Group Title: Journal of Economic Entomology, 99 (4). pp. 1291-1298.
Title: Evaluation of predatory mites and acramite for Control of Twospotted Spider Mites in Strawberries in North Central Florida
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Title: Evaluation of predatory mites and acramite for Control of Twospotted Spider Mites in Strawberries in North Central Florida
Series Title: Journal of Economic Entomology, 99 (4). pp. 1291-1298.
Physical Description: Book
Creator: Rhodes, Elena M.
Liburd, Oscar E.
Affiliation: University of Florida -- Entomology and Nematology Department
Publication Date: 2006
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Subject: Prostigmata   ( lcsh )
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HORTICULTURAL ENTOMOLOGY


Evaluation of Predatory Mites and Acramite for Control of Twospotted
Spider Mi:i in Strawberries in North Central Florida

ELENA M. RHODES AND OSCAR E. LIBURD
Department of Entomology and Nematology, University of Florida, Gainesville, FL 32611



J. Econ. Entomol. 99(4): 1291-1298 (2006)
ABSTRACT Greenhouse and field experiments were conducted from 2003 to 2005 to determine the
effectiveness of two predatory mite species, 1: .. I,..' persimilis Athias-Ilenriot and Neoseiulus
caliornicwu (McGregor), and a reduced-risk miticide, Acramite 50 WP (1 .. ), for control of
twospotted spider mite, Tetranychus urticae Koch, in strawberries (Fragaria x nanassa Duchesne). In
greenhouse tests, three treatments consisting of releases of P. persimilis, N. califomicus, and an
untreated control were evaluated. Both species of predatory mites ,.:- .. -'I. reduced twospotted
spider mite numbers below those found in the control during the first 3 wk of evaluation. However,
during week 4, twospotted spider mite numbers on the plants treated with P. persimilis increased and
did not differ significantly from the control. Field studies used releases of P. persimilis and N.
. applications of Acramite, and untreated control plots. Both N. californicus and P. persimilis
significantly reduced populations of twospotted spider mite below numbers recorded in the control
plots. During the 2003 2004 field season P. persimilis took longer than N. californicus to bring the
twospotted spider mite population under control (<10 mites per leaflet). Acramite was effective in
reducing twospotted spider mite populations below 10 mites per leallet during the 2003-2004 :;. i.1
season but not during the 2004 2005 field season, possibly because of a late application. These findings
indicate that N. californicus releases and properly timed Acramite applications are promising options
for twospotted spider mite control in strawberries for growers in north Florida and other areas of the
southeast.

KEY WORDS Acramite, Neoseiulus californicus, ; ., persimilis, twospotted spider mite,
strawberry


The twospotted spider mite, Tetranychus urticae Koch,
is the key arthropod pest affecting strawberries (Fra-
garia x ananassa Duchesne) in Florida (Mossler and
Nesheim 2003). Their feeding causes injury to chlo-
i. 11 containing mesophyll cells within the leaf tis-
sue, which results in a decrease of photosynthetic
capacities of infested leaves (Sauces et al. 1982). Early
season infestations of twospotted spider mites cause
reductions in photosynthesis and transpiration at a
much lower population level than the population level
that causes the same level of injury later in the season
(Sances et al. 1981).
Development from egg to mature adult takes -19 d
(Mitchell 1973), although this time can be as short as
5 d (Ho and ILo 1979). Optimal conditions for devel-
opment are high temperatures (up to 38C) and low
humidity (White and Liburd 2005). Their high fecun-
dity and short life cycle 'i: twospotted spider mites
to quickly become resistant to miticides used to con-
trol them. The use of reduced-risk miticides and pe-
riodic releases of predatory mites may give growers
more options for management of twospotted spider
mites.


Acramite 50 WP (bifenazate) (Crompton, Morgan-
town, WV) is a reduced-risk miticide that has shown
promising results in strawberries (Liburd et al. 2003).
This miticide has alow toxicity toward predatory mites
(White 2004). Acramite can only be applied twice in
a season at 0.85-1.125 kg of product per hectare with
applications at least 21 d apart (Price 2002, Mossler
and Nesheim 2003). In laboratory studies using leaf
disks, White (2004) recorded higher rate of twospot-
ted spider mite mortality using Acramite 50 WP com-
pared with the conventional miticide Vendex (fen-
butatin oxide) (DuPont, Wilmington, DE).
Phytoseiulus persimilis Athias-HIenriot and Neoseiu-
lus californicus (McGregor) are predatory mites be-
longing to the family Phytoseiidae. They have been
shown to effectively control twospotted spider mites
on strawberry and other crops in various parts of the
world (Port and Scopes 1981. Cross 1984, Charles et al.
1985, Charles 1988, Waite 1988, Easterbrook 1992, De-
cou 1994, Cross et al. 1996, Garcia-Mari and Gonzalez-
Zamora 1999, Easterbrook et al. 2001, Barber et al.
2003, Liburd etal. 2003). .. .. .. 1:. ... .1
califo~nicus are also effective in controlling twospot-
ted spider mites on strawberries in California (Oat-


0022-0493/06 1291-1298$04.00/0 0 2006 Entomological Society of America






JOURNAL OF ECONOMIC ENTOMOLOGY


man et al. 1967, 1968, 1976, 1977a,b; Trumble and
Morse 1993). Phytoseiulus persimilis is a type I spe-
cialist that feeds exclusively on Tetranychus mites
(McMurtry and Croft 1997). In contrast, N. califonzi-
cus has traits of both a type II specialist and a type III
generalist (Croft et al. 1998). N. califonnicus prefer
spider mites as food but can subsist on other sources
of food such as thrips and pollen when mite popula-
tions are low, and they also prey upon other predatory
mite species (Gerson et al. 2003).
In Florida, seasonal releases of P. persimilis have
been fairly successful in controlling twospotted spider
mite populations in the southern regions of the state
(Decou 1994). However, releases of P. persimilis have
not been effective in northern Florida, possibly be-
cause it may not be able to survive the winter typical
of this area (White 2004). Preliminary studies by
Liburd et al. (2003) indicate that N. californicus may
be able to effectively control twospotted spider mite
in north Florida strawberries where P. persimilis is
ineffective.
The objective of these experiments was to compare
the efficacy of both species of predatory mites for
control of twospotted spider mites under controlled
greenhouse conditions and under field conditions in
north Florida. Under controlled greenhouse condi-
tions, P. persimilis would be expected to perform as
well as or better than N. californicus because temper-
ature effects are not a factor.
In the field, our hypothesis was that N. californicus
would provide better control of twospotted spider
mites compared with P. persimilis. Furthermore, both
N. califonzicus and Acramite would significantly re-
duce populations of twospotted spider mites in straw-
berries to below levels found in untreated (control)
plots.

Materials and Methods
Colony. A twospotted spider mite colony reared on
strawberries was maintained in the laboratory to en-
sure that only twospotted spider mites predisposed to
strawberries were used in the experiments. The col-
ony consisted of mite-infested strawberry plants that
were kept under a photoperiod of 14:10 (L:D) regime
at a temperature of -270C with 65% RH. Plants were
watered twice weekly.
Predatory mites used in all experiments were ob-
tained from Koppert Biological Systems (Romulus,
MI). Predatory mites were used within 48 h of their
arrival dates.
Greenhouse Experiment. Fifteen mite-free 'Festi-
val' strawberry plants with a total of four trifoliates
were placed into previously constructed mite-free
cages (Fig. 1). Each plant was placed into an individual
cage (60 by 30 by 20 cm). Cages were constructed of
nylon fabric. Velcro was placed on three sides of the
cage to gain easy access to the plants. Each cage was
attached to a pot containing a strawberry plant by
using a pull cord sown into the bottom of the cage.
Cages were used to keep both twospotted spider mites
and predatory mites from dispersing between plants.


Fig. 1. Mite-free cages (60 by 30 by 20 cm) containing
strawberry plants.


Ten twospotted spider mites were released onto each
plant and allowed to multiply for 1 to 2 wk (this varied
depending on when the predatory mite shipment ar-
rived). Before the release of predatory mites, a leaflet
was collected from each plant. The number of
twospotted spider mite motiles and eggs on each leaf-
let was counted, and the mean number per leaflet was
calculated. The average release rate was approxi-
mately a 10:1 ratio (10 twospotted spider mite to one
predatory mite). Also, a representative sample (one
bottle from each species of predatory mite) was ob-
served for at least 15 min to ensure that the shipped
predatory mites were active and in good condition
before use.
Experimental design was a completely randomized
block with three treatments. Each treatment was rep-
licated five times. Treatments included 1) 10 P. per-
similis adults released per infested plant (-100
twospotted spider mites), 2) 10 N. californicus adults
released per infested plant (100 twospotted spider
mites), and 3) untreated (control) plants (100
twospotted spider mites) without predatory mites.
Sampling. Each week the population of predators
and twospotted spider mites was sampled by taking
one leaflet from each plant (five leaflets from each
individual treatment) and counting the numbers of
twospotted spider mites as well as predatory mites
(motiles and eggs). The term motiles refers to all life
stages except eggs. Samples were recorded for 4 wk.
Three trials of this experiment were run: the first trial
was conducted in February/ March 2004, the second in
December 2004/January 2005, and the third in March/
April 2005.
Data Analysis. Twospotted spider mite motile and
egg data were log transformed. Mean twospotted spi-
der mites per leaflet was compared each week across
treatments by using an analysis of variance (ANOVA),
and means were separated using a least significant
difference (LSD) test (SAS Institute 2002).


Vol. 99, no. 4






RHODES AND LIBURD: CONTROL OF TWOSPOTTED SPIDER MITES


Field Experiment. The field experiment was con-
ducted at the University of Florida, Plant Science
Research Unit, Citra, FL. Festival strawberry plants
were planted in mid-October. They were grown as an
annual crop on raised beds covered by black plastic
mulch. A combination of overhead and drip irrigation
was used for the first 3 wk to help establish the trans-
plants. After this establishment period, only drip irri-
gation was used. Strawberries were planted in plots 7.3
by 6.1 m consisting of six rows 0.5 m wide with 0.5-m
row spacing. There were 24 plots arranged in a 4 by 6
grid and spaced 7.3 m apart. The experiment was a
completely randomized block design with six repli-
cates. Four treatments were evaluated and included 1)
two releases of P. persimilis (P), 2) two releases of N.
californicus (N), 3) two applications of the reduced-
risk miticide Acramite 50 WP at the rate of 1.125 kg of
product per hectare (A), and 4) an untreated control
(C). In the 2003-2004 field season, all treatments were
applied during the weeks of 11 December and 11
February. In the 2004 2005 field season, all treatments
were applied on 9 December and again on 10 March.
Predatory mites were applied at a rate of one bottle
(2000 mites) per plot by hand by gently rotating the
bottle over each plant. Acramite was sprayed using a
11-liter (3-gal) backpack sprayer.
Sampling. Sampling was initiated once the plants
had established (-3 wk after planting). Each week, six
leaves per plot (36 leaves per treatment) were col-
lected randomly from the middle and lower strata of
each plant and brought back to the laboratory where
the number of twospotted spider mite motiles and
eggs and predatory mite motiles and eggs on each leaf
were counted under a dissecting microscope. Only P.
persimilis and N. californicus mites were counted.
Data Analysis. The twospotted spider mite motile
and egg data were separated into five periods based on
treatment application dates and time during the sea-
son. This was also important to examine trends
throughout the 4-mo field season. These periods were
1) pretreatment (3 wk before any treatment), 2) ear-
ly-season (posttreatment to week 7), 3) mid-season
(week 8 to when the second application was applied
in 2003-2004 at week 12), 4) early-late season (week
13 to when the second treatment was applied in the
2004-2005 season at week 16), and 5) late season
(weeks 17-19). The late season was split into two
periods because of the difference in timing of the
second application between seasons. For example,
during the 2003-2004 season, the second application
occurred at the beginning of the late season, whereas
in the 2004 2005 field season, the second application
occurred in the middle of the late season. In each
period, data were log transformed and then treatments
were compared using an ANOVA, and means were
separated using an LSD test (SAS Institute 2002).
Yield. Yield data were collected beginning in early
January in both seasons. Strawberries were harvested
weekly by hand. Marketable strawberries from the
four inner rows were weighed on abench scale (Met-
tier Toledo, Columbus, OH). Damaged, rotten, and
severely deformed strawberries were discarded. The


a.

25 -

S20- P
S15 a
,
i 105- a
E
U) 5
SU ab b
0
0 1 2 3 4
Week after predatory mite release


b.
50-^

4 40o

a 30 -

S
g 20 -

CO 10 -


0 1 2 3 4
Week after predatory mite release
Fig. 2. Weekly mean SEM twospotted spider mite
(TSSM) (a) motiles and (b) eggs per leaflet in each treat-
ment in the greenhouse experiment. Week 0 is the initial
sample taken before predatory mites were released. Means
on the same date followed by the same letter are not signif-
icantly different by LSD (P = 0.05) (C, control; P, P. persi-
milis; and N, N. californicus).

two outer rows served as border rows and the straw-
berries from these rows also were discarded.
Data Analysis. The mean yield per treatment for
the whole season was compared across treatments
using an ANOVA (SAS Institute 2002). A LSD test was
used to show treatment differences for the yield data.
This was done for both field seasons.

Results
Greenhouse Experiment. There were no significant
differences in twospotted spider mite motile and egg
populations among treatments when the initial sample
was taken (week 0) (motiles: F = 1.1; df = 2, 24; P =
0.3638; eggs: F = 0.94; df = 2, 24; P = 0.4046) and 1 wk
after predatory mite release (motiles: F = 0.08; df = 2,
24; P = 0.9257; eggs: F = 0.45; df = 2, 24; P = 0.6441)
(Fig. 2). Two weeks after the release of predatory
mites, P. persimilis significantly reduced twospotted
spider mite motile and egg numbers below numbers in
the control (motiles: F = 3.4; df = 2, 24; P = 0.0513;
eggs: F = 4.4; df = 2, 24; P = 0.0230). However,
twospotted spider mite numbers on plants where N.
californicus was released were intermediate, averaging


August 2006






JOURNAL OF ECONOMIC ENTOMOLOGY


1.3 + 1.3 motiles and 9.7 + 8.4 eggs per leaflet (Fig. 2).
Both species of predatory mites significantly reduced
numbers of twospotted spider mite motiles and eggs
below the control by week 3 (motiles: F = 6.2; df = 2,
24; P = 0.0068; eggs: F = 6.0; df = 2, 24; P = 0.0075).
Twospotted spider mite motile and egg numbers on
plants where P. persimilis were released began to in-
crease at week 4 and were not significantly different
from the control. At this time, there were significantly
fewer motiles per leaflet in the N. californicus treat-
ment compared with the other two treatments (mo-
tiles: F = 4.3; df = 2, 24; P = 0.0256). However, the
difference in egg numbers was not significant (eggs:
F = 1.9; df = 2, 24; P = 0.1717).
One week after predatory mite release, there were
an average of 0.1 + 0.1 P. persimilis motiles per leaflet
in the P. persimilis treatment. This increased to 0.5 -
0.2 motiles per leaflet at week 2 and 0.7 0.6 motiles
per leaflet at week 3. The population then decreased
to 0.3 + 0.2 motiles per leaflet at week 4. Only one P.
persimilis egg was recorded in the study at week 4 in
the third trial of the experiment.
In contrast, N. californicus numbers in the N. cali-
fornicus treatment were similar throughout the study.
There were an average of 0.7 0.5, 0.7 0.4, 0.6 0.3,
and 0.6 0.3 motiles per leaflet at weeks 1, 2, 3, and
4, respectively. In total, seven eggs were laid during
the experiment: four at week 1 in the first trial, one at
week 2 in the third trial, and two at week 3 in the
second trial.
Field Experiment. 2003-2004 Field Season. There
were no significant differences in motile and egg num-
bers among treatments in the pretreatment period
(motiles: F = 0.8; df = 3, 15; P = 0.5041; eggs: F = 0.7;
df 3, 15; P = 0.5805) or in the early-season (motiles:
F =0.7; df = 3, 15; P = 0.5511; eggs: F = 1.4; df = 3,
15; P = 0.2755) (Fig. 3). The N. californicus and Ac-
ramite treatments had significantly fewer motiles and
eggs per leaflet than the control plots during the mid-
season (motiles: F = 3.9; df = 3, 15; P = 0.0297; eggs:
F = 5.0; df = 3,15; P= 0.0137) and during the early-late
season (motiles: F = 5.1; df = 3, 15; P = 0.0121; eggs:
F = 4.0; df = 3, 15; P = 0.0280) (Fig. 3). During these
two periods, twospotted spider mite numbers in the P.
persimilis treatment were fairly high but were not
significantly different from twospotted spider mite
numbers in the N. californicus treatment. Also, num-
bers of twospotted spider mite in the P. persimilis
treatment were not significantly different from those
in the control with the exception of the early-late
season in terms of the numbers of motiles (Fig. 3).
There were no significant differences in twospotted
spider mite motile and egg numbers among treatments
in the late season (motiles: F = 1.2; df = 3, 15; P =
0.3461; eggs: F = 0.68; df = 3, 15; P = 0.5777).
Predatory mites were observed from 17 December
until 17 March (mid- and early-late seasons). In the P.
persimilis treatment, the P. persimilis population be-
gan increasing on 7 January reaching a peak of 1.4 +
0.4 motiles and 1.6 + 0.4 eggs per leaflet on 20 January.
The population remained high for several weeks then
declined sharply on 20 February and remained low.


pretrt early mid early-late late
Period


pretrt early mid early-late late
Period
Fig. 3. Mean SEM twospotted spider mite (TSSM) (a)
motiles and (b) eggs per leaflet for five periods of the 2003
2004 field season. Means on the same date followed by the
same letter are not significantly different by LSD (P = 0.05)
(C, control; P, P. persimilis; N, N. californicus; A, Acramite).


No P. persimilis were recorded after 17 March. In the
N. californicus treatment, the N. californicus popula-
tion began to increase at 17 December, reaching a
peak of 1.0 + 0.3 motiles and 0.6 0.1 eggs per leaflet
on 2 February. The population declined sharply at 25
February. After this date, no N. californicus were re-
corded with the exception of a small number noted on
30 March (no twospotted spider mites were recorded
after 25 February in this treatment either). Very small
numbers of motiles of both species were recorded
from the Acramite treatment the week after each
release. However, no eggs were ever recorded. Small
numbers of both species dispersed into the control
plots and were recorded from 2 February until 9
March. The combined population never exceeded
0.4 + 0.3 motiles or 0.2 + 0.2 eggs per leaflet.
2004 -2005 Field Season. During the 2004 2005 field
season, the twospotted spider mite population peaked
much later in the season. There were no significant
differences in twospotted spider mite motile and egg
numbers in the pretreatment period (motiles: F = 0;
df = 3, 15; P = 1; eggs: F = 0; df = 3, 15; P = 1) or in
the early-season (motiles: F = 0.47; df = 3, 15; P =
0.7073; eggs: F = 0.57; df = 3, 15; P = 0.6432) (Fig. 4).


Vol. 99, no. 4


E bb






RHODES AND LIBURD: CONTROL OF TWOSPOTTED SPIDER MITES


pretrt early mid early-late late
Period


pretrt early mid early-late late
Period

Fig. 4. Mean SEM twospotted spider mite (TSSM) (a)
motiles and (b) eggs per leaflet for five periods of the 2004
2005 field season. Means on the same date followed by the
same letter are not significantly different by LSD (P = 0.05)
(C, control; P, P. persimilis; N, N. californicus; and A, Ac-
ramite) .

There were also no significant differences in twospot-
ted spider mite motile or egg numbers in the mid-
season (motiles: F = 1.9; df = 3, 15; P = 0.1699; eggs:
F = 2.4; df = 3, 15; P = 0.1129). However, twospotted
spider mite egg numbers showed a trend of being
higher in the Acramite treatment compared with the
N. californicus treatment during the mid-season (Fig.
4). The control treatment had significantly higher
numbers of twospotted spider mite motiles and eggs
than the N. californicus and P. persimilis treatments in
the early-late season (motiles: F = 11.1; df = 3, 15; P
= 0.0004; eggs: F = 13.0; df = 3, 15; P = 0.0002) and
the late season (motiles: F= 14.6; df = 3,15; P = 0.0001;
eggs: F = 14.6; df = 3, 15; P = 0.0001) (Fig. 4). During
the early-late season, numbers of twospotted spider
mite motiles in the Acramite treatment did not differ
significantly from those found in the control. How-
ever, there were significantly less twospotted spider
ii ..in the Acramite treatment during this period
than in the control. The Acramite treatment had sig-
nificantly higher numbers of twospotted spider mite
motiles and eggs than both predatory mite treatments
in the early late season. In the late season, the Ac-
ramite treatment had significantly more twospotted


Table 1. Mean total marketable strawberry yield from each
treatment for the 2003-2004 and 2004-2005 field season

Mean I SEM total yield (kg) of
Treatment strawberries
2003-2004 2004-2005
Acramite 93.9 1 4.9ab 39.2 1 1.8
N. californicus 98.3 1 3.8a 35.1 1.4
P persimilis 83.1 4 4.8b 37.2 1 0.9
Control 83.1 7 7.6b 38.9 1 1.1

Means within a season followed by the same letter are not signif-
icantly different (LSD test; P = 0.05).

spider mite motiles than both predatory mite treat-
ments and significantly more twospotted spider mite
eggs than the N. californicus treatment (Fig. 4).
Predatory mites were observed from 12 January
until 30 March (mid, early-late, and late seasons).
Observations were sporadic until 23 February. In the
P. persimilis treatment, P. persimilis motiles and eggs
were recorded continuously from 9 March until 30
March. The population never rose above 0.1 + 0.1
motiles or 0.1 + 0.03 eggs per leaflet. In the N. cali-
fornicus treatment, N. californicus motiles and eggs
were recorded continuously from 23 February until 25
March. The population never rose above 0.2 + 0.1
motiles or 0.3 + 0.1 eggs per leaflet. In total, two P.
persimilis motiles and four P. persimilis eggs were
recorded in the Acramite treatment on 25 March, and
one egg was recorded on 16 March. Both species
dispersed into the control plots and were recorded
from 23 February until 30 March. The combined pop-
ulation reached a peak of 0.4 + 0.1 motiles and 0.4 +
0.1 eggs per leaflet on 25 March and then declined
slightly by 30 March.
Yield. In the 2003-2004 season, the mean total mar-
ketable yield from the P. persimilis treatment was not
significantly different from the control (Table 1). The
total marketable yield for the N. californicus treatment
averaged significantly higher than the P. persimilis and
control treatments (F = 3.36; df = 3, 15; P = 0.0376).
The total marketable yield for the Acramite treatment
was not significantly different from the other three
treatments.
In contrast, in the 2004 2005 season, there were no
significant differences in mean total marketable yield
between the four treatments (F = 2.6; df = 3, 15; P =
0.0907) (Table 1). Mean total marketable yield per
treatment was much lower than the previous season.
Marketable yields averaged between 35 and 40 kg per
treatment compared with 85-90 kg per treatment in
the 2003-2004 field season.

Discussion
Greenhouse Experiment. Our results indicate that
both species of predatory mites significantly reduced
twospotted spider mite numbers below those found in
the control. N. californicus suppressed twospotted spi-
der mite populations for a longer time period and its
population remained relatively constant throughout
the experiment. However, these predators initially


August 2006






JOURNAL OF ECONOMIC ENTOMOLOGY


took longer (1 wk) than P. persimilis to reduce
twospotted spider mite numbers below the control.
This was not surprising because P. persimiis is known
to quickly knock down twospotted spider mite pop-
ulations under controlled conditions (Gilstrap and
Friese 1985). In our studies, P. .. J: seems to be
unable to sustain suppression of twospotted spider
mite beyond 3 wk in the greenhouse. The twospotted
spider mite population increased at week 4, whereas
the P. persimiils population declined. The reason for
this may be related to its biology. As a type I specialist,
P. persimilis cannot survive in areas where its food
supplies are low. Therefore, once the twospotted spi-
der mite population has been greatly reduced, P. per-
similis must disperse to a new location to :;i..1 food
(McMurtry and Croft 1997). Both species of predatory
mites laid very few eggs. We do not know why this
occurred, but we speculate that there was not enough
twospotted spider mite prey for either species to pro-
duce large numbers of eggs.
Fi..I b ,..li ;,,. i, In both field seasons, there were
fewer twospotted spider mites in the N. californicus
treated plots compared with the P. persinilis-treated
plots, although the difference in the 2004 2005 season
was not significant. This suggests that N califoirnicus
may be better at reducing or regulating field popula-
tions of twospotted spider mite than P. persimilis. In
this study, we do not think that environmental con-
ditions contributed to the better performance of N.
californicus because similar results were obtained in
the greenhouse under controlled conditions.
Acramite was highly effective in the 2003-2004 sea-
son, but did not adequately control twospotted spider
mite populations in the 2004 2005 season. This was
: ..... ,1 a result of timing. In the 2003 2004 season,
the first spray knocked down the twospotted spider
mite population and the second spray in early Feb-
ruary kept the numbers low. During the 2004-2005
season, in contrast, there were no detectable twospot-
ted spider mite populations when Acramite was first
sprayed. By the time twospotted spider mite popula-
tions began to increase, there was little residual ac-
tivity i in the Acramite plots. We delayed the sec-
ond application of all treatments until March during
the 2004 2005 field season so that twospotted spider
mite populations could have a chance to increase.
Because the Acramite plots were essentially controls
(1. ..- of delayed application), the twospotted spi-
der mite population in these plots exploded and was so
high that the second application of Acramite could not
...i; :,,1 reduce twospotted spider mite levels.
This illustrates the importance of properly timing Ac-
ramite applications for growers. Using Acramite in
combination with other management tactics (preda-
tory mites) to control twospotted spider mites might
avoid such problems, although further research would
be needed to demonstrate this possibility.
There were several differences in the twospotted
spider mite populations between the two field seasons.
In the 2003-2004 season, twospotted ;.1 .. 1 .
present in the plots from the start of the experiment,
they began to increase when the temperature began


to increase, and then declined by the last few weeks
of the season. In contrast, no detectable numbers of
twospotted spider mites were found in the 2004-2005
season until mid-January, and the numbers of twospot-
ted spider mite did not increase greatly until late
February. Overall, both twospotted spider mite and
predatory mite numbers were much lower in the
2004-2005 season. White and Liburd ,-.. ) found
that twospotted spider mites prefer hot, dry condi-
tions, and the late summrner and fall 2004 were incred-
.1.i wet because of hurricanes Francis and Jeanne.
This may have knocked back the surrounding popu-
lation oftwospotted spider i. ... ... ......
bers to disperse into the strawberries when they were
planted.
Another difference in the c. ::' .1 spider mite
populations between seasons was that the presence of
the green form of twospotted spider mite was much
more prevalent than the red form in 2003-2004. This
trend reversed itself in 2004 2005. The red and green
forms were considered separate species until Hino-
moto et al. (2001) showed that reproductive incom-
patibility between the two forms is incomplete. Using
cytochrome oxidase subunit I sequences, Hinomoto et
al. (2001) demonstrated that the two forms belong to
the same species from a phylogenetic standpoint. We
do not know why the green form predominated in
2003 2004 and the red predominated in 2004 2005.
This would be an interesting topic to research further.
Predatory mite population trends also differed
greatly between seasons. In 2003-2004, both species
were abundant when twospotted spider mite popula-
tions were high, declining when prey populations di-
minished. In 2004 2005, predatory mites also were
present when twospotted spider mite populations
were high (compared with the rest of the season).
However, overall predatory mite populations were
much lower than those in the previous season. This
was probably because of the overall much lower
twospotted spider mite population in the 2004 2005
season.
Yield. During the 2003 2004 field season, the N.
californicus treatment had a significantly higher mean
total marketable yield than both the control and P.
persimilis treatments. However, during the 2004 2005
field season, there were no significant differences
among treatments. The difference in twospotted spi-
der mite population trends between the two seasons
may i.. 1 .11 explain why there were significant dif-
ferences between treatments in 2003-2004 but not in
2004-2005. Sances et al. (1981) found that higher
numbers of twospotted spider mites are needed to
cause a similar level of damage when infestation oc-
curs later in the season. That the twospotted spider
mite population in 2004 2005 was so low and twospot-
ted spider mites occurred so late in the season sug-
gested that the numbers of twospotted spider mites
were not high enough to affect the .p. ...;: of straw-
berries produced during the 2004 2005 field season.
Other factors that may have contributed to the lack of
significant differences in the 2004-2005 field season
were a greater amount of fungal problems, including


Vol. 99, no. 4






RHODES AND LIBURD: CONTROL OF TWOSPOTTED SPIDER MITES


Anthracnose fruit rot, Colletolrichum acutatum Sim-
monds, and gray, mold ., ., .... Pers; bird dam-
age; and weed infestation in 2004 2005. These factors
also may have lead to the overall lower marketable
yields in 2004 2005 compared with 2003 2004.
Overall, releases of N. californicus seem to be a
viable tactic for north Florida strawberry growers.
Acramite is very effective ,; :!t'l', '. i -timed and
applied properly. Application of Acramite followed by
the release of an effective predatory mite such as N.
be an effective management tactic for
strawberry growers in north Florida. Further research
on combination treatments is needed before definite
conclusions can be drawn.


Acknowledgments
We thank Scott Taylor and the staff at the Citra Plant
Science Research and Education Unit located in Citra, FL,
for assistance in managing our research plots. We also thank
Alejando Arevalo, Marinela Capana, and Raymond Littell for
help with the statistical analysis. Finally, we thank all the staff
and students of the University of Florida Small Fruit and
Vegetable Integrated Pest Management Laboratory in
Gainesville Florida, .I1: Crystal Kelts, for assistance in
the fieldwork. We thank Robert Meagher and Donald Dick-
son for editing the earlier drafts of this manuscript. Funding
for this project was provided by an EPA Region four Straw-
berry IPM Grant 736404813.


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Received 22 September 2005; accepted 2 March 2006.




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