VARIATION IN FOOD QUALITY AND TEMPERATURE CONSTRAIN FORAGING OF GREGARIOUS CATERPILLARS

Hemileuca lucina (Saturniidae) specializes on the shrub Spiraea latifolia (Rosaceae). First-instar larvae, which in nature encounter only new leaves, showed no preference for new leaves over mature ones. Third-instar larvae, which normally encounter both leaf types, exhibited a preference for new leaves. Larvae in groups were more likely to survive and exhibited less variation in biomass gained than solitary larvae. An experiment determined the interactions of temperature (with daytime temperatures of 20°, 25°, or 30°C), group size (solitary or in groups of 10), and diet (new or mature leaves) on 3rd instar larvae. At a daytime temperature of 20°, representing overcast and cool spring conditions, growth rates were similar for all group size-diet combinations; there was no advantage in feeding on new leaves or in larval groups. 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 (25°) for field conditions. At 30°, representing the average temperature reached through basking, larvae eating new leaves grew significantly faster than those on mature leaves, regardless of larval group size. Thus, in the absence of predators, in which case larvae eat new leaves, bask and remain aggregated, 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. -from Authors