Leaf Dip Bioassay to Determine Susceptibility of Tobacco Hornworm (Lepidoptera: Sphingidae) to Acephate, Methomyl and Spinosad

The tobacco hornworm, Manduca sexta (L.), is an annual economic pest of flue-cured tobacco in Georgia and throughout the southeastern U.S. Recent concerns about controlling hornworms with standard insecticides have been reported by producers and the Cooperative Extension Service. Therefore, a tobacco leaf dip assay was developed to determine the dosage-mortality responses of tobacco hornworms to three standard tobacco insecticides: acephate, methomyl, and spinosad. Larvae, 4 to 5 days old (second instar) and weighing 20 to 40 mg, were reared from eggs collected on field-grown tobacco. Serial concentrations of selected insecticides were prepared in 3.8-L containers, and untreated tobacco leaves were immersed in the solution for 5 s, then air dried. Larvae were placed directly on the treated foliage and examined for mortality after 24, 48 and 72 h of exposure. Hornworm larvae were highly susceptible to foliage dipped in spinosad, with LC50's of 4.3 × 10−4 at 24 h, 1.0 × 10−6 at 48 h, and 5.7 × 10−7 at 72 h ...