Baseline Susceptibility of Tobacco Hornworms (Lepidoptera: Sphingidae) to Acephate, Methomyl and Spinosad in Georgia

The tobacco hornworm, Manduca sexta (L.), is a serious defoliating pest of flue-cured tobacco in Georgia. Isolated producer reports of increased difficulty in controlling this pest with standard insecticides have created concern. Therefore, a topical application technique was used to determine the dosage-mortality responses of M. sexta to three commonly-used insecticides for Georgia tobacco: acephate, methomyl and spinosad. Larvae, 4 to 5 days old (second instar) and weighing 20 to 40 mg, were collected from tobacco plants. Serial concentrations of selected insecticides were applied topically to the larvae. The larvae were subsequently examined for mortality up to 72 h after exposure. Hornworm larvae were highly susceptible to spinosad, with LD50's of 0.059, 0.002, and 0.0004 μg/larva at 24, 48, and 72 h, respectively. Susceptibility to methomyl was intermediate, ranging from 0.123 to 0.176 μg/larva at 72 h, and acephate was the least toxic, with an LD50 of approximately 1.0 μg/larva. The LD50 values for methomyl and acephate remained constant from 24 to 72 h exposure. Location of the hornworm population within the state (south-central, eastern, and southeastern) did not appear to influence the overall susceptibility of the larvae. Baseline data are now established for the three commonly used insecticides for hornworm control, and can be used to document insecticide resistance if it occurs.