period of storage.
Additional aspects of culturing the twospotted spider mite
and its predator can be found in the following: Anonymous
(1975), Gilstrap (1977), Hoy et al., (1982), Kamburov (1966),
McMurtry and Scriven (1965 Scopes (1968), Scriven and
McMurtry (1971), Theaker and Tonks (1977).
We especially thank M. J. Tauber, Cornell University, for
his expert advice and helpful comments during the preparation
of the manuscript. We also thank the following individuals for
critically reviewing the manuscript: W. W. Allen, R. G.
Helgesen, M. A. Hoy, N. W. Hussey, J. E. Laing, R. K.
Lindquist, R. J. McClanahan, J. A. McMurtry, R. F. Mizell, J.
C. van Lenteren and J. F. Price. Preparation of portions of
this Bulletin was financed by a grant from the Elvenia J.
Slosson-University of California Endowment Fund for
Environmental Horticulture. Photographs were taken by
Jack K. Clark.
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This publication was produced at an annual cost of $4982 or
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biological control of one of the major plant pests found in
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