Foraging behaviour of two parasitoid species, Cotesia vestalis and Didegma semiclausum, was studied under conditions of intra- and interspecific competition at three different temperatures, 22, 27 or 33 degrees Celsius. The two species have overlapping host niches but differ in their thermotolerance thresholds. Because the parasitoids were allowed to forage at different densities and density may influence parasitism success through interference competition, we first investigated the effect of parasitoid density on parasitism success. Adult parasitoid females were released in different numbers (1, 2, or 4) and allowed to forage for 3h in a cage with a plant infested with 30 hosts (2nd to 3rd instar Plutella xylostella caterpillars). Diadegma semiclausum was also tested at a density of 8 females. After the 3-h foraging period, the adult parasitoids were removed and the cages with parasitized caterpillars were maintained in a greenhouse at 22 degrees Celsius until parasitoid cocoons were formed. All cocoons, moth pupae and dead caterpillars were counted. Parasitism success was then calculated as the number of cocoons that were formed out of the 30 caterpillars that had initially been placed on the plants. We then continued by comparing parasitism success when both species were foraging together (one or two parasitoids of each species) at 22, 27, 33 degrees Celsius. Parasitoid were only exposed to the higher temperature regimes during foraging (3h). Cages with pararasitized caterpillars were maintained at 22 degrees Celsius until cocoon formation and parasitism success was determined as described for the density treatments. Host density was 30 in all cages and cages were considered the unit of replication.