Susceptibility of Tobacco Thrips (Thysanoptera: Thripidae) Adults to Selected Insecticides and Development of Three Diagnostic Doses, 2016

The TT are a key economic pest of cotton that have caused enormous losses to cotton yields in Missouri, Tennessee, Arkansas, and Mississippi. Development of baseline data for monitoring for resistance is necessary to develop and initiate sustainable management solutions for control of these thrips on cotton. To characterize TT response to key insecticides and develop accurate diagnostic doses for use on field populations, we (1) used diet-incorporated bioassays to describe the responses of adults of a susceptible laboratory colony to key insecticides including acetamiprid (Intruder WSP), clothiandin (Poncho), dicrotophos (Bidrin 8), imidacloprid (Gaucho 600), sulfoxaflor (Transform WG), thiamethoxam (Cruiser 5FS), and thiodicarb+imidacloprid (Aeris), (2) developed three diagnostic doses LD₅₀, LD₇₅, and LD₉₅, and (3) verified diagnostic doses with ~600 TT from a single susceptible population using diet-incorp- orated/residual bioassays to determine whether observed survivorship differed from the expected level of 50%, 25%, and 5%, respectively. Rearing methods and an artificial diet of cabbage leaves were used to maintain the susceptible TT colony. Commercial formulations of Gaucho 600, Aeris, Poncho, Cruiser 5FS, Transform WG, and Intruder WSP were diluted in deionized water for use in diet-incorporated bioassays. Baseline dose response curves were estimated and used for estimation of diagnostic doses LD₅₀, LD₇₅, and LD₉₅ values. A series of dilutions were prepared from a 1,000 ppm stock solution. Based on preliminary data, 6 to 9 concentrations with 0.01–50 Poncho, 0.01–100 Cruiser 5FS, 0.01–50 Gaucho 600, 0.01–100 Aeris, 0.01–10 Intruder WSP, 0.01–10 Transform WG, and 0.01–10 ppm Bidrin 8 and deonized water as an untreated check were used in each bioassay. The dilutions of insecticide were combined at 15% with an artificial rearing diet. The diet contained 1.24-ml green food coloring per 1 liter of water as a food attractant. The representative chemical (150 µl) of each concentration was placed into the cap of a 1.5-ml microcentrifuge tube, and a small piece (2.5 × 2.5 cm) of parafilm sealed the solution in the cap. Five thrips were placed into the 1.5-ml microcentrifuge tube, and the cap sealed the aphids with diet-incorporated treatment. Vials were then placed into an environmental chamber at 26.8 ± 1°C, 60% RH, and a photoperiod of 16:8 h (L:D), and mortality was assessed after 48 h by prodding the thrips with a blunt probe and observing their movement. A thrips was considered to be alive if it was able to move its antennae, legs, or head when prodded. Thrips that exhibited only rapid twitching of the legs, abdomen, or antennae were considered to be moribund and combined with dead insects (no movement) for analysis. Each bioassay had a minimum of four replicates. Dose–response (mortality) relationships were estimated with PoloJR (LeOra Software, LLC, Parma, MO). Data were analyzed assuming the probit model. Plots of standardized residuals were examined for outliers. Only concentrations between the lowest concentration that caused 100% mortality and the highest concentration that caused 0% mortality were used in analyses. Slopes LD₅₀, LD₇₅, and LD₉₅ values were estimated for each bioassay (Table 1). Diagnostic doses were administered to ~100 thrips per dose on a single date, and we compared observed survivorship with expected survivorship. Mortality was corrected with formula of Abbott (1925). Means of survivorship were tested against the expected level with a one-sample t-test and a two-tailed probability distribution (PROC TTEST, SAS Institute; P < 0.05). Results reported here provide baseline data for future monitoring of resistance development.

To determine the baseline susceptibility of TT to these insecticides, we estimated the statistical parameters of concentration– response relationships observed in binary bioassays. We selected insecticides commonly used as seed treatments and foliar treatments. The foliar neonicotinoids (Transform WG and Intruder WSP) and the foliar organophosphate, Bidrin 8, were the most toxic to TT. Gaucho 600, a neonicotinoid, most toxic of all the seed treatments (Table 1). The most effective chemical, Transform, is currently under review by the EPA and is permitted for use by Section 18 emergency exemption only. Cruiser 5FS was by far the least toxic chemical with an LD₅₀ of 45.03 ppm and 195-fold difference to Transform, another neonicotinoid analog. Likewise, Poncho (clothiandin) was 88-fold less toxic than Transform. Aeris, a commercially available premix of imidacloprid and thiodicarb showed similar toxicity to that of Poncho. Baseline susceptibility data are important to allow future monitoring of development of resistance resulting from selection pressure from insecticide use in the field. Our results provide such data for Transform WG, Bidrin 8, Intruder WSP, Gaucho 600, Aeris, Poncho, and Cruiser 5FS. Continued monitoring of TT response to these reduced-risk insecticides will be essential to detect and manage resistance. This research was supported in part by industry gifts of pesticide and research funding.

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