Virulence of the entomopathogenic fungus, Nomuraea rileyi, to various larval stages of the corn earworm, Helicoverpa armigera (Lepidoptera : Noctuidae)

The entomopathogenic fungus, Nomuraea rileyi, was found to be less virulent to younger instars of the corn earworm, Helicoverpa armigera, than the old er ones, the LC50 value to the Ist instars being ca. 625 fold higher than t hat to the 5th. However, body surface (mm(2)) was positively correlated to larval stage in days (r=0.982). Furthermore, determination of conidial numb ers on the body surface of each stadium also revealed a positive correlatio n between the conidial loading quantity and the body surface (r=0.986). The refore, it is evident that higher virulence of N. rileyi to older instars i s due to their larger body surfaces. This permits adherence of more conidia on their body, indicating that body surface is an important factor affecti ng virulence of N. rileyi to N. armigera. In addition, the higher virulence at the 5th stadium appears to be related to its longer duration, providing sufficient time for conidia to penetrate into the hemocoel to cause mortal ity. During the 4th stadium, a lower virulence was obtained for those larva e close to molting than newly emerged ones, indicating that ecdysis is an i mportant factor preventing conidial penetration into the hemocoel.