Abamectin is effective in suppressing chilli thrips in strawberry fields of Florida

Abstract

Since 2015, chilli thrips, Scirtothrips dorsalis Hood (Thysanoptera: Thripidae), has emerged as the most important economic pest of open-field strawberry production in Florida. During the long production season spanning from September to April, few insecticide modes of action are effective in suppressing this invasive pest that easily develops resistance to insecticides. There is no information available regarding the efficacy of abamectin (IRAC Group 6) in suppressing S. dorsalis in strawberry fields, although this product is labeled for and is highly effective in twospotted spider mite, Tetranychus urticae Koch (Trombidiformes: Tetranychidae), suppression in strawberry. Therefore, the objective of this study was to investigate the efficacy of abamectin (Agri-Mek SC, 3.5 fl oz/A), compared with spinetoram (Radiant SC, 10 fl oz/A) and nontreated control in suppression of S. dorsalis adults and larvae in strawberries. On 17 October 2022, bare root strawberry cultivar, ‘Florida Brilliance’, was planted on raised strawberry beds maintained according to commercial production standards with a plot size of 12 plants per plot at the Gulf Coast Research and Education Center (GCREC), Wimauma, University of Florida. On 7 February 2023, treatments were initiated after pretreatment sampling indicated a well-established S. dorsalis field population in experimental plots. Five randomly selected strawberry trifoliates in each plot were sampled to assess S. dorsalis population before treatment. The treatments were applied in a randomized complete block design with four replicates. All products were tank mixed with the surfactant LI700 at the rate of 0.125% v/v and applied using a backpack sprayer calibrated to 50 gallons per acre and at 40 PSI. Only a single application was done. Post-treatment sampling was conducted on 7, 14, and 21 days after treatment (DAT). Tissue samples consisting of 5 young, fully expanded strawberry trifoliates/plot were brought to the Entomology laboratory at the GCREC and washed with 70% ethanol for counting thrips life stages under stereomicroscope. Trifoliates were sampled due to feeding preference of S. dorsalis on leaves. Marketable fruit yield (gm) was also assessed per strawberry plot every week. The effect of abamectin and spinetoram on the S. dorsalis field population was analyzed using a repeated-measures analysis of variance (ANOVA), where the main effects of insecticide treatments on S. dorsalis adults and larvae were assessed (Proc Mixed, SAS Institute, Cary, NC, USA). For yield data, ANOVA was conducted to assess the main and interaction effects of treatment and date of harvest. AICC fit statistics were used to decide to transform data into natural log for meeting the normality assumptions for adults and square root transformation for larvae. Yield data were not transformed for ANOVA. Means were separated using the Tukey HSD test (α ≤ 0.05) when appropriate. Untransformed means data is presented in tables. Abamectin was as effective as spinetoram in the suppression of S. dorsalis adults (F = 8.81, df = 2, 41, P = 0.0007) and larvae (F = 7.34, df = 2, 42, P = 0.0018) in strawberry trifoliates compared with nontreated control plants (Table 1). There was a significant interaction effect of treatment and date of harvest on strawberry marketable fruit yield with highest yield seen in abamectin and spinetoram-treated plants on 21 DAT (F = 7.75, df = 6, 33, P < 0.0001, Table 2). Overall, fruit yield was higher in insecticide-treated plots with no difference between spinetoram and abamectin treatments (F = 17.21, df = 2, 33, P < 0.0001). None of the treatments showed any signs of phytotoxicity on strawberry plants after application. It should be noted that abamectin is toxic to pollinators and needs to be used according to manufacturer’s label instructions only.1 Mean (±SE) chilli thrips, Scirtothrips dorsalis Hood, in strawberry trifoliates treated with abamectin and spinetoram during field season 2022–2023 in Florida Means with the same letter in the column are not significantly different (Tukey’s HSD, P > 0.05). aThe surfactant LI700 was included with all treatments at the rate of 0.125% (v/v). Mean strawberry marketable fruit yield (±SE) from plants treated with abamectin and spinetoram for S. dorsalis management during field season 2022–2023 in Florida Means with the same letter in the column are not significantly different (Tukey’s HSD, p > 0.05). aThe surfactant LI700 was included with all treatments at the rate of 0.125% (v/v)

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