Leaf Quality and Enemy Avoidance by the Larvae of a Pyralid Moth

Low nutritional quality of mature plant leaves generally leads to slow growth of herbivores. Slow growth, in turn, is thought to increase the vulnerability of herbivores to their annual enemies. I investigated the relative importance of nutritional quality and enemy avoidance to the larvae of a pyralid moth, Omphalocera munroei, that feed almost exclusively on the old leaves of their host plant (Asimina spp.). The effect of leaf age on development rate, the importance of predators and parasitoids as mortality agents, and the interactions between leaf quality and enemy avoidance were studied in field manipulations using larvae placed on naturally growing plants. Larvae of O. munroei developed 20% more slowly on old leaves than they did not young foliage. Predators, which accounted for most of the mortality of O. munroei caterpillars, reached maximum densities at the same time that O. munroei populations peaked. Two aspects of larval feeding behavior, leaf tying and gregarious feeding, reduced the impact of natural enemies. Leaf ties built of old leaves were more effective at reducing predating than were those built of young leaves, probably because old leaves maintained their shape, while young leaves wilted, following feeding by O. munroei. The greater rigidity of old leaves, however, required that O. munroei feed in groups of at least 20 caterpillars, which could then pull the leaves together. A group size of 20 coincided with the mean size of egg masses laid by females in the field. Given a choice, larvae preferred nutritionally poor old leaves arranged to form a shelter rather than nutritionally superior young leaves without shelter, indicating that protection from enemies was more important than nutrient to the larvae. The results also suggest that slow growth need not invariably lead to greater exposure of herbivores to natural enemies.