Conidia of Metarhizium anisopliae need water activity (Aw)>0·98 for germination irrespective of whether they are formulated in an aqueous or oil carrier. The presence of nutrients in the carriers or culture medium accelerated germination but did not increase the range of Aw (or humidities) over which conidia could germinate. Insect extract stimulated more conidia to germinate than yeast extract. Oils appeared to extract fungistatic and stimulatory compounds from the insect cuticle. The ratio of these compounds depends on both the insect species and the carrier. Conidia of M. anisopliae formulated in oils were observed with SEM to flow over the surface of insect and plant cuticles. The conidia were probably deposited at sites which are conducive for germination and infection and consequently increased the overall mortality of insects. In contrast, aqueous formulations of conidia remain as drops on the leaf surface immediately after application with a pipetteman or sprayer.