VARIATION IN FOOD QUALITY AND TEMPERATURE CONSTRAIN FORAGING OF GREGARIOUS CATERPILLARS

We examined the separate and combined effects of changing food quality, thermal environment, and larval behavior on the foraging pattern of a gregarious caterpillar, Hemileuca lucina (Saturniidae), which specializes on the shrub Spiraea latifolia (Rosaceae). First-instar larvae, which encounter only new leaves under field conditions, showed no preference for new leaves over mature ones. In contrast, third-instar larvae, which normally encounter both leaf types, exhibited a preference for new leaves, in both 12-h and instar- length choice tests. In laboratory tests, larvae in groups were more likely to survive and exhibited less variation in biomass gained than solitary larvae. An experiment was con- ducted to determine the interactions of temperature (with daytime temperatures of 200, 250, or 300C), group size (solitary or in groups of 10), and diet (new or mature leaves) on third-instar larvae. The effect of diet and group size on relative growth rate depended on the daytime temperature. At a daytime temperature of 200, representing overcast and cool spring conditions, growth rates were similar for all group size-diet combinations. Thus, at such a cool temperature, there was no advantage in feeding on new leaves or in larval groups. In contrast, solitary larvae fed mature leaves gained less mass and developed more slowly than the other group size-diet combinations at the mean maximal day temperature (250) for field conditions. At 300, representing the average temperature reached through basking, larvae eating new leaves grew significantly faster than those on mature leaves, regardless of larval group size. These results indicate that in the absence of predators, in which case larvae eat new leaves, bask, and remain aggregated, the larvae can grow twice as fast and attain more biomass per instar than solitary larvae eating mature leaves in the shade, which is often the case when they avoid predators.