PATCH EXPLOITATION BY THE PARASITOIDS COTESIA-RUBECULA AND COTESIA-GLOMERATA IN MULTI-PATCH ENVIRONMENTS WITH DIFFERENT HOST DISTRIBUTIONS

1. We analysed the foraging behaviour of two closely related parasitoi d species (Cotesia rubecula and Cotesia glomerata) with respect to lea ving tendencies from patches in different environments. We investigate d how intrapatch experiences like contact with feeding damage and enco unters with hosts influence patch leaving decisions. We also estimated the effect of experiences in previously visited patches on leaving de cisions in the present patch. 2. For this analysis we applied the prop ortional hazards model (Cox 1972) to data collected in three Versions of a multiple patch set-up. These set-ups consisted of different host species or combinations of host species: (1) Pieris rapae, (2) Pieris bl brassicae and (3) both P. rapae and P. brassicae. The larvae of the se hosts differ in their spatial distribution on plants: P, brassicae occur in clusters and the distribution of larvae is heterogeneous; P. rapae larvae feed solitarily, 3. The specialist parasitoid C. rubecula used a simple strategy: highest leaving tendency on empty leaves, low er leaving tendency on leaves infested with the non-preferred host P. brassicae, lowest leaving tendency on leaves infested with the preferr ed host, P. rapae. In the environment with both host species, the leav ing tendency only decreased on leaves infested with P. rapae. 4. The g eneralist C. glomerata used a more complex set of rules. (a) Multiple ovipositions on the present patch decreased the leaving tendency on le aves containing the gregarious host. (b) Once the parasitoid had encou ntered two or more hosts, it had a lower leaving tendency during subse quent patch visits, (c) The leaving tendency increased with the number of visits on infested leaves. In environments where the less preferre d host P. rapae was present, C. glomerata switched to the same simple type of rule as used by C. rubecula. 5. Neither of the two Cotesia spe cies used a count-down rule, in which ovipositions increase the leavin g tendency. We discuss how patch exploitation by both Cotesia species compares to the patch exploitation mechanisms as proposed by Waage (19 79) and Driessen et al. (1995). 6. We formulate an 'adjustable termina tion rate' model for patch exploitation in both Cotesia species in mul ti-patch environments.