Heliconius Caterpillar Mortality during Establishment on Plants With and Without Attending Ants

Rates of disappearance (mortality) in Heliconius caterpillars of various sizes were investigated by experimental placement on Passiflora host plants growing in their native rain forest habitat. Survivorship of experimental larvae was similar to that of Heliconius larvae in natural populations: 60% mortality during the initial 2 d, followed by 30% mortality during each 2—d period thereafter. Survivorship was not dependent on larval size or instar, nor was it related to larval growth rate or host plant isolation from surrounding foliage. However, survivorship was appreciably lower on host plants on which ants were attending extrafloral nectaries (70% vs. 45% mortality in first 2 d), indicating that ants may be a primary source of high mortality. I conclude that for these Heliconius, high first—instar mortality is not due to the small size of the caterpillars but rather is a consequence of ant attendance to some host plants; most caterpillars on plants with ants are killed in the first instar, while caterpillars on ant—free plants have much lower mortality and make up the majority of the cohort in later instars.