Persistence of Metarhizium flavoviride and consequences for biological control of grasshoppers and locusts

The residual infectivity of an oil formulation of the fungal entomopathogen Metarhizium flavoviride was measured during a field trial against the rice grasshopper, Hieroglyphus daganensis, in north Benin. The pattern of infectivity was shown to decline exponentially following application, with a half-life of 6.8 days. In this environment, infections due to residual spores from the spray were identified as a key route of infection and accounted for 40-50% of the total infection measured 12 days after application. To examine the within- and between-season consequences of such residual infection, a simple host-pathogen model was developed. The model revealed that even very small increases in residual activity could provide large increases in total mortality and that under certain conditions, residual infection was essential for effective pest control. This aspect of the activity of mycopesticides is rarely considered. The implications of these results are discussed in the context of developing optimum spray strategies for locust and grasshopper control under different ecological conditions.