Evaluation of Insecticide Resistance in Populations of Tobacco Thrips, Frankliniella fusca (Hinds), and Plant Density Effects on Thrips Injury to Cotton

Two field studies were performed in Tennessee to evaluate the effects of plant density and seed spacing on thrips injury to upland cotton (Gossypium hirsutum L.). The plant density study consisted of seeding rate treatments that were low, normal, high, and very high, relative to university Extension recommendations. In the plant spacing study, treatments were implemented using nearly identical seeding rates but with relatively uniform seed spacing versus a clumped, “hill-dropped” spacing. In both studies, the seed was either treated with an insecticide (imidacloprid) and a fungicide or only with a fungicide. Due to variable environmental conditions, the results were not consistent for each test. In the plant density tests, where thrips pressure did not overwhelm the potential effects of the plant population treatments, more adult thrips were seen in plots with lower plant populations. In the seed spacing study, thrips injury ratings at the first true leaf stage showed higher injury for isolated plants than clumped plants. Future research investigating oviposition rates on newly emerged seedlings could aid in determining why thrips injury is higher in lower plant populations.

Standardized field trials and bioassays were done to evaluate the resistance of Tobacco Thrips, Frankliniella fusca (Hinds), to insecticides. From 2018 to 2021, field trials and bioassays were performed on tobacco thrips populations in the southern U.S. Classes of insecticides evaluated in these studies included organophosphates (acephate, dicrotophos, and dimethoate), a synthetic pyrethroid (lambda-cyhalothrin), a neonicotinoid (imidacloprid), and a spinosyn (spinetoram). Acephate is the most commonly applied foliar insecticide; however, observations in Tennessee suggest that its efficacy has declined. Discriminating dose bioassay and field trial results suggest that tobacco thrips have developed resistance to acephate and likely other organophosphate insecticides; however, this resistance seems to be concentrated in the upper Mid-South (Arkansas, Tennessee, and Mississippi Delta Region). Further research is needed to determine the heritability and mechanism of this resistance.