1. The effects of prey species and leaf age used by prey on performance of two generalist invertebrate predators were studied. The focal plant was Plantago lanceolata, which contains iridoid glycosides.
2. Diet of the herbivorous prey influenced their growth rate.
3. The generalist herbivore (Vanessa cardui) and the novel-plant feeder (Manduca sexta) contained very low levels of iridoid glycosides in their haemolymph, whereas the specialist (Junonia coenia) levels were 50–150-fold higher.
4. Predatory stinkbugs (Podisus maculiventris) fed either the novel-plant feeder or the specialist exhibited similar developmental rates. However, stinkbugs ate less of the generalist but grew faster. The growth rate of the stinkbugs was higher when the caterpillar species were raised on the new-leaf powder diet, which contained twice as much protein and iridoid glycosides as the mature-leaf powder diet.
5. Jumping spiders (Phidippus audax) ate more mealworms (Tenebrio molitor) than specialist J.coenia caterpillars, fed either new- or mature-leaf powder diets, and could not gain weight when fed J.coenia.
6. These results indicate that prey quality was not determined solely by the iridoid glycoside concentration in the diet.