Effects of environmental factors on virulence of the entomopathogenic fungus, Nomuraea rileyi , against the corn earworm, Helicoverpa armigera (Lep., Noctuidae)

The effects of environmental factors on infection of the entomopathogenic fungus, Nomuraea rileyi , isolated from the corn earworm, Helicoverpa armigera , in Taiwan, to its host insect were studied in the laboratory. The fungus caused higher larval mortality at 20°C than at 30°C when 5 × 10 6 conidia/ml were sprayed on the fourth instar. However, mortality of the fifth instar injected with 1 × 10 3 conidia/larva was not significantly different when the inoculated larvae were incubated from 15 to 30°C. The fungal development in inoculated larvae was best at 20 and 25°C after shifting from 20°C to either lower or higher temperatures. The germination rate was higher at 20 and 25°C than at 30 or 35°C. Conidial germination was better on the wash‐off of insect cuticle than on Sabouraud maltose agar with yeast extract. Sporulation on chill‐dried cadavers was maximal at 95 or 100% relative humidity than at lower levels of relative humidity. The time required for sporulation was 2 days less at 100% than at 95% relative humidity. Although photoperiod did not affect fifth instar mortality caused by N. rileyi , the median lethal time (LT 50 ) values were shorter upon incubating under light than in darkness. Incubation of infected cadavers under 12 or 24 h light resulted in 20‐fold more conidial production than under full darkness. Therefore, illumination is necessary for development of this isolate on insect cadavers.