Mortality of Maruca vitrata (Lepidoptera: Crambidae) larval stages induced by different doses of the entomopathogenic fungi Metarhizium anisopliae and Beauveria bassiana

The dose-dependent mortality of second (3-4 days old), third (5-6 days old) and fourth (7-9 days old) larval instars of Maruca vitrata as induced by the entomopathogenic fungal isolates, Metarhizium anisopliae Ma 29 and Beauveria bassiana Bb 115 was determined in a laboratory experiment. For each fungal isolate, five doses (0, 2x104, 2x105, 2x106 and 2x107 conidia/larva, respectively) were applied topically to one individual larva at the rate of 2 µl of an oil formulation. Observations were focused on mortality and sporulation of treated larvae. Cox regression was used to determine the lethal dose 50 (LD50). Results revealed high mortality of the third larval stage (L3) at 2 x 10? conidia for Ma 29 and Bb 115 with 49% and 96% mortality, respectively. No significant difference in mortality was observed between larval stages exposed to 2x107 conidia/ larva. The sporulation rate did not differ significantly between larval stages. Regardless of application doses, the Mean Survival Times (MSTs) of larvae were significantly different between doses for both fungal species. The lowest MSTs were recorded for the younger larvae (L2, L3) at higher doses (2x106 and 2x107 conidia/larva). Larval mortality increased with increasing dose, regardless of fungal species and larval stages, suggesting a dose-dependent response of M. vitrata larvae.