The germination behaviour of Metarhizium anisopliae on the surface of aphid and flea beetle cuticles

Aphids and flea beetles were differentially susceptible to the same isolates of the entomopathogenic Metarhizium anisopliae. Differences in susceptibility reflected the response of the pathogen to cuticular cues. More conidia adhered and germinated readily on the surfaces of live aphids and at sites under the elytra of live beetles. On dorsal elytra and the ventral thorax of live beetles poor adhesion and germination was attributed to fungistasis because no such behaviour was noted on dead beetles. The cuticle also influenced appressorium morphology and the mode of pathogenesis. Fewer, smaller appressoria were produced on aphid as opposed to beetle cuticle. Metarhizium anisopliae also penetrated the cuticle directly, i.e. without producing appressoria. Sites under the elytra of live flea beetles were more vulnerable to infection than sites on the dorsal elytra or ventral thorax resulting in higher mortalities and lower LT50 values.