The performance of natural enemies, such as
parasitoid wasps, is affected by differences in the quality of
the host’s diet, frequently mediated by species or
population-related differences in plant allelochemistry.
Here, we compared survival, development time, and body
mass in a generalist herbivore, the cabbage moth, Mamestra
brassicae, and its solitary endoparasitoid, Microplitis
mediator, when reared on two cultivated (CYR and STH)
and three wild (KIM, OH, and WIN) populations of
cabbage, Brassica oleracea. Plants either were undamaged
or induced by feeding of larvae of the cabbage butterfly,
Pieris rapae. Development and biomass of M. brassicae
and Mi. mediator were similar on both cultivated and one
wild cabbage population (KIM), intermediate on the OH
population, and significantly lower on the WIN population.
Moreover, development was prolonged and biomass was
reduced on herbivore-induced plants. However, only the
survival of parasitized hosts (and not that of healthy larvae)
was affected by induction. Analysis of glucosinolates in
leaves of the cabbages revealed higher levels in the wild
populations than cultivars, with the highest concentrations
in WIN plants. Multivariate statistics revealed a negative
correlation between insect performance and total levels of
glucosinolates (GS) and levels of 3-butenyl GS. However,
GS chemistry could not explain the reduced performance
on induced plants since only indole GS concentrations
increased in response to herbivory, which did not affect
insect performance based on multivariate statistics. This
result suggests that, in addition to aliphatic GS, other non-
GS chemicals are responsible for the decline in insect
performance, and that these chemicals affect the parasitoid
more strongly than the host. Remarkably, when developing
on WIN plants, the survival of Mi. mediator to adult
eclosion was much higher than in its host, M. brassicae.
This may be due to the fact that hosts parasitized by Mi.
mediator pass through fewer instars, and host growth is
arrested when they are only a fraction of the size of healthy
caterpillars. Certain aspects of the biology and life-history
of the host and parasitoid may determine their response to
chemical challenges imposed by the food plant.