none The behavioural interactions of the larval parasitoid Apanteles euphydryidis Muesebeck (Braconidae) and its web-making, aggregated host Euphydryas phaeton Drury (Nymphalidae) were examined to evaluate a prediction of optimal searching theory for parasitoids. Webs (host patches) were manipulated by changing the density of webs and the density of host larvae within webs. As predicted by optimal searching theory, these parasitoids aggregated at high host density and, correspondingly, the level of parasitism was highest at these webs. However, the parasitoids were using different webs rather than attending the same webs day after day and they were more aggregated at webs at the end of the season than anticipated. The results suggest that generally parasitoids were moving among host webs more frequently than expected, as a consequence of the continually changing state of the host patches. Alternative explanations for searching behaviour to that of optimal searching theory are suggested for aggregated host-parasitoid systems in which numbers of available, acceptable hosts fluctuate greatly in time and space.