THE SEARCH PATTERN OF A PARASITOID WASP, APHELINUS-ASYCHIS, FOR ITS HOST

We examined the within-patch foraging behaviour of a parasitoid wasp, using an experimental system that consisted of Aphelinus asychis females searching for pea aphids in a petri dish. The wasps' encounter rate with aphids agreed with an expected random distribution in 18 of 22 cases examined. We used a simulation model to find the general conditions under which a random or a systematic search pattern would be favoured by natural selection. We compared the influence of four combinations of searching behaviour (random vs systematic) and target movement (moving vs non-moving) on the numbers of targets encountered, of hosts accepted for oviposition and of prey consumed by a forager. Systematic searching was the best strategy if hosts did not move within the patch, whereas random searching was the best strategy for a forager searching for random-moving targets. The dispersion (near-random vs clumped) of the target population on the forager's payoff was of relatively greater importance at low than at high density. We discuss our findings with regard to the definition of patch and the assumptions of random and systematic searching by foragers in mathematical models.