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cS AGRICULTURAL RESEARCH & EDUCATION CENTER
'Bradenton AREC Mimeo Report GC-1972-6 August, 1972
MANAGEMENT OF SPIDERMITE POPULATIONS ON FLORIDA STRAWBERRIES
S. L. Poe
Since 1932 the two spotted spidermite, Tetranvchus urticae Kochhas been one
of the most important chronic pests on strawberries grown in Florida. Whereas
other species of mites may occasionally reach large population proportions, they
are of secondary importance.
Strawberry production begins when plants are set in the field during October
or early November. These plants may come from local nurseries, or from nearby
states or from as far away as California. In years past, research workers in Flor-
ida noted large populations of mites on fruiting plants from December through March.
A rapid decline in mite population size led these workers to believe that frequent
heavy summer rains were detrimental to the populations.
Spidermites are not insects and cannot be controlled as such. They are secre-
tive and hide on the under surface of leaves. They feed by inserting needle-like
..outhparts into cells of the leaf. All stages,,^ phe, ,ycle are present on
leaves concurrently egg, larva, nymph 1, nymph 2, and nymph 3, male and female.
.iites in all these stages except the egg feed onthe .foi~lige. Between each stage
of development an immobile state occurs when the--oid skhin-iE' shed and a new one
fcrmad. During this period of molting, toxicants are less effective than when in-
dividuals are active. Application of a miticide '2'CErl'c ide may result in the
death of active mites but not the eggs or molting individuals.
Several possibilities exist for management of mite populations on fruiting
plants. Some of these, such as use of resistant varieties developed through breed-
ing, or biological control agents hold promise and have been studied during the
past two years. Effective nanagcement of the two spotted spidermite on strawberries
is also dependent upon (1) adequate knowledge of its biology, life history, and
population dynamicL relative to the crop and (2) judicious use of miticides.
Routine samples have been taken since 1969 on a regular schedule in field
plots in growers fields and on research stations, from plants of many varieties,
grown under varied conditions and treated with various chemical miticides to care-
fully monitor mite populations.
Figure 1 shows a typical population curve of mites and eggs on plants through-
out an entire year. There is a conspicuous absence of mites in the summer months
(plants in the nursery) and an increase through February (plants in a fruiting
field), then a decline in April and May. The ratio of eggs to mites in an increas-
ing population always favors the eggs or young, in a declining population the con-
verse is true, there are more active mites than eggs. Thus, April and early May
populations show nearly equal ratios with more mites than eggs appearing in late
May. This leads to the absence of a population from June through August. The
egg:mite ratio indicates that outside or external climactic factors are not regu-
lating the population size but that the numbers decline through a loss of oviposi-
tion potential or fecundity. This is brought about through the inadequacy of the
host plant. Subsequent work has indicated the validity of this hypothesis. Vege-
tative plants are not good hosts for spidermite population development whereas
fruiting plants are excellent hosts and support large populations.
Tests of acaricides sprayed throughout the season in 1969, Table 1, show the
results of treatment with two promising compounds co::pared to treatment with
dicofol (Kelthane -). Mite populations on plants where effective miticides were
used remained small, less than 10 per leaf, throughout the season. However, un-
treated or dicofol treated plots had high numbers of mites per leaf.
Table 2 shows population data from 1970 season tests using several materials
recommended for use on strawberries, along with two promising candidate materials.
Both of the new materials, Plictran(R) and Omiite(R), provided control superior to
the other materials but the populations were higher than in 1969 throughout the
entire season for all treatments. Significant in this test is the inadequacy of
ethion and dicofol when compared to naled (Dibrom(R)). The latter material pro-
vided a much greater measure of control throughout the season than did other recom-
Table 3 gives data comparing Omite at two rates with naled and a check. The
difference between rates of Omite were not apparent in this test. Naled provided
inadequate suppression of mite populations.
Further effects of Omite on large populations can be seen in the data of Table
4. Application of Omite at the rates shown was made with pre- and post-treatment
counts. Substantial population reduction was achieved with all treatments after 8
days, however this test was conducted during the latter part of March and extension
of control effects to 8 or more days may be misleading as depicted by the follo--
A separate group of samples was taken 0, 1, 2, 3, and 4 days after sprays
were applied to determine the effect on mite populations. These data are shown
on Fig. 2. The points on the graph were made from additive data of 10 materials
or 40 plots per each point. There was an immediate unexplained increase in the
population of both eggs and mites 1 day after application, then a gradual decide
in numbers to the third day after treatment. Samples on the fourth day after
treatment showed the population to be again increasing. These data indicate
the maximum time an acaricide may be expected to remain effective after treatment.
Based on data of this report and other unpublished information several point;
of summarization and recommendations to the grower are in order.
1. Since mite populations build up quickly in fruiting fields but not in
summer nurseries the plants set into the field in October should be mite-free. A
few mites on these plants in the beginning can lead to large populations and ser-
ious problems by mid-December.
2. Where large populations of mites are present, back-to-back sprays 4 days
apart should be instituted to "wipe out" the populations. lore time is required
for a few mites to reach damaging population levels than for many mites to attain
the same level, therefore maintain the population at a low level at all times.
3. A spray schedule should be based on close observation of the field popu-
lations and remedial action taken at once when needed. A new material, Omite, is
now available, however its use should be governed by common sense and sound judg-
a) Obtain thorough coverage of the entire plant; wet upper and lower leaf
surfaces with the toxicant, If necessary, add a sticker and increase
gallonage per acre.
b) Base spray practices upon need and do not delay application.
c) Keep foliage of plants open when possible through plant spacing and
avoid dense, heavy mats of foliage to facilitate application.
4. Do not rely exclusively on one acaricide for population suppression.
Frequently, use of one material alternately followed by a second or a third will
result in better population management than any one material alone. This will
also avoid maintaining a heavy selection pressure against a single compound.
Alternate sprays of Omite, naled and dicofol should provide adequate control.
5. Do not set strawberries in fields adjacent to garden crops or on lrnc-
cleared of such ideal mite hosts as eggplant since mites readily move from one
crop to another.
6. Use acaricides only according to label recommendations.
Table 1. Effect of acaricides on two spot spidermite on
'Florida 90' strawberries. Treatments applied
weekly for 10 weeks.
Treatment Rate Season avg: no. individuals/leaf
Material ai/100 Ral eggs mites
Morestan 25WP 0.25 11Ob1 9a1
Plictran 50WP 0.37 53a 9a
Kelthane 18% EC 1.0 156c 73b
Untreated 142c 161c
Numbers in each column followed by identical letters do not differ
Effect of acaricides on two spot .piderr.ite on
'Tioga' strawberries. Treatments applied weekly
for 10 weeks.
Material ai/100 gal
Ciite 30 WP
Numbers in each
column followed by identical letters do
Table 3. Effect of acaricides on spidermita population control
on 'Tioga' strawberries. Treatments applied weekly.
Rate Season vrf: no. of individuals/leaf
Tinterial ni/1QO Ral es ____s mites
Numbers in each column followed by identical letters do not differ
Table 4. Effect of Omite on two spot spidermite populations on 'Tioga'
strawberries. Single application.
% change in population
Rate Prespray count eggs mites
Treatment ai/100 gal eggs mites 2 days 8 days 2 days 8 days
Omite 30WP 0.5 860 472 -53 -85 -82 -92
Omite 30WP 1.0 532 216 -49 -83 -50 -92
Omite 30WP 1.5 364 260 -41 -62 -75 -97
Omite 30WP 2.0 696 436 -55 -82 -90 -99
CGaite 30WP 2.5 514 130 -41 -87 -69 -94
Omite 30WP 3.0 568 292 -42 -78 -87 -99
Omite 10% Dust 7.4 790 296 -49 -89 -72 -95
Untreated 393 109 + 1 +25 +45 +92
Fig. 1. Population of mites on strawberries grown
during surrmer and winter.
M J J
A D1 D2 J F
MI M2 A
per leaf after
of two spotted spidermites
2 3 4
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