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 (ml/ml). Hornworms were moderately susceptible to both methomyl and acephate, with 72 h LC50's of 1.0 × 10−4 (ml/ml) and 1.2 × 10−4 (mg/ml), respectively. Although the LC50's for spinosad continued to drop from 24 to 72 h, the LC50's for methomyl and acephate remained relatively constant from 24 to 72 h. Hornworm larval feeding was disrupted within 1 h of exposure to the higher concentrations of spinosad and methomyl, with cessation of feeding accompanied by larvae moving off the treated foliage. Feeding disruption on acephate foliage was not as evident during the initial 4 h of exposure. Baseline results from leaf dip assays are now established for three commonly used tobacco hornworm controls and can be used to document insecticide resistance.