PERSISTENCE OF METARHIZIUM-FLAVOVIRIDE AND CONSEQUENCES FOR BIOLOGICAL-CONTROL OF GRASSHOPPERS AND LOCUSTS

The residual infectivity of an oil formulation of the fungal entomopat hogen Metarhizium flavoviride was measured during a field trial agains t the rice grasshopper, Hieroglyphus daganensis, in north Benin. The p attern of infectivity was shown to decline exponentially following app lication, with a half-life of 6.8 days. In this environment, infection s 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 c onsequences of such residual infection, a simple host-pathogen model w as developed. The model revealed that even very small increases in res idual activity could provide large increases in total mortality and th at under certain conditions, residual infection was essential for effe ctive pest control. This aspect of the activity of mycopesticides is r arely considered. The implications of these results are discussed in t he context of developing optimum spray strategies for locust and grass hopper control under different ecological conditions.