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.

2000. Effects of plant phenology, nutrients and herbivory on growth and defensive chemistry of plantain, Plantago lanceolata. -Oikos 88: 371 -379.To assess the combined effect of herbivory, nutrient availability and plant phenology on plant mass and defensive chemistry, we conducted a field experiment with plantain (Plantago lanceolata: Plantaginaceae) using three levels of herbivory, three levels of fertilizer and two harvest dates. Shoot mass of the no-herbivory plants showed a nonlinear response to increased fertilizer such that mass with high fertilizer was no greater than that with low fertilizer. In contrast, shoot mass of the low-herbivory plants (12% damage) was not influenced by fertilizer, but for high-herbivory plants (23% damage), there was a positive linear response to increased fertilizer. Increasing nutrient levels caused a decrease in iridoid glycoside concentration. Herbivory did not induce higher iridoid glycoside concentration in leaves of any age. But increasing herbivory resulted in a decrease in the concentration of catalpol in new leaves. Another experiment assessed how leaf age and plant age affected plant defensive chemistry. Total iridoid glycosides increased over 5 weeks, but catalpol only increased in new leaves. Overall, the order of importance in determining variation in iridoid glycoside concentration was plant phenology, nutrient availability and, to a much lesser extent, herbivory.