Coexistence and niche segregation by field populations of the parasitoids Cotesia glomerata and C-rubecula in the Netherlands: predicting field performance from laboratory data

Field experiments with foraging parasitoids are essential to validate the c onclusions from laboratory studies and to interpret differences in searchin g and host selection behaviour of parasitoid species. Furthermore, field ex periments can indicate whether the parameters measured in the laboratory ar e relevant to elucidation of the ecological processes under study, such as adaptation or species interactions. In previous extensive laboratory studie s we studied plant- and host-searching behaviour, host acceptance, host sui tability; host plant preference, and learning of two congeneric parasitoids of Pieris caterpillars: the generalist Cotesia glomerata, which has been r eported to attack several Pieridae species, and C. rubecula, a specialist o f the small cabbage white Pieris rapae. In the present field study our aim was to verify the importance of these previous laboratory findings for expl aining the performance of these two species in the field. We investigated e xperimentally whether parasitism on three Pieris species varied with parasi toid species and with food plant of the caterpillars. We exposed different types of host plants, infested with different Pieris species, to parasitism by natural populations of Cotesia species, by setting the experimental pla nts out in Brussels sprouts cabbage fields. Furthermore we made direct obse rvations of parasitoid foraging in the field. In general, the field results confirmed our predictions on the range of host plant and host species used in the field. The two Coresia species appear to coexist through niche segr egation, since C. glomerata was mainly recovered from P. brassicae and C. r ubecula from P. rapae. Although C. glomerata is a generalist at the species level, it can be a specialist at the population level under certain ecolog ical circumstances. Our study shows the importance of variation in host pla nt attraction and host species acceptance in restricting host plant and hos t diet in the field. Furthermore the results suggest that, at least in the Netherlands, specialisation of C. glomerata on P. brassicae may occur as a result of C. rubecula outcompeting C. glomerata in P. rapae larvae.