Evaluation of biorational insecticides against two-spotted spider mite (Acari: Tetranychidae) on Tomato

Abstract

Tomato (Solanum lycopersicum) is one of the most popular and potentially profitable crops grown in high tunnel (polyethylene-covered structures) production. The two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), is a major threat to tomato production in high tunnels in the Southeastern United States. The mite’s remarkable potential for the rapid evolution of resistance makes the chemical control less effective. The study aimed to identify potential alternatives to conventional acaricides for effective management of T. urticae. Thus, the primary objectives of the research were to: (1) conduct laboratory evaluation of susceptibility of two-spotted spider mite to biorational acaricides; (2) to conduct a field evaluation of biorational acaricides as stand-alone treatments or in rotation with predators for managing two-spotted spider mite in high tunnel tomato production. In chapter II, we examined the susceptibility of T. urticae to seven commercially available biorational acaricides using leaf-spray application bioassays in the laboratory: Mycotrol® ES (Beauveria bassiana strain GHA), Molt-X® EC (Azadirachtin), Grandevo® WDG (Chromobacterium subtsugae), Venerate® XC (Burkholderia spp. Strain A396), TetraCURB™ Concentrate EC (Rosemary oil), TetraCURB™ Organic EC (rosemary oil, clove oil, and peppermint oil), and SuffOil-X® EC (mineral oil). These acaricides were first evaluated at the label recommended rate against the adult, nymph, and egg stages of T. urticae, followed by multiple-concentration assays to determine the lethal concentration needed to achieve 50% mortality (LC50) as well as lethal time to 50% mortality (LT50) for promising formulations. At the label recommended rates, all tested formulations were toxic to at least one life stage of T. urticae. Among all the materials tested, TetraCURB™ Concentrate was the most effective and caused 100% mortality in adults and nymphs within three days of exposure. SuffOil-X® and TetraCURB™ Organic were the next best treatments with at least 90% and 60% mortality in nymphs and adults, respectively. Eggs of T. urticae were relatively less susceptible, and 50% suppression in egg hatchability was observed only with TetraCURB™ Concentrate and SuffOil-X®. The results of multiple concentration bioassay with adult mites indicated that TetraCURB™ Concentrate was over two times (LC50 = 0.47 gallon/acre) more toxic than SuffOil-X® (LC50=1.21 gallons/acre) or TetraCURB™ Organic (LC50 = 0.96 gallons/acre), and acted three times (LT50=0.50 days) as fast as SuffOil-X® (LT50=1.65 days), and 6.25 times faster than TetraCURB™ Organic (LT50=3.12 days). In chapter III, field experiments were conducted in two growing seasons (2018-2019) in Alabama to evaluate biorational insecticides that were tested in laboratory study (chapter II). Specifically, six acaricides mostly approved by the Organic Materials Review Institute, (OMRI) were evaluated including SuffOil-X® EC (mineral oil), Mycotrol® ES (Beauveria bassiana strain GHA), Molt-X® EC (Azadirachtin), Grandevo® WDG (Chromobacterium subtsugae), TetraCURB™ Organic EC (mixture of rosemary oil, clove oil, and peppermint oil), and TetraCURB™ Concentrate EC (rosemary oil). In addition, the predatory mite, Phytoseiulus persimilis Athias-Henriot (Acari: Phytoseiidae) was evaluated to compare its efficacy with the selected biorational acaricides in the high tunnel. The biorational insecticides were applied as stand-alone treatments at label-recommended rates on a weekly schedule. In the second year, some of the treatments that were identified in the previous season as the most effective treatments were further evaluated in rotation (alternation). Insecticide efficacy was determined by comparing densities of T. urticae adults, nymphs, and eggs in treated and untreated control plots. TetraCURB™ Concentrate, SuffOil-X® EC, and predatory mite (Phytoseilus persimilis) treatments consistently performed well in suppressing T. urticae populations. TetraCURB™ Concentrate can be applied in rotation with SuffOil-X® EC for effective management of T. urticae in high tunnel tomato production. The knowledge obtained from this research will help the southern vegetable farmers to combat the pest problem by developing an effective integrated pest management strategy against T. urticae in high tunnel tomato production.