Influence of the aphid pathogen Pandora neoaphidis on the foraging behaviour of the aphid parasitoid Aphidius ervi

1. The parasitoid Aphidius ervi and the entomopathogenic fungus Pandora neoaphidis both require successful invasion of an aphid host to complete their life cycle. A shorter developmental period allows P. neoaphidis to out-compete A. ervi. Aphidius ervi may reduce this fitness cost by avoiding aphid colonies containing P. neoaphidis. Here the response of A. ervi towards P. neoaphidis was assessed using sequential experiments designed to replicate different stages of parasitoid foraging behaviour. 2. Entry rate experiments showed that A. ervi entered aphid colonies containing P. neoaphidis-sporulating cadavers and that there was no significant difference in the attraction of A. ervi to aphid-damaged Vicia faba plants containing either healthy Acyrthosiphon pisum or P. neoaphidis-sporulating cadavers. 3. Observational behavioural experiments indicated that the presence of P. neoaphidis did not affect the search time or total foraging time of A. ervi on V. faba plants infested with either healthy A. pisum or P. neoaphidis-sporulating cadavers. 4. In Petri dish bioassays using aphids infected with P. neoaphidis over a period of 120 h, A. ervi showed no difference in attack rate against uninfected aphids or living aphids infected with P. neoaphidis for 1, 24, 48, 72, or 96 h. However, sporulating cadavers (120 h infection) were not attacked. 5. Aphidius ervi appears only able to detect the presence of P. neoaphidis once the host is dead and sporulation has started. The fitness of A. ervi may therefore be severely reduced when foraging in P. neoaphidis-infected aphid colonies.