1. Plant resistance against herbivores can act directly (e.g. by producing
toxins) and indirectly (e.g. by attracting natural enemies of herbivores). If plant
secondary metabolites that cause direct resistance against herbivores, such as
glucosinolates, negatively influence natural enemies, this may result in a conflict
between direct and indirect plant resistance.
2. Our objectives were (i) to test herbivore-mediated effects of glucosinolates on
the performance of two generalist predators, the marmalade hoverfly (Episyrphus
balteatus) and the common green lacewing (Chrysoperla carnea) and (ii) to test
whether intraspecific plant variation affects predator performance.
3. Predators were fed either Brevicoryne brassicae, a glucosinolate-sequestering
specialist aphid that contains aphid-specific myrosinases, or Myzus persicae, a nonsequestering
generalist aphid that excretes glucosinolates in the honeydew, reared
on four different white cabbage cultivars. Predator performance and glucosinolate
concentrations and profiles in B. brassicae and host-plant phloem were measured, a
novel approach as previous studies often measured glucosinolate concentrations only
in total leaf material.
4. Interestingly, the specialist aphid B. brassicae selectively sequestered glucosinolates
from its host plant. The performance of predators fed this aphid species was
lower than when fed M. persicae. When fed B. brassicae reared on different cultivars,
differences in predator performance matched differences in glucosinolate profiles
among the aphids.
5. We show that not only the prey species, but also the plant cultivar can have
an effect on the performance of predators. Our results suggest that in the tritrophic
system tested, there might be a conflict between direct and indirect plant resistance.