The western spruce budworm, Choristoneura occidentalis Freeman, was used to test the hypothesis that insect herbivore performance is dependent on dietary nitrogen in a positively linear or convex manner. Artificial diets with seven levels of N were tested both with or without mineral supplements to determine if minerals had important effects on the budworm's response to N. A three-generation bioassay quantified survival rates for all life stages and pupal weights for insects reared on diets with 1.2-7.6% N dry mass. Data were used to estimate population growth over three generations by determining the number of F1, F2, and F3 offspring produced (i.e., fitness The budworm's response to increased dietary N was neither positively linear nor convex. Larvae reared on diets with very high (3.9 and 7.6%) concentrations of N, compared with N levels in Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco, foliage, had good survival and reproduction, whereas larvae on diets with N levels near foliage quantities (1.2-3.0%) had verv poor performance, with the exception that performance was good on the mineral-limited 1.2% N diet. The addition of minerals to the diets had important effects on responses to N, implicating an interaction between N and minerals. Regression analysis of data on estimated population growth on the 14 diets tested indicated that the ratio of Zn to N was the best predictor of budworm fitness, not the actual N content of the diet. The response to Zn/N ratios was concave, implying proportions less-than-or-equal-to 0.00085 or greater-than-or-equal-to 0.00282 had positive effects on fitness, whereas ratios from 0.001 to 0.0025 had negative effects. It was hypothesized that host plant N determines the amount of food ingested, which in turn affects the amounts of other nutrients consumed. Thus, a proper balance of many different nutrients appears to be the most important factor in the nutritional ecology of insect herbivores.
