The effects of quantity and quality of diet nitrogen on the growth, efficiency of food utilization, nitrogen budget, and metabolic rate of fifth-instar Spodoptera eridania larvae (Lepidoptera: Noctuidae)

Abstract

Relative growth rate and relative nitrogen accumulation rate for fifth-instar Spodoptera eridania larvae vary less than 20% on artificial diets in which protein content varies more than 250%, due to compensatory adjustments in consumption rate and changes in efficiencies of food and nitrogen utilization. The substitution of zein for two-thirds of the casein in a diet containing 26.0% protein results in a 25% decrease in both relative growth and nitrogen accumulation rates, due to reduced values of relative consumption rate, relative nitrogen consumption rate, approximate digestibility, approximate digestibility of nitrogen and efficiency of conversion of digested nitrogen. Although larval growth rate is relatively independent of diet nitrogen, larval composition is highly dependent upon both nitrogen quantity and quality. As diet nitrogen increases, larval nitrogen content increases and fat content decreases. Larvae on diets containing a mixture of casein and zein produce 3.5-3.9 times as much uric acid and respire at rates significantly higher than larvae on diets containing the same total amount of protein but lacking zein. Thus, there is a measurable metabolic cost associated with processing low quality protein. We conclude, however, that elevated metabolic rates of larvae on diets containing nutritionally unbalanced protein are not the cause of reduced growth rates. On these diets, growth is limited byan amino acid present in limiting quantities, and the elevated metabolic rate is due, at least in part, to increased synthesis of uric acid. Finally, we discuss the possibility that variation in amino acid profiles across host plant species might be a factor favouring specialization in insect herbivores.