Abstract
The entomopathogenic fungus, Nomuraea rileyi, was foung to be less virulent to younger instars of the cornearworm, Helicoverpa armigera, than the older ones, the LC50 value to the 1st instars being ca. 625 fold higher than that to the 5th. However, body surface (mm2) was positively correlated to larval stage in days (γ=0.982). Furthermore, determination of conidial numbers on the body surface of each stadium also revealed a positive correlation between the conidial loading quantity and the body surface (γ=0.986). Therefore, it is evident that higher virulence of N. rileyi to older instars is due to their larger body surfaces. This permits adherence of more conidia on their body, indicating that body surface is an important factor affecting virulence of N. rileyi to H. armigera. In addition, the higher virulence at the 5th stadium appears to be related to its longer duration, providing suffciient time for conidia to penetrate into the hemocoel to cause mortality. During the 4th stadium, a lower virulence was obtained for those larvae close to molting than newly emerged ones, indicating that ecdysis is an important factor preventing conidial penetration into the hemocoel.
