Encounters With Parasitized Hosts: To Leave or Not To Leave a Patch

ENCOUNTERS WITH PARASITIZED HOSTS: TO LEAVE OR NOT TO LEAVE A PATCH by J.C. VAN LENTEREN (Department of Population Biology, University of Leiden, P.O. Box 9516, 2300 RA Leiden, The Netherlands. Present address: Laboratory of Entomology, Wageningen Agricultural University, P.O. Box 8031, 6700 EH Wageningen, The Netherlands) SUMMARY Several factors determine the amount of foraging time which the parasitoid Leptopilina heterotoma will spend on a patch with hosts, like the composition of the host medium, the age and size of the host patch, the number of searching conspecifics, and the number and quality of host larvae. How encounters with already parasitized hosts in- fluence foraging time is the topic of this paper. When parasitized hosts are stung early during a visit to a patch, an increase in foraging time is the result compared with visit times on patches without hosts. Fre- quent contacts with parasitized hosts, however, lead to a decrease of foraging time. When a host patch is being exploited by L. heterotoma the ratio of parasitized hosts to unparasitized hosts increases. In such a situation, with a low density of unparasitized and a high density of parasitized hosts, visit times were shorter than on patches with only a low density of unparasitized hosts. The difference in searching time could not be attributed to the parasitoid measuring an increase in interval time between con- tacts with unparasitized hosts in the situation where a combination of parasitized and unparasitized hosts was offered: interval times were the same whether parasitized hosts were present or absent. The shorter visit times on patches with a combination of parasitized and unparasitized hosts could neither be attributed to a difference in the structure of the patch nor to differences in kairomone composition. It is the con- tacts with parasitized hosts which causes a shortening of the searching time on patches where a large proportion of the hosts is parasitized. KEY WORDS: Leptopilina, parasitoid, patch-time allocation. INTRODUCTION The problem of how parasitoids allocate foraging time to patches of different quality has received much interest over the past two decades, particularly in the context of optimal-foraging theory and population dynamics (e.g. WAAGE, 1979, and HASSELL, 1978, respectively). The allocation of foraging time involves two decisions: (1) which patch to visit (patch selection), and (2) how long to stay in a patch (patch time). Bakker's research group has previously published about both aspects (e.g. VAN ALPHEN, 1982; VAN ALPHEN & VET, 1986; BAKKER et al.,