Stabilization of the Rate of Nitrogen Accumulation by Larvae of the Cabbage Butterfly on Wild and Cultivated Food Plants

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.