Evaluation of toxicity of selected insecticides against thrips on cotton in laboratory bioassays

Authors: Lopez, Juan De Dios; Fritz, Bradley - Brad; Latheef, Mohamed - Ab; Lan, Yubin; Martin, Daniel - Dan; Hoffmann, Wesley

Submitted to: Journal of Cotton Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/25/2008
Publication Date: 10/2/2008
Citation: Lopez, J., Fritz, B.K., Latheef, M.A., Lan, Y., Martin, D.E., Hoffmann, W.C. 2008. Evaluation of toxicity of selected insecticides against thrips on cotton in laboratory bioassays. Journal of Cotton Science. 12:188-194.

Interpretive Summary: Western flower thrips are a pest of cotton in the United States that degrades cotton quality and yield and vectors plant diseases. Studies were conducted to evaluate the effectiveness of three classes of insecticides (organophosphates, spinosyn, and neonicotinoids) in controlling both immature and adult Western flower thrips. Adult vial testing showed that all technical insecticides had different lethal dosages with spinosad being the most toxic and imidacloprid being the least toxic. Spray table assessments showed no significant effects of spray droplet size on thrips control. However, increased spray rates provided better control at lower active ingredient rates, indicating that the increased spray rate resulted in better coverage and improved efficacy. Results of these laboratory studies will be used as a basis for selection of insecticides to control Western flower thrips in producer-grown cotton.

Technical Abstract: Adult vial technique (AVT) and spray table bioassays were conducted to evaluate toxicity of selected insecticides against immature and adult Western flower thrips (WFT), Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae). In AVT, technical insecticides comprising of organophosphates (dicrotophos and methamidophos), spinosyn (spinosad) and neonicotinoids (thiamethoxam and imidacloprid) were used. The LC50 values for contact with the insecticides were all significantly different, with spinosad being most toxic and imidacloprid being least toxic to F. occidentalis. Spray table tests were conducted using formulated insecticides comprised of dicrotophos, methamidophos and spinosad on thrips-infested greenhouse-grown cotton plants. Multiple application treatments of varying droplet sizes and densities were made at 19 L/ha (2 gpa) and thrips sampled at 1, 3 and 7 days after treatment (DAT). Droplet characteristics (size and density) and DAT did not significantly influence post-treatment thrips densities on cotton plants. Average numbers of thrips/plant on methamidophos- and spinosad-treated cotton plants were significantly fewer than those on dicrotophos-treated and untreated plants. Spray table treatments were also made to examine effects of spray rate [19(2) and 47(5) L/ha(gpa)] on thrips control at 3, 5, 7 and 14 DAT on cotton plants at different active ingredient (a.i.) rates of spinosad. Increased spray rate [47 L/ha (5 gpa)] provided better control regardless of DAT at a lower spinosad a.i. rate indicating that improved coverage increased efficacy at marginal control a.i. concentrations.