Susceptibility of Tarnished Plant Bug (Hemiptera: Miridae) Adults to Selected Reduced-Risk Insecticides and Development of Three Diagnostic Doses, 2016

The damage from TPB feeding mostly causes pinhead squares to abort, causing significant yield loss. To characterize TPB response to key insecticides including acephate (Orthene), acetamiprid (Intruder WSP), bifenthrin (Brigade 2EC), imidacloprid + bifenthrin (Brigadier), zeta-cypermethrin (Mustang Maxx), dicrotophos (Bidrin 8), dicrotophos + bifentrhin (Bidrin XPII), dimethoate (Dimethoate EC), and sulfoxaflor (Transform WG) and develop accurate diagnostic doses for use on field populations, we 1) used residual bioassays to describe the responses of adults of a susceptible laboratory colony to these chemicals, 2) estimated LD50s, LD75s, and LD₉₅s; 3) verified diagnostic doses with ~600 TPB from a single susceptible population using residual bioassays to determine whether observed survivorship differed from the expected level of 50%, 25%, and 5%, respectively. Rearing methods and artificial diet of wheat germ and lima bean were used to maintain the laboratory colony. A series of dilutions were prepared from a 1,000 ppm stock solution. Commercial formulations of Transform WG, Brigade 2EC, Orthene 97, Mustang Maxx, Bidrin 8, Bidrin XPII, Dimethoate 4EC, Brigadier, and Intruder 70 WSP were diluted in de-ionized water for use as residual application. Baseline dose response curves were estimated (LD₅₀s, LD₉₀s) (Table 1) and used for estimation of diagnostic doses (LD₅₀s, LD₇₅s, LD₉₅s). Results reported here provide baseline data for future monitoring of resistance development.

Fifteen adult plant bugs were collected from the susceptible lab-reared colony by a hand-held aspirator. A vented Petri dish (60 mm) was sprayed to 100% coverage using a Crown Spray-Tool system (Woodstock, Illinois) and allowed to dry. A green bean (ca. 2 cm) was dipped for 10 s into each representative insecticide solution with which 0.25% Silwet L-77 surfactant was added to allow even distribution of the insecticide on the bean. The bean was then placed on a paper towel to dry and then added to each Petri dish. Dishes were aerated with mesh screen and sealed with parafilm to prevent escape. Treated adults were held at 26.8 ± 2ºC, 70% RH, and a photoperiod of 14:10 (L:D) h, and mortality was assessed after 48 h by prodding the plant bugs with a blunt probe and observing their movement. A plant bug was considered to be alive if it was able to fly, walk, or move its antennae, legs, or head when prodded. Plant bugs that exhibited no walking and only twitching of the abdomen or legs were considered to be moribund and combined with dead insects (no movement) for analysis. Each bioassay for the laboratory colony 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 (PROC TTEST, SAS Institute, P < 0.05). Slopes, LD₅₀, LD₇₅, and LD₉₅ values were estimated for each bioassay (Table 1). Diagnostic doses were administered to ~100 TPB 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).

To determine the baseline susceptibility of TPB to these insecticides, we estimated the statistical parameters of concentration–response relationships observed in binary bioassays. Although we selected conventional and reduced-risk insecticides for evaluations, the two older classes of pyrethroid and organophate (i.e., bifenthrin (Brigade 2 EC) and acephate (Orthene)) continue to offer two of the highest toxicities to tarnished plant bug. The most effective chemical, Transform, is currently under review by the EPA and is permitted for use by Section 18 emergency exemption only. Intruder was by far the least toxic chemical with an LD₅₀ of 19.39 ppm and 388-fold difference to Transform, another neonicotinoid analog. Dimethoate was 15.4-fold less toxic than Orthene, another organophosphate. 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 dicrotophos, dicrotophos + bifenthrin, bifenthrin, acetamiprid, acephate, ζ-cypermethrin, dimethoate, imidacloprid + bifenthrin, and sulfoxaflor. Continued monitoring of TPB 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.

Reference Cited