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.Electronic supplementary materialThe online version of this article (doi:10.1007/s10886-012-0065-2) contains supplementary material, which is available to authorized users.
of Arabidopsis thaliana that differ in their GLS composition and concentrations and one 32 transformed line that constitutively produces higher concentrations of aliphatic GLS were 33 used, the latter allowing a direct assessment of the effects of aliphatic GLS on insect 34 performance. 35Feeding by the generalist S. exigua and the specialist P. rapae induced both higher 36 aliphatic and indole GLS concentrations in the A. thaliana ecotypes, although induction was 37 stronger for indole than aliphatic GLS. For both herbivores a negative correlation between 38 performance and aliphatic GLS concentrations was observed. This suggests that the specialist, 39 despite containing a nitrile-specifier protein (NSP) that diverts GLS degradation from toxic 40 isothiocyanates to less toxic nitriles, cannot completely inhibit the formation of toxic GLS 41 hydrolytic products, or that the costs of this mechanism are higher at higher GLS
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