DIET HETEROGENEITY - IMPLICATIONS FOR GROWTH OF A GENERALIST HERBIVORE, THE GYPSY-MOTH

Within-plant variation in food nutritional quality is a fundamental feature in the foraging environment of herbivores, but little is known as to how such small-scale variability affects individuals or structures populations of herbivores. The selective pressure favoring choosy herbivores may depend on the relative performance of random foragers, which will experience chronic changes in food quality, vs. non-random foragers. To determine the effect of involuntary variation in nitrogen intake on larvae of a polyphagous insect, gypsy moth (Lymantria dispar (L.)), I reared individuals on one of three diet treatments: (1) Constant nitrogen: larvae received 3.0% N diet continuously; (2) Low variance: larvae were switched each day in an alternating sequence of 2.25% and 3.75% N diet; and (3) High variance: as in Low variance, except larvae were alternated between 1.25% and 4.75% N diet. Despite equal relative consumption rates (RCR) and nitrogen consumption rates (RNCR) across all treatments, larvae experiencing variation in the diet suffered reduced pupal mass and extended development time. Evidently growth performance cannot be predicted based solely on mean diet nitrogen concentration, which was equal among all treatments (3.0% N). Further, growth on a mixture of foods cannot be predicted based on known responses to the individual foods. Reduced growth associated with the chronic variation likely was caused, in part, by nonlinearity in the relationship between nitrogen and food utilization efficiency, and by disruption of the compensatory feeding response. These results suggest that intraplant variation in food quality has the potential to exert strong selective pressure for regulation of variation in the total diet.