Effect of formulation, application and rain on the persistence of the entomogenous fungus Metarhizium anisopliae on oilseed rape

The effect of simulated rain on the persistence of oil and wafer formulatio ns of conidia of the entomogenous fungus Metarhizium anisopliae when applie d to oilseed rape foliage was investigated, using third instar larvae of th e mustard beetle (Phaedon cochleariae) as the target host. Rain significant ly (P < 0.01) reduced the susceptibility of the beetle larvae to M. anisopl iae but the amount of inoculum removed was influenced by the formulation. L arvae exposed to plants treated with conidia formulated in aqueous Tween, S hellsol T, or sunflower oil/Shellsol T resulted in 55, 82.5 and 72.5%, mort ality, respectively. The mortality for these respective formulations was re duced by 42, 57 and 51% if the plants were exposed for I h to simulated rai n. Laboratory and field studies showed that more inoculum collected beneath plants sprayed with conidia formulated in Shellsol T or aqueous Tween than in the more viscous sunflower/Shellsol T mixture. Mortality studies on lea ves taken from field plots suggested that conidia on leaf surfaces could be replenished by repeated application. The number of conidia isolated from f ield plots was greater where inoculum was applied bi-weekly than once weekl y.