Insect species
To measure plant resistance, we used the large cabbage butterflyPieris brassicae (Lepidoptera: Pieridae) and the African cotton leaf worm Spodoptera littoralis (Lepidoptera: Noctuidae) as specialist and generalist chewing herbivore insects, respectively.P. brassicae is a specialist herbivore that feeds exclusively on plants producing GSLs, especially on species of the Brassicacea (Chew 1988), which act as oviposition and feeding stimulants for specialist herbivores (Huang et al. 1994). The caterpillars used in this experiment were originated from a rearing culture on Brassica rapa ssp. chinensis (Brassicaceae). S. littoralis is a generalist herbivore, known to feed on species belonging to more than forty families of plants (Brown & Dewhurst 1975) and is widely used for performing plant resistance bioassays. Eggs were obtained from Syngenta, Stein AG, Switzerland, and newly hatched S. littoralis larvae to be used in the bioassays were reared on corn-based artificial diet until the beginning of the experiment to avoid previous acclimation to a GSL-based diet. Moreover, we used the cabbage aphids Brevicoryne brassicae (Heteropetera: Aphididae) and green peach aphid Myzus persicae (Heteroptera: Aphididae) as specialist and generalist phloem-feeding insects, respectively. Brevicoryne brassicae is a specialist aphid that feeds exclusively on Brassicaceae plants, while M. persicae has been recorded to feed on more than 120 species and 30 families of plants worldwide (van Emden et al.1969). The aphids used in this experiment were originated from a rearing culture on radish plants (Raphanus sativus , Brassicaceae).