Coexistence and niche segregation by field populations of the parasitoids Cotesia glomerata and C-rubecula in the Netherlands: predicting field performance from laboratory data
Jbf. Geervliet et al., Coexistence and niche segregation by field populations of the parasitoids Cotesia glomerata and C-rubecula in the Netherlands: predicting field performance from laboratory data, OECOLOGIA, 124(1), 2000, pp. 55-63
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.