Diet heterogeneity: Effects on gypsy moth feeding behavior and growth.

Abstract

Variation in nutrients is common within and among plants and has been hypothesized to have defensive value against insect herbivores. I investigated the effects of dietary variation in protein concentration on the feeding behavior, growth and survival of gypsy moth, Lymantria dispar (Lepidoptera: Lymantriidae) in laboratory experiments using artificial diets ranging from low to high protein. The results indicate that dietary variation in protein can either suppress or enhance insect fitness, depending on the insect's foraging behavior. Minimization of dietary variation can reduce fitness if accomplished by selecting a homogeneous diet consisting of either high or low protein food. When provided diets homogeneous in protein concentration, larvae consuming high protein food grew rapidly relative to larvae consuming low protein food, but had smaller energy reserves and lower survival under starvation conditions. This tradeoff between rapid growth and starvation tolerance should reduce directional selection towards preference for exclusively high or low protein foods. Variation may suppress the growth of insects if they forage randomly. In no-choice experiments, daily change in food protein concentration reduced growth below that of larvae reared on homogeneous diets of identical mean protein concentration. Decreased growth likely arose from two sources: (1) nonlinearity in the relationship between protein concentration and food utilization efficiency, and (2) longer lags in compensatory feeding for protein as larvae switched from high to low protein diet than vice versa. There was no cost of switching per se. In contrast, variation may improve growth if insects forage non-randomly and choose food to meet changing nutritional requirements. Larvae that non-randomly mixed foods differing in protein content utilized consumed food more efficiently and achieved higher growth rates than larvae that consumed homogeneous diets. As larvae developed, they shifted preference from protein to lipid, a behavior that probably reflects changing nutritional and metabolic demands of a growing larva. By making choices, insects can avoid detrimental effects of intraplant variation in food nutrient composition, benefit from this same variation, and reduce the effectiveness of plant variation as a defensive trait.