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 × 10⁶ conidia/ml were sprayed on the fourth instar. However, mortality of the fifth instar injected with 1 × 10³ 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 (LT₅₀) 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.