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Experimental and Applied Acarology ISSN 0168-8162 Exp Appl AcarolDOI 10.1007/s10493-014-9853-4

Comparison of conventional and integratedprograms for control of Tetranychusurticae (Acari: Tetranychidae)

Larissa Akemi Iwassaki, Mário Eidi Sato,Fagoni Fayer Calegario, Marcelo Poletti& Aline de Holanda Nunes Maia

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Comparison of conventional and integrated programsfor control of Tetranychus urticae (Acari: Tetranychidae)

Larissa Akemi Iwassaki • Mario Eidi Sato •

Fagoni Fayer Calegario • Marcelo Poletti •

Aline de Holanda Nunes Maia

Received: 24 March 2014 / Accepted: 18 September 2014� Springer International Publishing Switzerland 2014

Abstract The twospotted spider mite (TSSM), Tetranychus urticae Koch, is one of the

main pests on strawberry crops in Brazil. TSSM can be difficult to control due to acaricide

resistance. The objective of this work was to compare the effect of conventional and

integrated strawberry production (ISP) systems on mite abundance and acaricide resis-

tance. The control of TSSM in ISP was based on the release of Neoseiulus californicus

(McGregor) or application of a selective acaricide (propargite), when TSSM monitoring

indicated the timing for the release of predaceous mites (1–3 mites per leaflet on 30 %

leaflets) or chemical intervention ([10 mites per leaflet). Only acaricides (abamectin,

fenpyroximate) were applied in the conventional system. Integrated control of TSSM were

sufficient to maintain a significantly lower pest infestation level, resulting in a sixfold

reduction in the frequency of acaricide applications, and consequently, a lower selection

pressure for acaricide resistance. Strategies for the management of TSSM in strawberry

fields are described and discussed.

Keywords Twospotted spider mite � Strawberry � Biological control � Neoseiulus

californicus � Chemical control � Pesticide resistance

L. A. Iwassaki (&) � M. E. SatoInstituto Biologico, APTA, Rodovia Heitor Penteado km 3.5, Caixa Postal 70, Campinas,SP CEP 13001-970, Brazile-mail: iwassaki.akemi@gmail.com

F. F. Calegario � A. H. N. MaiaEmpresa Brasileira de Pesquisa Agropecuaria, Embrapa Meio Ambiente, Caixa Postal 69, Jaguariuna,SP CEP 13820-000, Brazil

M. PolettiPROMIP - Comercio, Pesquisa e Desenvolvimento de Agentes Biologicos Ltda. ME,Caixa Postal 111, Engenheiro Coelho, SP CEP 13165-000, Brazil

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Exp Appl AcarolDOI 10.1007/s10493-014-9853-4

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Introduction

The twospotted spider mite (TSSM), Tetranychus urticae Koch (Acari: Tetranychidae),

is a worldwide pest for many plant species, including several agricultural crops (Zhang

2003). This tetranychid mite causes significant damage to various crops in Brazil, such

as apple, peach, papaya, kidney bean, cotton and ornamental plants (de Moraes and

Flechtmann 2008). On strawberries, the spider mite feeds on the undersurface of the

leaves, causing curling and discoloration. Their rapid developmental rate and high

reproductive potential enable them to reach damaging infestation levels very rapidly

under good growing conditions (Nyoike and Liburd 2013). In Brazil, the TSSM has

been controlled almost exclusively using acaricides. However, chemical control of the

pest is becoming increasingly difficult in the field due to the development of resistance

to various acaricides (Sato et al. 2005, 2009; Nicastro et al. 2010, 2013; Tirello et al.

2012). Another problem associated with the indiscriminate use of pesticides is the

resurgence of pests due to the suppression of natural enemies (Van de Vrie et al. 1972;

Dutcher 2007).

The Brazilian production of strawberry comes mainly from the states of Minas Gerais,

Rio Grande do Sul and Sao Paulo, which account for almost 80 % of the national pro-

duction (IBGE 2006). In 2006, a project entitled the Integrated Production of Strawberry

was initiated in the State of Sao Paulo (SP; Calegario 2012; Embrapa Meio Ambiente

2013) with the aim of preserving and/or enhancing the mortality factors of the pests, using

several resources (including biological and chemical control), in an integrated manner, and

based on technical, economic, ecological, and social parameters (Tomczyk et al. 1991;

Gerson et al. 2003; Bacci et al. 2007).

One of the strategies adopted by the ISP for the control of spider mites in Brazil is the

release of predaceous mites of the species Neoseiulus californicus (McGregor) in the field.

Neoseiulus californicus is a phytoseiid mite which can provide effective biological control

of tetranychid mites on strawberry and several other cultivated plant species (Strong and

Croft 1995; McMurtry and Croft 1997; Greco et al. 2005; Sato et al. 2007; Gomez-Moya

and Ferragut 2009; Saber 2012).

This study aims to compare the strategies used for the control of T. urticae in two

strawberry production systems: an area conducted under the technical standards of ISP

(Brasil, 2008) and a commercial strawberry field using a conventional production system,

in SP.

Materials and methods

Mite strain

Neoseiulus californicus (Neomip Max�) were provided by Promip Trade, Research and

Development of Biological Agents (SP, Brazil).

Chemicals

The acaricides abamectin (Vertimec� 1.8 % EC, Syngenta Crop Protection), propargite

(Omite� 720 EC, Chemtura Chemical Industries of Brazil) and fenpyroximate (Ortus� 5 %

SC, Arysta Lifescience), were commercially available in SP and were purchased in Atibaia

County, SP.

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Field experiment

Two strawberry production fields located in Atibaia County, SP, were compared with

respect to the strategies for the control of spider mites: one field adopted the ISP system

[Sao Joao farm; Demonstration Unit for ISP System: 23�040 S, 46�400 W], whereas the

other was a commercial strawberry field using a conventional production system (Tanaka

farm: 23�050 S, 46�400 W). These fields were chosen due to the short distance between

them (B8 km), in order to reduce possible climatic differences which could affect man-

agement strategies.

Seedlings of ‘Oso Grande’ strawberry cultivar were used in both areas as follows:

• Sao Joao farm-IPA (integrated production area): 4,214 plants (0.35 9 0.35 cm),

transplanted on 9 plots (1 9 57 m), in the fourth week of March 2008.

• Tanaka farm-CPA (conventional production area): 4,000 plants (0.30 9 0.30 cm),

transplanted on 8 plots (1 9 45 m), in the third week of March 2008.

In both areas, strawberries were planted in three rows on raised beds, mulched with

black polyethylene plastic, in open-field cultivation. For the statistical analysis, the

strawberry plots were considered blocks in each production system, totaling 8–9 replicates

per treatment.

For IPA, the strategies for the control of TSSM were based on the results of the weekly

monitoring of pest abundance. The releases of predaceous mites (N. californicus) were

made when the infestation level of TSSM reached 1–3 mites per leaflet, on at least 30 % of

the leaflets. Neoseiulus californicus were released at a rate of approximately 3.0 mites per

m2, when the spider mite infestation reached the action level (1–3 mites/leaflet; 30 % of

leaflets). After the first release of N. californicus, the subsequent release was made only

when the predaceous mites were not sufficient to reduce the spider mite infestations in

3 weeks.

These N. californicus releasing rates were chosen based on the reports of Zalom (2002)

and Fraulo and Liburd (2007). Zalom (2002) considered an economic threshold level

(ETL) of 5–10 mites per mid-tier leaflet for the first 4 months following the transplant of

strawberry. Once harvest begins, strawberry plants become more tolerant of mite feeding

and treatment thresholds increase to an average of 15–20 mites per mid-tier leaflet (Zalom

2002). Fraulo and Liburd (2007) mentioned an ETL of 23–27 active mites per leaflet or

70–80 mites per leaf.

According to Greco et al. (2011), at proportions of leaflets infested with TSSM higher

than 0.30, the pest can reach the ETL [50 mites/leaflet (Wyman et al. 1979)] in less than

7 days. Previous studies in strawberry fields (Oso Grande cultivar) in the SP indicated that

when N. californicus was released at TSSM infestation levels of 1–5 mites per leaflet, the

spider mite abundance was maintained below ten mites per leaflet during all the strawberry

season (Berton et al. 2007).

In order to minimize the effect of wind on the dispersion of phytoseiid mites and favor

the establishment of the predators in the strawberry field, a fence with a plastic sheet

(1 m high) was installed on July 14, around the experimental area.

For IPA, when the TSSM infestations reached levels above ten mites per leaflet for

more than 1 week, the decision of chemical intervention was taken to prevent possible

serious damages caused by the spider mites (Wyman et al. 1979; Chiavegato and Mischan

1981). In this case, when the acaricide applications were made on the strawberry field (July

28 and August 18), TSSM abundances were of 35–59 mites per leaflet, with more than

80 % of infested leaflets. These infestation levels (C35 mites/leaflet) were similar to the

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ETL (40 mites/leaflet) adopted by Sato et al. (2007) for the acaricide application against

TSSM, in strawberry fields where N. californicus had been released.

The selective acaricide propargite (Sato et al. 2002) was applied at the recommended

concentration [216 (AI) mg/l] for the control of TSSM in strawberry fields in Brazil

(AGROFIT 2014). In the conventional production area (CPA), the control of TSSM was

performed by the farm owner, using only chemical acaricides, without considering any pest

abundance monitoring. The acaricide applications were made almost every week,

according to the owner, from June to September 2008.

Assessments of abundances of phytophagous and predaceous mites were made weekly

(for 32 weeks) in both areas, from the seedling transplanting period to the end of the

strawberry harvest (April–November). The leaflet sampling method was similar to that

proposed by Greco et al. (2011), collecting one leaflet from a mature leaf in each row at

10-m intervals (ten footsteps, each of approximately 1 m).

On each sampling date, 80 (n = 10 leaflets/plot 9 8 plots for CPA) or 81 (n = 9

leaflets/plot 9 9 plots for IPA) strawberry leaflets were collected and placed in paper bags,

in Styrofoam boxes with ice, and taken to the laboratory in Campinas City. Mites (T.

urticae and N. californicus) on each leaflet were counted under a dissecting microscope

within 24 h. The phytoseiid mites collected from leaves were mounted in Hoyer’s medium

on microscope slides for subsequent identification. Samples of TSSM (males) were also

mounted on slides for identification.

Data on the meteorological (temperature and rainfall) conditions in Atibaia County

along the field experiment period were provided by Integrated Agrometeorological

Information Center (CIIAGRO) of Instituto Agronomico de Campinas (IAC; CIIAGRO

2008).

Acaricide resistance monitoring in Tetranychus urticae

Samples of TSSM mites were taken from both strawberry fields at the beginning of the

spider mite infestation (March for IPA; May for CPA) in each area and at the peak of the

strawberry harvesting period (August for IPA; September for CPA) for the monitoring of

acaricide resistance in TSSM. The study was carried out for the acaricides abamectin,

propargite and fenpyroximate. After collection, the mites were transferred to jack bean

plants (Canavalia ensiformis L.) and maintained for a period of 17–25 days under labo-

ratory conditions (at 25 ± 2 �C) before performing the toxicity tests.

These tests were based on the method described by Knight et al. (1990). Sixty female

TSSM adults were placed on a bean leaf disc (4 cm diameter) on water-soaked cotton in a

Petri dish (9 cm diameter). A prepared suspension of acaricide (2 ml) was sprayed onto the

leaf disc mites using a Potter spray tower (Burkard Manufacturing, Uxbridge, UK) at

68.9 kPa. Preliminary tests indicated that 1.6 ± 0.1 mg/cm2 of distilled water was sprayed

on the leaf disc with this volume and pressure.

Only one concentration [in mg/l of AI (active ingredient)] was used for each acaricide,

which was equivalent to the discriminating concentration [abamectin (4.79); fenpyroxi-

mate (46.3); propargite (40.3)] of each acaricide (Sato et al. 2004, 2005). Only distilled

water was applied for the control. Thereafter, the mites on the leaf disc were kept at

25 ± 1 �C with a 14 h photoperiod for 48 h (for abamectin and fenpyroximate) or 72 h

(for propargite) after treatment.

Individual mite survival was determined by touching each mite with a fine brush. Mites

that were unable to walk at least a distance equivalent to their body length were considered

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dead. Tests in which control mortality was equal to or higher than 10 % were not con-

sidered in this study. Each experiment was replicated four times.

Statistical analyses

For the field experiment, repeated measures ANOVA was used to analyze and compare the

numbers of mites (T. urticae and N. californicus) between the treatments (IPA and CPA)

and evaluation dates (periods). For comparison of treatments the post hoc Tukey’s multi-

comparison test was applied. Profile Analysis was used for comparisons among evaluation

dates (periods).

The relationship between the time series of the number of spider mites and predators

was analyzed using cross-correlation function analysis, estimating the coefficient of cor-

relation (r) and lag between the temporal series. If the lag is zero, it means that the highest

correlation (between mite species) was observed for the same evaluation dates. For lag 0,

the data of abundance streams in real-time. If the lag is 1, for instance [for the species T.

urticae (A) and N. californicus (B)], the fluctuation of mites of the species B is correlated

with the fluctuation of mites of the species A in lag 1, or 1 week (interval between

evaluations) after the evaluation period for the species A. Lag 3 correspond to a 3-week

interval, and lag 5 to a 5-week interval. Data were square-root-transformed for homoge-

neity of variance (homoscedasticity) and normal distribution.

For the monitoring of acaricide resistance in TSSM, the percentage of acaricide-resis-

tant mites was estimated using weighted least squares (Stokes et al. 2000), considering that

the number of alive individuals in each treatment was a random variable with binomial

distribution. Comparisons between pairs of treatments were made via v2 test (Wald 1943),

using the PROC CATMOD of SAS System (SAS Institute 2008).

Results

Significant differences were observed for the two treatments (IPA and CPA)

[F1,151 = 331.79; P \ 0.001], evaluation dates (periods) [F31,4681 = 206.53; P \ 0.001]

and interactions (treatments 9 periods) [F31,4681 = 48.49; P \ 0.001], indicating that the

abundance of TSSM were different between treatments (conventional vs. integrated pro-

duction systems) during this period. The abundance of TSSM in IPA was lower than in

CPA from week 22 to 32 after the strawberry transplanting. The highest abundance in CPA

was 140.9 mites (active stages) per leaflet in week 23, whereas the highest abundance for

IPA was in week 21, with 59 mites per leaflet (Fig. 1). The average abundance of TSSM

(33.0 mites/leaflet) in CPA was 2.29 higher than for IPA.

The TSSM infestation in the strawberry integrated production area (IPA) was observed

from the first sampling date, on 1 April. Four releases of the N. californicus were neces-

sary, based on the infestation levels of the spider mite. For IPA, the predaceous mites were

released at rates of 29–3.5 mites per m2, on 11 April (week 3), 21 May (week 8), 29 July

(week 18) and 19 August (week 21), when the TSSM infestations were above the action

level (1–3 mites/leaflet; 30 % of leaflets). The last two releases (July and August) were

carried out 24 h after spraying of the selective acaricide (propargite) on the strawberry

field.

The two acaricide applications (weeks 18 and 21) were performed when TSSM

abundance reached levels above ten mites per leaflet for more than 1 week. The appli-

cations of the acaricide propargite significantly reduced (P \ 0.05) the abundance of

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TSSM (weeks 19 and 22) without causing any significant negative effect on the abun-

dance of N. californicus.

Additional acaricide treatments or N. californicus releases were not necessary due to the

gradual reduction of TSSM infestation in the field after the end of August. In the second

0

20

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Apr Mai Jun Jul Aug Sep Oct Nov

Rainfall (mm)

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Tetranychus ur�cae

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Neoseiulus californicus

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Fig. 1 Number of active forms of Tetranychus urticae and Neoseiulus californicus per leaflet on strawberryplants, in integrated production (IPA) and conventional production areas (CPA); rainfall and temperatureconditions in Atibaia County, state of Sao Paulo (SP), in 2008

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half of October, the infestation of spider mites reduced to zero on the strawberry plants of

IPA (Fig. 1).

For CPA, the beginning of the TSSM infestation was registered in the second half of

May (week 8); however, the grower only initiated pest control 1 month later, when he

started applying acaricides. According to him, the acaricide applications were done on an

almost weekly basis between the months of June and September. Altogether, 12 sprays

were used, ten of abamectin and two of fenpyroximate. Even with those applications, the

TSSM infestation increased in the field, reaching 100 % of infested leaflets and up to 140.9

mites per leaflet in September (week 23; Fig. 1).

Although predaceous mites were not released in CPA, N. californicus were found on

strawberry plants from week 24 (September 2008) of the experiment (Fig. 1). In both

strawberry fields, significant differences in N. californicus abundances were also observed

for treatments (IPA vs. CPA) [F1,151 = 80.87; P \ 0.001], periods [F31,4681 = 37.06;

P \ 0.001] and their interaction [F31,4681 = 35.27; P \ 0.001]. Neoseiulus californicus

mites were observed in IPA from April to November with higher abundances than in CPA

in week 3 and weeks 15–28 after transplanting. The highest abundance of N. californicus

was observed on 22 September (week 26), with 4.1 mites per leaflet in IPA (Fig. 1).

Significant correlations were observed between TSSM and N. californicus on straw-

berry plants for IPA (r = 0.792, df = 30, P \ 0.05; in lag 3) and for CPA (r = 0.818,

df = 30, P \ 0.05; in lag 5). The correlation between TSSM and N. californicus in lag 3

indicates that the abundances of N. californicus followed variations in TSSM infestations

in the strawberry field (IPA), but with a lag of 3 weeks.

Acaricide resistance monitoring

In the CPA, the percentage of abamectin-resistant TSSM increased significantly

(P B 0.0001) from May to September, varying from 7.1 to 69.0 %. For fenpyroximate-

resistant mites, the increase was from 24.2 to 68.3 % in the same period. For propargite,

which was not used by the grower during the evaluated period, a decrease in the percentage

of resistant mites was observed (Table 1). In the case of IPA, no significant change was

Table 1 Mean (±SE; n = 4) percentage of resistant mites for the acaricides abamectin, fenpyroximate andpropargite, in populations of Tetranychus urticae collected at the beginning and at the peak of infestation instrawberry conventional production (CPA) and integrated production areas (IPA) and P values associatedwith the v2 Wald test for comparisons between periods of tests with each acaricide, and for each productionsystem, in Atibaia County, SP, in 2008

Productionsystem (Area)

Acaricide Resistant mites (%) v2 (df = 1) P

May September

CPA Abamectin 7.1 ± 0.02 69.0 ± 0.03 388.33 \0.0001

Fenpyroximate 24.2 ± 0.03 68.3 ± 0.03 131.37 \0.0001

Propargite 51.2 ± 0.03 14.6 ± 0.02 86.18 \0.0001

March August

IPA Abamectin 21.7 ± 0.03 16.2 ± 0.02 2.30 0.13

Fenpyroximate 17.9 ± 0.02 20.0 ± 0.03 0.34 0.56

Propargite 10.0 ± 0.02 10.8 ± 0.02 0.09 0.77

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observed (P C 0.13) in the percentage of acaricide-resistant mites during the strawberry

growing season (Table 1).

Discussion

Considering the abundance of TSSM in CPA, high infestation levels (80–140 mites/leaflet)

were observed from 25 August (week 22) to 6 October (week 28), causing severe damage

to the strawberry plants during this period. According to Chiavegato and Mischan (1981),

high infestation levels of TSSM can significantly affect strawberry production (up to 80 %

loss), leading to the death of plants if high infestation levels are maintained for long

periods in the field.

For IPA, the maximum infestation level of TSSM was 59 mites per leaflet, with an

average abundance of 14.9 mites per leaflet, which did not cause any visual damage to the

strawberry plants. The presence of N. californicus mites in IPA may have affected the

abundance and the behavior of TSSM, reducing the damage caused by the pest mite. The

TSSM can recognize infochemicals (kairomones) from its predators (phytoseiid mites) or

cues from injured spider mites and consequently avoid feeding or ovipositing in areas

exposed to these cues (Grostal and Dicke 1999).

The potential of N. californicus as a biological control agent has been documented

widely (Sabelis and Janssen 1994; Rhodes and Liburd 2006; Greco et al. 2005, 2011; Sato

et al. 2007; Kim et al. 2013). The results of management adopted in IPA are congruent with

those reported by Sato et al. (2007) and Greco et al. (2011), which indicate that this

predator is capable of maintaining low TSSM densities at appropriate relative prey–

predator ratios.

The first release of N. californicus carried out in the third week (April) was not suffi-

cient for a rapid establishment of the predaceous mite in IPA. One of the possible problems

associated with the establishment of the phytoseiid mites on the strawberry plants was the

presence of winds (C10 km/h) on the area of the beds, in conditions of low abundance

(0.19 mites/leaflet) of the spider mite. The influence of winds on the aerial dispersal of

phytoseiid mites was reported by several authors (Tixier et al. 1998; Jung and Croft 2001).

The aerial dispersal is mainly passive and by wind, but some active take-off behaviors have

been reported (Sabelis and Afman 1994). As prey food levels decline, with starvation and

the presence of wind, aerial take-off of phytoseiids occurs (Jung and Croft 2001).

The fence with a plastic sheet installed on 14 July around the experimental area in IPA,

in order to protect the plants from the wind, may have influenced the establishment of N.

californicus in the field. The abundance of N. californicus increased very slowly

(0.01–0.07 mites/leaflet in 14 weeks) from April to the beginning of July on the strawberry

plants in IPA; however, a relatively rapid increase (0.15–0.37 mites/leaflets in 2 weeks) in

its abundance was noticed in the second half of July (weeks 16–18), after the installation of

the fence. This fortnight interval was one of the driest periods of the year in Atibaia County

(CIIAGRO 2008).

Abundance of N. californicus equal to or above 1.0 mites per leaflet was observed in

IPA from 18 August (week 21), coinciding with the period of the highest rates of rainfall

(40–80 mm/month) and consequently higher humidity. The positive effect of higher

humidity on the establishment of phytoseiid mites was also reported by several authors

(Gerson et al. 2003; de Vis et al. 2006).

The applications of the acaricide propargite did not affect the abundance of N. cali-

fornicus. The low toxicity of propargite to this strain of N. californicus has already been

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reported by Sato et al. (2002). The median lethal concentration (LC50) of propargite in

adults of N. californicus was 2 g AI/l, under laboratory conditions (Sato et al. 2002), which

is almost 109 higher than the recommended concentration of this acaricide for the control

of TSSM in strawberry fields in Brazil (AGROFIT 2014). This strain of N. californicus has

been shown to be resistant or tolerant to pesticides from several chemical classes (e.g.

pyrethroids, organophosphates, spiromesifen) (Poletti and Omoto 2005; Sato et al. 2007,

2011), facilitating the integration of the use of this natural enemy with pesticide appli-

cations for insect pest control in strawberry fields.

The use of pesticide-resistant predaceous mites for the control of spider mites has also

been reported for several species of phytoseiid mites [e.g. Amblyseius fallacis (Garman),

Metaseiulus occidentalis (Nesbitt), Typhlodromus pyri Scheuten, Phytoseiulus persimilis

Athias-Henriot] in different crops (Hoy 1990; Blommers 1994; Hardman et al. 2000; Lee

et al. 2001).

Although propargite has been used as the selective acaricide for the control of TSSM,

several other acaricides have low toxicity to N. californicus (Sato et al. 2002; Castagnoli

et al. 2005; Liburd et al. 2007) and may be useful for the management of the spider mite in

strawberry fields. Bifenazate, for instance, presents low toxicity towards the predaceous

mite, and has been recommended for use in combination with N. californicus for the

control of TSSM on strawberries (Rhodes and Liburd 2006; Liburd et al. 2007; Greco et al.

2011).

The presence of phytoseiid mites in the area of the conventional production (CPA)

was only observed on 8 September (week 24), probably due to the interruption of the

acaricide applications, which was associated with an increase in the rainfall frequency.

Abamectin, which was the most frequently applied acaricide on the strawberry plants in

CPA, was classified as slightly harmful (Meyer et al. 2009; da Silva et al. 2012) or

harmful (Kaplan et al. 2012) to N. californicus. This acaricide caused significant mor-

talities (C60 %) to the active forms of the phytoseiid mite, especially on the first day

after treatment (Sato et al. 2002; da Silva et al. 2012). The other acaricide used by the

grower (fenpyroximate) was considered to be less toxic to N. californicus than abamectin

(Sato et al. 2002).

In the case of CPA, although the grower used ten applications of the acaricide aba-

mectin and two of fenpyroximate on the strawberry beds, the TSSM abundance was not

satisfactorily controlled, due to the evolution of resistance to these acaricides, as confirmed

in the study of acaricide resistance monitoring. Abamectin and fenpyroximate resistance in

TSSM was also reported by several authors (Campos et al. 1995; Stumpf and Nauen 2002;

Suh et al. 2006; Yorulmaz and Ay 2009; Ay and Kara 2011), including cases of popula-

tions found in commercial strawberry fields in SP (Sato et al. 2009; Nicastro et al. 2010).

The initial frequency of abamectin resistance in TSSM from CPA was relatively low

(7.1 %), in spite of the fact that this compound has been used on strawberries for more than

20 years in Atibaia County. This result is probably due to the instability of abamectin

resistance (Sato et al. 2005; Nicastro et al. 2010) associated with the strawberry cultivation

system in this county. Sato et al. (2005) mentioned a decrease in abamectin resistance

frequency from 75 to \15 % in 6 months under laboratory conditions. The strawberry

cultivation period in Atibaia is normally from April to November. During the other period

of the year (5–6 months), there was no acaricide application in the field. In this aspect, the

interval from one season of strawberry to another may contribute significantly to the

reestablishment of susceptibility in TSSM. The immigration of susceptible (or resistant)

mites from other host plants may also affect the reestablishment of susceptibility in field

conditions (Miller et al. 1985).

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For fenpyroximate, the resistance was shown to be more stable in TSSM under labo-

ratory conditions. In a population with an initial resistance frequency of 75 %, the per-

centage of resistant mites decreased to 49.6 % within 1 year (Sato et al. 2004). This

stability is one of the factors that may have influenced the reestablishment of susceptibility

in the field, leading to a higher initial resistance frequency (24.2 %) for fenpyroximate in

TSSM in CPA.

Considering the evolution of acaricide resistance along the cultivation period, a similar

frequency of resistance of TSSM to fenpyroximate and abamectin (about 69 %) was

observed in September, 4 months after the first sampling, despite the fact that abamectin

was sprayed 59 more often than fenpyroximate in this period (May–September). One

possible explanation for the rapid evolution of fenpyroximate resistance is the dominance

of resistance. Studies of the genetics of fenpyroximate resistance in a strain of TSSM

collected from a commercial strawberry field in SP indicated that the fenpyroximate

resistance was controlled by one major, incompletely dominant factor (Sato et al. 2004).

The stability of fenpyroximate resistance may have also contributed to the rapid evolution

of resistance in the field.

For the acaricide propargite, although the grower did not apply this chemical in CPA

during the studied period, the compound had been used for many years in this area. The

high initial percentage (51.2 %) of propargite-resistant mites in CPA is probably due to the

migration of resistant mites from other host plants (Miller et al. 1985), such as roses and

other ornamental crops, cultivated in several commercial fields in Atibaia. The reduction in

the percentage of resistant mites from May to September was probably related to the

instability of propargite resistance in the absence of selection pressure (Grafton-Cardwell

et al. 1987).

In the case of IPA, although the number of acaricide applications was 69 lower than in

CPA, the TSSM infestations were significantly lower on the strawberry plants. This

reduction in TSSM abubdance was probably due to the release of the predaceous mite N.

californicus on the strawberry beds (Greco et al. 2004, 2005; Sato et al. 2007).

The strategies used in the area of integrated production of strawberry, including mon-

itoring the abundance of TSSM and use of the predaceous mite N. californicus in asso-

ciation with the selective acaricide propargite, were efficient to keep the TSSM infestation

levels lower than in the area of conventional production, allowing a reduction in the use of

acaricides by 69 in comparison with CPA. Another advantage associated with the use of

predaceous mites was the reduction in selection pressure for the evolution of pesticide (e.g.

abamectin and fenpyroximate) resistance in TSSM and other target pests [e.g. broad mite,

Polyphagotarsonemus latus (Banks; Acari: Tarsonemidae)] in IPA.

Acknowledgments The authors are thankful to the FAPESP (Sao Paulo Research Foundation) for fundingthis research (Processes # 2007/08612-4 and 2012/17972-2) and for the scholarship to the first author(Process # 2008/02103-3). We would also like to thank CNPq-Brazil (The National Council for Scientificand Technological Development) for the research fellowship provided to the second author, to Dr. HelymarMachado (UNICAMP) for the assistance in the statistical analysis, and to the growers for providing theexperimental areas.

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