Abstract. Concentration in the spring of feeding by caterpillars of the winter moth, Operophtera brumata L., and other species of Lepidoptera on oak trees in England is believed to be related to seasonal changes in the texture and chemical composition of the leaves. Increasing leaf toughness is a proximate, though probably not ultimate, factor preventing late larval feeding by the winter moth, the commonest spring species on oak. Early feeding coincides with maximum leaf protein content and minimum leaf sugar content, which suggests that availability of nitrogen, rather ·than of carbohydrate, may be a limiting factor for spring-feeding larvae. The content of oak leaf tannins, which inhibit the growth of winter moth larvae, increases during the summer and may render leaves less suitable for' insect growth by further reducing the availability of nitrogen and perhaps also by influencing leaf palatability. Oak trees are extensively damaged by insect attack, and it is likely that leaf tannins have a defensive function against insects as well as against other herbivores and against pathogens.

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Larvae of Pieris rapae (Lepidoptera: Pieridae) were reared on several crucifer species and varieties which differed in their nitrogen content, either naturally or because they had been fertilized with NH4NO3. Growth rates, food consumption rates, and efficiencies of food utilization were determined for fifth instar larvae on all plants, using standard methods. Larvae on low—nitrogen plants consumed food faster, utilized it less efficiently, and utilized N more efficiently than larvae on high—nitrogen plants. As a result, growth rates and rates of accumulation of N into larval biomass were as high on plants of 1.5% (dry wt) N as on plants of 4.8% (dry wt) N. There was no correlation between larval growth rate and reported patterns of glucosinolate content in the food plants. We attribute low N accumulation rates and growth rates on Lepidium virginicum, Lunaria annua, and Thlaspi arvense to the presence in these plants of unique secondary compounds or atypical glucosinolate breakdown products. Larvae fed on Dentaria diphylla and on the most highly fertilized collards (6.1% dry wt N) accumulated N and grew at unusualy high rates. This may be because N in these plants was more digestible than in the other food plants. We suggest that larvae of P. rapae adjust their feeding rates to maximize the rate at which they can accumulate N, and thus the rate at which they can grow, on any given food plant. There is a limit to the rate at which N can be accumulated, because N is utilized less efficiently as it is consumed faster. We conclude that the growth of P. rapae larvae is limited by the availability of N in their food plants, that N budgets for such larvae are of greater ecological significance than energy budgets, and that, in this case at least, natural selection favors the rate (power output) rather than the efficiency of a biological process.