In recent years, chilli thrips, Scirtothrips dorsalis Hood (Thysanoptera: Thripidae), has developed into a significant economic pest of horticultural crops in Florida (Seal et al. 2006a; Panthi et al. 2020). Larvae and adults feed on plant tissues, causing leaf distortion, petiole and vein bronzing, cracking and bronzing, and size reduction of fruit, resulting in plants and plant parts becoming unmarketable (Seal et al. 2006b).

Spinetoram and cyantraniliprole insecticides are effective in S. dorsalis adult and larval suppression in strawberries, Fragaria × ananassa Duchesne (Rosaceae) (Lahiri & Panthi 2020). Also, there is an increasing demand for organic strawberry currently, but limited options are available to manage S. dorsalis. Generalist predatory mites such as Amblyseius swirskii Athias-Henriot and Neoseiulus cucumeris (Oudemans) (Arachnida: Phytoseiidae) are effective in S. dorsalis suppression on sweet pepper, Capsicum annuum L. (Solanaceae) (Arthurs et al. 2009). Additionally, several studies have shown the repellant and pesticidal properties of certain Capsicum accession extracts to suppress pests such as twospotted spider mites, Tetranychus urticae Koch (Arachnida: Tetranychidae), Anopheles stephensi Liston, and Culex quinquefasciatus Say (both Diptera: Culicidae) (Antonious et al. 2006; Madhumathy et al. 2007). No information is available currently regarding the potential of these techniques for strawberry pest management in Florida. Therefore, the objective of this study was to evaluate the efficacy of A. swirskii and Capsicum oleoresin to manage S. dorsalis infesting strawberry.

Materials and Methods

Potted strawberry plants of variety ‘Florida Radiance’ (Chandler et al. 2009) maintained in nylon mesh cages (0.027 m³) (BugDorm, BioQuip Products, Rancho Dominguez, California, USA) in the greenhouse located at University of Florida, Gulf Coast Research and Education Center, Wimauma, Florida, USA, were used for this experiment at natural photoperiod and ambient temperature of 24.5 ± 0.5 °C, and relative humidity of 76.0 ± 0.5% (HOBO U23 Pro v2, Onset Computer Corporation, Bourne, Massachusetts, USA). Each experimental cage had a single plant. A laboratory colony of S. dorsalis was maintained on potted cotton, Gossypium hirsutum L. (Malvaceae) in a growth room at 25 ± 5 °C, 60 ± 5% relative humidity, and 14:10 h (L:D) photophase. For the experiment, a single fresh strawberry plant with a minimum of 5 expanded trifoliates was inoculated with 20 newly emerged adult female S. dorsalis and left undisturbed for 7 d. Four treatments: A. swirskii, Capsicum oleoresin extract-based biopesticide (Captiva® Prime, Gowan Company, Yuma, Arizona, USA), conventional insecticide spinetoram (Radiant® SC, Dow AgroSciences LLC, Indianapolis, Indiana, USA), and control sprayed with tap water only, were tested for S. dorsalis adult and larval suppression. Each treatment had 5 replicates. Seven d after S. dorsalis inoculation, 20 adult female A. swirskii individuals of unknown age (from a commercially available source, BioBee Biological Systems, Atlanta, Georgia, USA) were released per plant in treatment nylon mesh cages. On the same d, insecticide treatments also were applied using spray bottles of volume 210 mL (8 fl oz) (Air-spray^TM, Tech Spray, Inc., Peachtree City, Georgia, USA). The manufacturer’s recommended rate of application (731 mL per ha or 10 fl oz per acre) was followed for both insecticide applications.

The number of S. dorsalis adults and larvae, and plant damage rating was recorded by collecting 3 randomly selected strawberry leaflets (1 trifoliate = 3 leaflets) per cage both before and after the treatment. Insect count data were collected at pre-treatment, and 7, 14, 21, and 28 d after treatment. Plant damage rating data were collected at pretreatment, 7, and 28 d after treatment. The 3 freshly collected leaflets per cage were pooled and washed in 70% ethanol to count all thrips adults and larvae.

To analyze the effect of treatments on each date (pre-treatment, 7, 14, 21, and 28 d after treatment) on adult and larval S. dorsalis count, and untransformed plant damage rating, an analysis of variance (ANO-VA) was conducted on natural log-transformed data of insect count. Significant differences were subjected to a separation of means test (Tukey 1953) (PROC MIXED, SAS Institute, Cary, North Carolina, USA). The mean and standard error of untransformed data is presented in the figures.

Results

Seven d after 20 newly emerged S. dorsalis adult females were released on each of the 20 caged strawberry plants, an average of 2 adults and 6 larvae were present per 3 leaflets (1 trifoliate), and no observable plant damage was recorded. There was a significant effect of treatments on S. dorsalis adults on 7, 14, and 21 d after treatment (F_3,12 = 14.18, P = 0.0003; F_3,12 = 3.97, P = 0.0354; F_3,12 = 5.7, P = 0.0116, respectively; Fig. 1). All 3 treatments provided significant adult suppression by 7 d after treatment, so that adult S. dorsalis infestation was 21 times higher on control plants. However, spinetoram alone continued to provide over 7 times higher adult suppression compared to control plants on 14 d after treatment. Also, there was significant effect of treatments on S. dorsalis larvae on 7, 14, and 21 d after treatment (F_3,12 = 5.63, P = 0.012; F_3,12 = 13.48, P = 0.0004; F_3,12 = 6.78, P = 0.0063, respectively; Fig. 1). Spinetoram alone provided more than 12 times higher larval suppression on 7 d after treatment compared to control. On 14 and 21 d after treatment, both A. swirskii and spinetoram provided more than 5 times higher larval suppression compared to control. There were no statistical differences in S. dorsalis adult and larval numbers on pretreatment date (F_3,12 = 1.95, P = 0.1751; F_3,12 = 1.08, P = 0.3961, respectively) and on 28 d after treatment (F_3,12 = 1.93, P = 0.1778; F_3,12 = 2.89, P = 0.0797, respectively; Fig. 1).

Fig. 1.

Mean (± SE) adult and larval Scirtothrips dorsalis Hood per 3 strawberry leaflets on caged strawberry plants treated with (1) Capsicum oleoresin based biopesticide, (2) predatory mite Amblyseius swirskii, and (3) conventional insecticide spinetoram, and compared with control plants for 7, 14, 21, and 28 d after treatment. Means with the same letter are not significantly different (P < 0.05; Tukey HSD).

On the pre-treatment date, all plants had a plant damage rating of ‘0’; therefore, statistical analysis could not be conducted. There were no significant differences among treatments and the control damage rating (Mean ± SE: 1.2 ± 0.58) on 7 d after treatment either (F_3,12 = 0.35, P = 0.7893; Fig. 2). However, significant difference among treatments were evident on 28 d after treatment (F_3,12 = 45.26, P < 0.0001; Fig. 2). Spinetoram treated plants had the lowest damage rating (0.2 ± 0.2), followed by A. swirskii treated plants (2.4 ± 0.24), when compared to control plants (3.4 ± 0.24). However, the Capsicum oleoresin extract was ineffective in suppressing plant damage when compared to control plants.

Discussion

The results of this study indicate that the predatory mite, A. swirskii and the Capsicum oleoresin extract are as effective as spinetoram in the suppression of S. dorsalis adults up to 7 d after treatment only. Also, A. swirskii is as effective as spinetoram in suppression of larval S. dorsalis for at least 21 d after treatment. Plant damage can be suppressed as effectively by A. swirskii as spinetoram for up to 28 d after treatment due to effective larval suppression. Therefore, A. swirskii can be included as an effective biological control agent of S. dorsalis as a replacement for repeated use of synthetic insecticides.

This is especially relevant for organic strawberry production because there is only 1 effective insecticide (spinosad) currently labeled for use in strawberries for thrips management. The variable performance of plant essential oils or extracts for thrips management may be attributed to higher volatility, the ability of thrips to adapt rapidly to the negative stimuli in the absence of acceptable hosts, short residual properties, formulation of the product, or lower initial toxicity (Cloyd & Chiasson 2007; Koschier 2008).

Incorporation of a biological control agent (A. swirskii in this case) will reduce instances of disruptive effects of synthetic insecticides, on both biological control agents and natural enemies (Brødsgaard 2004). The Capsicum oleoresin extract can be used in organic strawberry fields during the early season to target adult S. dorsalis population, but additional plant rescue efforts will be needed 7 d after treatment.

The compatibility of A. swirskii with Capsicum oleoresin extract and synthetic insecticides for S. dorsalis control in open field strawberry fields needs to be evaluated. Thrips predators such as mites, N. cucumeris and A. swirskii, are available commercially for management of S. dorsalis and other phytophagous thrips species commonly occurring in strawberry fields. The possibility of synergistic relationships between A. swirskii and other biological control agents and natural enemies also needs to be explored. The resulting findings will benefit both small fruit and vegetable cropping systems.

This work was supported in part by the USDA National Institute of Food and Agriculture Hatch Project No. FLA-GCR-005888, industry sponsors, and University of Florida, IFAS, Gulf Coast Research and Education Center. Authors thank Vance Whitaker for strawberry plants, and Marissa Cassaway for technical assistance.

Fig. 2.

Mean (± SE) plant damage rating caused by feeding of Scirtothrips dorsalis Hood on caged strawberry plants treated with (1) Capsicum oleoresin based biopesticide, (2) predatory mite Amblyseius swirskii, and (3) conventional insecticide spinetoram, and compared with control plants for 7 and 28 d after treatment. Means with the same letter are not significantly different (P < 0.05; Tukey HSD). Images of strawberry damage rating (0–4) shows the scale used to assign damage rating to plants.