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).