INFLUENCE OF DIET QUALITY, DEVELOPMENTAL STAGE, AND TEMPERATURE ON FOOD RESIDENCE TIME IN THE GRASSHOPPER MELANOPLUS-DIFFERENTIALIS

Predictions from an insect-herbivore digestion model (Yang 1993) were empirically evaluated in this study using the nondiapausing strain of a generalist-feeding grasshopper, Melanoplus differentialis (Thomas) ( Orthoptera: Acrididae). Food residence time (FRT) within the digestive tract was manipulated by M. differentialis as a means of digestive co mpensation. Food residence time decreased linearly when diet was dilut ed from 5% total nitrogen (N) to 1% N. Food residence time was 16% sho rter in sixth-instar nymphs than in adults and was reduced 24% when am bient temperature increased from 30 degrees to 35 degrees C. Approxima te digestibility (AD) decreased linearly when diet was diluted from 5% N to 1% N. Adults digested food 43% move efficiently than sixth-insta r nymphs. Approximate digestibility was positively related to FRT. Dig estibility rate (AD per unit of FRT) was not affected by body size, se x, or temperature but increased linearly with diet quality. Diet dilut ion did not induce gut size changes when grasshoppers were confined on poor-quality,food for only 3 d. However, the allocation of total dry matter to digestive tissue varied greatly between developmental stages as well as between sexes. The proportion of gut dry weight to body dr y weight (G/B) was 30% higher in smaller-bodied sixth-instar nymphs th an in larger adults and was 6% higher in females than in males. Wet we ight gain increased with diet quality. Sixth-instar nymphs gained more weight per gram of body weight than adults at each level of N content . But the nymphs were affected more by poor food quality than adults. Temperature had no effect on weight gain at either 3% or 5% N levels. But low food quality (1% N) combined with high temperature resulted in greatly reduced weight gain, Correct interpretation of differences in RD requires knowledge of the underlying whole-organism digestive resp onses. The relative stability of digestibility rate exhibited by M. di fferentialis in response to changes in food quality, developmental sta ge, and temperature suggest that, within certain physiological limits, insects may regulate digestive means to maintain an optimal or near-o ptimal digestibility rate. Digestibility rate, indicating the amount o f limiting dietary component extracted per unit of time, should be a g ood index of an animal's digestive capability.