Can sequestration of glucosinolates be a good defence for a specialist aphid against its parasitoid?
Résumé
Brassicaceae plants are characterized by the presence of glucosinolates which are considered to be a good defence system against generalist herbivores. In contrast, some specialist herbivores are known to sequester the potentially toxic compounds. It has been hypothesized that sequestration may present two advantages: 1) no impact of the glucosinolates on the performance of the specialist herbivores, and 2) a negative impact on the performance of their predators and parasitoids. However, this last hypothesis has only been tested for a few generalist predators. Brevicoryne brassicae, the cabbage aphid, feeds on a wide range of Brassicaceae with different concentrations and compositions of glucosinolates. The main parasitoid of B. brassicae is Diaeretiella rapae. To ask whether D. rapae is affected by the glucosinolates sequestered in its host, we measured the performance of the aphids and the parasitoids on four host plant species with contrasting levels of glucosinolates. Under controlled conditions, life history traits of B. brassicae and D. rapae did not differ significantly among host plants. However, adults of the parasitoids were larger when their hosts fed on the plant species with the highest concentrations of glucosinolates. In conclusion, the sequestration of plant chemical defences by the aphid does not seem to have a negative impact on its parasitoid.