'UNIVERSITY F Dover Research Report DOV 1994-1
'iFLORID July 1994
Institute of Food and Agricultural Scie ces
ManusoD 9l 5Glog5a Contero oV Twospoted
Spindle M5itesz on Strauwyberry ina FB5i(da
', .ston Science
,, University of Florida
M. van de Vrie and J. F. Price
University of Florida
In titute of Food and Agricultural Sciences
Agricultural Experiment Station
J. M. Davidson, Dean
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Dover Research Report DOV-1994-1
Institute of Food and Agri Itural Sciences June 1994
MANUAL OR BIOLOGICAL CONTROL OF TWOSPOTTED SPIDER MITES
ON STRAWBERRY IN FLORIDA
M. van de Vrie and J. F. Price
The twospotted spide mite, Tetranychus urticae Koch, is a major pest on
strawberry crops in lorida. Foliage damage by spider mite feeding often
results in reduced p ant vigor and berry production. Reliance on miticides
has become increasing ly difficult because spider mites develop resistance to
miticides, an import nt miticide has been withdrawn from the market, and
manufacturers are rel ctant to make the large investments necessary to
register new miticide on the relatively minor strawberry crop. Furthermore,
the application of mi icides can conflict with the harvest schedule. Mite
problems intensify du ing January through April when berries often are
harvested twice a wee.
Several disadvantages re associated with the use of miticides. They are
expensive and effective chemical spider mite control is difficult because good
coverage of the under des of the foliage is almost impossible to obtain.
This is especially tru when the older leaves contact with the plastic mulch.
In addition, as plants grow they form an almost closed leaf canopy, thus
making thorough cover e even more difficult.
To some extent, spider ite problems are the result of the application of
broad spectrum insecticides which are detrimental to natural enemies of spider
mites and insect pests. Elimination of natural enemies, which are commonly
present in undisturbed abitats, leads to a rapid development of spider mite
populations. Because o the absence of competition with other plant feeding
organisms and leaf disea es, the conditions for spider mite development are
Research at the Universi y of Florida's Gulf Coast Research and Education
Center, Bradenton and Do er, in cooperation with growers and a commercial
scouting company has dem stated that biological control of the twospotted
spider mite by releasing he predatory mite species, Phvtoseiulus persimilis,
is feasible in commercial strawberry production. This manual outlines the
procedures developed for biological control of the twospotted spider mite. It
includes directions for m nitoring spider mite and predatory mite populations,
handling the predaceous mites, and integrating biological mite control with
standard practices of inse t, mite and disease control. This manual should
assist the field manager o scout and should help to ensure success with
'Visiting Research Scientis and Associate Professor, respectively.
The Twospotted Spider Mite
Both male and femal spider mites develop through the following stages: egg,
larva, protonymph, eutonymph, and adult. All stages can be found in
web-covered colonies on the undersides of strawberry leaves. For untrained
observers, separate n of the various stages is difficult, with the exception
of the eggs. The e gs are spherical, about 0.14 mm in diameter and clear when
newly deposited. T ey turn opaque when incubation proceeds. The adults
usually have a dark spot on both sides of the body, hence their common name.
In Florida, populati ns of the twospotted spider mite reproduce throughout the
season. Non-active diapausing) females are reddish and can be found in dense
colonies. They do n t feed or reproduce.
Dispersal of the two potted spider mite is mainly by infested nursery stock.
Consequently, patchy infestations ("hot spots") are characteristic for this
pest in strawberry fi lds. Mites also can suspend themselves on silken
threads and be disper ed by air currents; dispersal by field workers is
possible. These dispersal systems, however, are of minor importance in
Temperature and host lant quality are important determinants for egg
production as well as for the rate of development. One female may produce as
many as 100 eggs during her life time; one generation may develop in about 7
days at 85'F.
The Predatory Mite
The developmental stag s of the predaceous mites are similar to that of the
twospotted spider mite: egg, larva, protonymph, deutonymph, and adult. Eggs
of Ph. persimilis are a ber and are always deposited in close proximity to the
food source, the spider mite colony. They can be distinguished from the prey
eggs by their amber col r, oval shape, and larger size. Immatures of the
predator are orange, ha e long legs, and are highly mobile.
The more rapid rate of development compared to that of its prey, enhances its
ability to suppress spid r mite populations. The predator may develop from
egg to adult in approxim tely half of the time required for the development of
spider mites under simil r conditions. Therefore, it is not surprising that
this species is a very e fective natural enemy of the twospotted spider mite.
All predator life stages, except the larvae which do not feed at all, feed on
all stages of the twospotted spider mite. They depend entirely on spider
mites as food; they are o ligatory predators and will starve or disperse if
spider mites are unavaila le.
Predatory mites have a hi h dispersal rate and capacity to find prey colonies
in strawberry fields. Th y disperse short distances by crawling from plant to
plant and long distances wind currents. Both systems contribute to a high
searching capacity by the redator.
Monitoring Spider Mite Densities
The comprehensive est services of an adequately trained and experienced field
scout are important to the success of pest management even when biological
control is not a pa t of the system. However, these services are critical
when biological con rol is used. A scout can be of enormous value in
determining the cor ect timing of a predator release, assessing the success of
the release, and gi ing early identification of problems (insects, diseases,
etc.) that may have hearing on the success of biological control and
ultimately the succe s of the crop.
To manage spider mit populations successfully, an appropriate monitoring
system is essential. Weekly monitoring usually will give sufficient
information, but during critical periods, for instance close to decision
making for the release increase the frequency to every 3 to 4 days. A hand
lens, with a magnific tion of 6X to 14X, is indispensable for recognition of
the various stages of mites. Always take a random, 100 leaflet (one of the
three blades of a who e strawberry leaf) sample and keep records of results.
Continue weekly monit ring for 6 to 8 weeks once the predators have been
Counting spider mites nd their eggs on leaves in the field is difficult and
time consuming. A rel tionship exists between the number of leaves infested
with spider mites and he average mite density per leaf, so that a simple
"absence/presence" sys em has been developed. The relationship between the
number of infested lea es and the average mite density per leaflet in a sample
of 100 leaflets is shove in Table 1.
Table 1. Relationship between the percentage infested leaves and the average
mite density n strawberry leaves.
% infe ted leaves density per leaflet
-5 2- 4
-10 5- 9
11 15 10 -13
16 20 13 -15
There are three periods d ring which assessment of spider mite populations is
critical. These are:
1. Before planting. S rawberry plants from nurseries often arrive
infested with spider mites. Careful examination of the plants is
essential to determi e their presence. Each plant shipment should be
inspected as differ t shipments from one nursery may have a different
history; records sho ld be kept of plant sources, cultivars and planting
locations. Fifty 1 0 plants from each shipment should be inspected;
if 10 or more plants re infested, a miticide should be applied as soon
as the plants are established. If fewer plants have spider mites,
frequent moni oring after planting is essential.
2. After plant e tablishment. When overhead sprinkling for crop
establishment as ended, it is important to monitor the spider mite
population den ity. Examination of 100 leaflets, taken at random across
the field, usu lly provides a reliable indication of mite densities.
Since mites co e with the plants, old leaves should be inspected during
the early part of the season. After the plants have grown two to four
new leaves, ta e fully expanded leaflets, avoiding the oldest and the
youngest leafl ts. Pick the leaflet, examine it and determine whether
any stage of spider mites is present. Record as (+) if so, and as (-)
if not. This s mple of infested and non-infested leaflets gives.a
sufficiently ac urate estimate of the average mite density per leaf, and
indicates the a tion to be taken. Watch for "hot spots" because spider
mites are often clumped in their distribution patterns. A sample data
sheet is illust ated at the end of this manual.
3. After predator r leases. After releasing the predatory mites, monitor
their presence a d effectiveness. Three to 6 weeks are needed to assess
their establish nt.
M nitoring Predatory Mite Densities
For monitoring the pre atory mite population the same procedure as for spider
mite density can be use However, as the numbers of predators shortly after
releasing are much lowe than that of the prey, their number should be
recorded per individual leaflet. A sample of 100 leaflets also provides a
reliable indication of heir presence and activity. During the first 2 weeks
after releasing, the pr dators are difficult to find; two to five mites and
some eggs in a 100 leaflet sample may be expected. After this period the
number of predaceous mit s and their eggs may increase greatly, depending on
the prey density.
Quality of the Predators to Be Released
The results in our studi s were obtained by releasing the "Koppert" strain of
Ph. persimilis. Predaceous mites are susceptible to various pesticides.
However, there are discre ancies as to the susceptibility or tolerance to
specific pesticides in di ferent strains of Ph. persimilis. Some of the
confusion might be attrib ted to differences is susceptibility to pesticides
between strains from diff rent origins; resistance or tolerance to various
insecticides and miticide as organophosphorus compounds, cyhexatin, and
others have been reported in different strains. Therefore, growers should
consult with their suppli rs as to the sensitivity of their strain to specific
A practical check on quali y or viability of the predators that can be
performed by the grower is to open the bottle, and keep it upright for a few
minutes. If the predators are in good condition a large number of them should
crawl around the opening o the bottle within a few minutes. These must be
mixed again into the vermi ulite before the bottle contents are dispensed.
Timing the Predator Release
The most critical phase in the implementation of biological control on
strawberry is the release phase. Proper timing of the predator release is
essential to achie e adequate control. Timing of releases is primarily based
on the spider mite density on the cultivar of concern, the plant size, the
possible presence f other pests which have to be controlled, and the costs
for obtaining and leasing the predators versus the anticipated benefits.
1. Spider mite nsity. The spider mite density at which predators should
be released d pends on cultivar susceptibility. 'Selva' and 'Seascape'
are more sensitive to spider mite damage than 'Oso Grande' or 'Pajaro';
'Sweet Charli seems to have an intermediate susceptibility.
Satisfactory biological control on 'Selva' and 'Seascape' was obtained
when the predators were released when 2 5% of the leaflets had spider
mites present. On 'Oso Grande' and 'Pajaro', good biological control
was obtained w en the predators were released when 5 to 10% of the
leaves had spi er mites. From limited experience, good control was
realized when he predators were released also when 5 to 10% of the
leaflets of 'S eet Charlie' were inhabited by spider mites.
If spider mite ensity exceeds these levels, a corrective spray with a
selective mitic de is recommended. For this purpose a low rate of
Vendex (fenbuta in-oxide, hexakis) can be used, because it does little
or no harm to t e predators. Predators can be released within a few
days after appl ing a low rate of Vendex. Also, when the development of
the spider mite population after releasing the predators is faster than
was anticipated, a "knock-down" spray with a low rate of Vendex may be
appropriate. Re ember that predators need time to become established
and have to disp rse and reproduce to sufficient numbers to become
2. Plant size. When the plants have two to four new leaves, the
environment is mo t suitable for the predators; if the number of new
leaves is higher he predators have to cover a larger area for detecting
the prey and it m take longer before adequate control is realized.
3. Presence of other ests. It is essential that no pesticides are used
shortly before or after releasing the predators. Usually a period of 4
6 days is suffic ent to accommodate the predatory mites. For
information on com atibility of pesticides with Ph. persimilis see Table
4. Economics of bioloqgcal control of spider mites. The economics of
biological control f spider mites depend on (1) costs of the predatory
mites and the labor involved for releasing; (2) the costs of chemical
control; and (3) th secondary advantages associated with biological
control. The costs f predatory mite releases and the costs of chemical
control are easily d rived. The secondary advantages of successful
biological control a e: more and larger leaves due to the reduction of
chemical pesticides; the presence of various parasites and predators of
insect pests such as aphids, thrips and worms, and the reduction of
pesticide residues on the fruit.
If the nursery has been sprayed with pesticides harmful to predators just
prior to plant harvest, the release of predators should be delayed until new
leaves are present. The grower should consult the supplier of the nursery
stock on recent applications of pesticides.
Application of fungicides for disease control may directly or indirectly
influence spider nite development. Control of insects by broad spectrum
insecticides also affects spider mite development by reducing their endemic
natural enemies. If possible, chemical control of diseases and insects should
be conducted soon after planting, leaving the plants without harmful residue
during the period of releasing the predators.
Method of Releasing the Predatory Mites
Predatory mites are delivered in plastic bottles containing about 2,000
predators in verm-culite. They should be released as soon as possible after
delivery. They m;y be stored in a cool place for a few hours until the
releases are made 50F is the optimum storage temperature. Before opening
the bottle, gently role and shake the contents to distribute the mites evenly
through the vermiculite. Sometimes the cap of the bottle contains an air
filter; this filter has to be removed and the cap replaced. Walk along the
beds and at every 4th or 5th plant spread the leaves with one hand and shake
about 1/4 teaspoon of the vermiculite and predator mite mixture into the
center of the plant. An average of one predator should be released per plant
possessing two to four new leaves; one bottle is sufficient for providing
2,000 plants with predators at the rate of one predator per plant. Leave the
empty bottle and he cap on the bed among the plants because some predators
may be left in the bottle and the cap. The predators will disperse naturally
to plants infested with spider mites.
Compatibility of Predators with Pesticides
Several fungicide;, miticides and insecticides are compatible with predators.
Captan wettable powder, wettable sulphur and Thiram are non-toxic to the
predators under c mmon use patterns. Javelin, Dipel, etc. (Bacillus
thurinqiensis) an] Vendex are also compatible with predators. Vendex may be
applied at low rates to reduce unexpected high densities of spider mites
before or after releasing the predators.
Benlate benomyll), Topsin (thiophanate-methyl), and captain flowable are
harmful to predac ous mites. Benlate and Topsin cause sterility in the female
predators and havy a long residual effect; captain flowable kills the mobile
stages of the pre ator but has a short residual effect. Lannate (methomyl) is
highly toxic to tie predators and to many other insect parasites and
predators, and has a long residual effect. Organophosphorus compounds, such
as Phosdrin (mevinphos), Malathion, and Diazinon, are toxic to the mobile
predator stages, but have a rather short residual life. Mobile predators in
the webbing on the underside of the leaves may survive to some extent. There
are potentials for using these disruptive insecticides once stability in the
prey : predator com lex has been established, but this needs further
The lowest recommend ed rate should be applied and combinations of pesticides
should be avoided w en possible. Once stability in the spider mite : predator
population has been achieved, application of the "intermediate" insecticides
may be possible when carefully timed and applied. According to our present
knowledge, from Marc onward, the chances for an upsurge of a mite population
under biological control is small, thus opening a period when more toxic
pesticides could be considered. Also, the dense leaf canopy provides a refuge
for some of the predators to survive. More detailed investigations are needed
to clarify this.
Some insecticides are extremely poisonous to predators, for instance, Lannate,
Sevin (carbaryl), Thi dan endosulfann) and Vydate (oxamyl). They also have a
long residual action nd are incompatible with biological control.
The present knowledge n pesticides that may be compatible with biological
control by Ph. persimi is in strawberry crops is summarized in the following
Table 2. Safe (S) and intermediately (I) safe pesticides for use with Ph.
persimilis p datory mites in strawberry crops.
Common name Trade name Effect PHI
Bacillus thuringiensis Javelin, etc. S 0
Fenbutatin-oxide, Hexaki Vendex S 1
Captan 50W Captan S 0
Copper Various coppers S 0
Fosetyl-Al Aliette S 0
Ipridione Rovral S 0
Sulphur Various sulphurs S 0
Thiram Thiram S 3
Avermectin Agri-Mek I 3
Diazinon Diazinon I 5
Malathion Malathion I 3
Mevinphos Phosdrin I 2
PHI = the number of days t at must elapse between application and harvest
1. Monitor spider mite p pulations, starting with the nursery stock.
2. Do not apply Benlate, Topsin, Lannate, Thiodan, or Sevin prior to
releasing the predato s.
3. Check viability of predators before releases.
4. Release preda ors at the rate of one per plant at the appropriate prey
density, and hen the plants have two to four new leaves.
5. Allow the pre ators 4 to 6 days to become established before applying
6. Apply a low ra e of Vendex if necessary to reduce high spider mite
7. Continue monitoring the spider mite : predator complex for 6 to 8 weeks
to obtain infor ation on the predators' effectiveness.
I ortant Points to Consider Each Month
October. Inspe t nursery stock for the presence of spider mites before
planting, check ach shipment, as they may have different histories.
Consult the supp ier of the nursery stock on the pesticide treatment
history. Start eekly monitoring of the spider mite populations as soon
as the plants ar established.
November. Contin e to monitor spider mites, as well as insects, and
release predators when the spider mite density is sufficient and the
plants have two t four new leaves. Apply a low rate of Vendex to
reduce unexpected high densities of spider mites. Control insects in an
early stage of de elopment by applying B. thuringiensis for worm control
or diazinon for a hid control.
December. Continue spider mite monitoring, make decisions for releasing
predaceous mites w en spider mite density has reached the indicated
levels and when th plants have two to four new leaves. Avoid
applications of pe ticides immediately prior to, and soon after
releasing the preda ors. Start monitoring the predators soon after
their release. Con ult you supplier of the predators about specific
insecticide suscept ability or tolerance.
January. Continue monitoring spider mite and predatory mite
development. Releases may be considered on fields where spider mites
approach the indicate d levels. Predators released in December should
have dispersed unifo mly and show the beginning of reducing spider mite
density. If needed, a corrective spray with a low rate of Vendex can be
applied if spider mi e populations become to high.
February March April. Monitoring spider mites and predatory mites
can be reduced to onc in 10 14 days. If needed, diazinon or
malathion may be appl ed for the control of aphids or fruit flies.
We wish to express ur appreciation to A. Williford, E. Jones, Elsberry
Brothers, T. Brock nd G. C. Decou for their cooperation in this research, and
to M. A. Hoy, D. N. aynard, E. E. Albregts and C. K. Chandler for reviewing
early versions of the's manuscript and for their suggestions for improvement.
The Florida Strawber y Growers' Association, Plant City, and Koppert
Biological Control, he Netherlands, provided partial support for this
TWOSPOTTED SPIDER MITES AND PREDATORY MITES
Az B C A B C A B C A B C A B C
1 21 41 61 81
2 22 42 62 82
3 23 43 63 83
4 24 44 64 84
5 25 45 65 85
6 26 46 66 86
7 27 47 67 87
8 28 48 68 88
9 29 49 69 89
10 30 50 70 90
1 31 51 71 91
12 32 52 72 92
13 33 53 73 93
14 34 54 74 94
15 35 55 75 95
16 36 56 76 96
17 37 57 77 97
18 38 58 78 98
19 39 59 79 99
20 40 60 80 100
A = sample leaflet
and their eggs
S% of leaves
P 7 % of leaves
number (1-100); B = presence (+) or absence (0) of
ler mites or their eggs; C = number of predatory mites
Infested with twospotted spider mites or their eggs
inhabited by predatory mites or their eggs
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