New insights into the infection of the American cockroachPeriplaneta americananymphs withMetarhizium anisopliaes.l. (Ascomycota: Hypocreales)

To study the marked resistance of Periplaneta americana to entomopathogenic Metarhizium anisopliae. The low susceptibility of 4th instar nymphs applied topically with conidia seemed to be related to an active removal of conidia by the cockroach and to a disabled or retarded germination and subsequent development of conidia on the cuticle (up to 80% germination in the next 7 days after application). Inhibitions or delays of germination were related to the composition of the epicuticular fatty acids (30·1% w/w oleic, 28·3% w/w linoleic, 24·5% w/w palmitic and 11·7% w/w stearic acid) reported here. Propagules invading the nymphs through the cuticle took at least 3 days to reach the haemocoel, and no propagules were found after day 8 post‐treatment. Strain IP 46 infected >50% of nymphs treated with doses ≥2 × 104 hyphal bodies (HB) nymph−1 and reduced the survival of nymphs ≤50%. Most nymphs (>70%) survived after injection of 6 × 103 and 2 × 103 HB nymph−1. Findings emphasize a distinct resistance of nymphs of the American cockroach to infections by M. anisopliae. Our findings provide support for the development of biological control of this synanthropic cockroach pest.