Effect of Beauveria bassiana Fungal Infection on Survival and Feeding Behavior of Pine-Tree Lappet Moth (Dendrolimus pini L.)

Research highlights: The pine-tree lappet moth, Dendrolimus pini, can cause serious needle defoliation on pines with outbreaks occurring over large geographical areas. Under laboratory conditions, the promising potential of the naturally occurring entomopathogenic fungus Beauveria bassiana was tested against D. pini larvae as a biological control method. Background and objectives: The aim of this study was to investigate the most effective concentration and treatment dose of B. bassiana conidial suspension and how it affected the survival and feeding behavior of the pest. Materials and methods: The first experiment applied the fungal suspension directly on the back of selected larvae, and in the second experiment, sporulating cadavers obtained in the first experiment were placed into Petri dishes with healthy individuals. Different doses per larvae [µL] and spore suspension concentration [spores/µL]) were used. The second experiment was designed to investigate the horizontal transmission of fungi by exposing individual caterpillars to a cadaver covered in B. bassiana mycelia. Mortality rates were analyzed by Chi-squared tests using absolute values for total mortality and B. bassiana- attributed mortality. The lethal time and feeding-disruption speed were analyzed with parametric and non-parametric tests with the aim to determine whether statistically significant differences were observed between treatments. Results: Analysis of lethal time revealed that the 20 µL dose and 7.9 × 104 concentration yielded highest mortality, but also the average dieback rate of 9.4 days was significantly faster compared to all other treatments. In order to see whether insects stopped feeding after infection with B. bassiana, larvae weight loss was examined. Results showed that effective treatments induced change in the feeding behavior of infected larvae. The feeding disruption caused by the 20 µL treatment occurred on average 5.5 days after exposure or 3.9 days before the final dieback of larvae.

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.