The cabbage aphid Brevicoryne brassicae is a specialist
herbivore that sequesters glucosinolates from its host
plant as a defense against its predators. It is unknown to what
extent parasitoids are affected by this sequestration.We investigated
herbivore-mediated effects of glucosinolates on the
parasitoid wasp Diaeretiella rapae and the predator Episyrphus
balteatus. We reared B. brassicae on three ecotypes of
Arabidopsis thaliana that differ in glucosinolate content and
on one genetically transformed line with modified concentrations
of aliphatic glucosinolates.We tested aphid performance
and the performance and behavior of both natural enemies.
We correlated this with phloem and aphid glucosinolate concentrations
and emission of volatiles. Brevicoryne brassicae
performance correlated positively with concentrations of both
aliphatic and indole glucosinolates in the phloem. Aphids
selectively sequestered glucosinolates. Glucosinolate concentration
in B. brassicae correlated negatively with performance
of the predator, but positively with performance of the
parasitoid, possibly because the aphids with the highest glucosinolate
concentrations had a higher body weight. Both
natural enemies showed a positive performance-preference
correlation. The predator preferred the ecotype with the lowest
emission of volatile glucosinolate breakdown products in each
test combination, whereas the parasitoid wasp preferred the A.
thaliana ecotype with the highest emission of these volatiles.
The study shows that there are differential herbivore-mediated
effects of glucosinolates on a predator and a parasitoid of a
specialist aphid that selectively sequesters glucosinolates from
its host plant.