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Group Behavior in Lepidoptera

There have been multiple observed instances of aggregate behavior in Lepidoptera. Such behavior has been suspected to develop as a response to certain environmental selection pressures, like the presence of predators, parasites and/or scarcity of food. Moths or butterflies in the larval stage will often stay together in groups to feed together for a significant portion of their lives. Later on, some Lepidoptera in their adult phase will exhibit group behavior like Migration, found in Monarch Butterflies

Group Behavior in Larvae

Both social and solitary larvae behavior can be found in Lepidoptera. Social, or gregarious feeding behavior usually occur in the early stages of larval life. Such behavior declines or changes to solitary by the last stage of the larva's life. Some species, like E. Chrysippe exhibit social behavior in all instars. There are certain costs and benefits associated with the development of aggregation tendencies. Whether or not a species develops these tendencies can be predicted by weighing their associated costs and benefits.

Benefits

Facilitated Feeding

Depending on the species, certain benefits are reaped more than others. A common benefit is facilitated feeding, where multiple larvae will make "feeding wounds" on objects that are tougher to consume, like sunflower leaves. The young larvae will collectively cut away trichomes, making the leaves much more easy to consume and digest. Such behavior is found in c. lacinia Facilitated feeding not only helps with the breakdown of food, it has also been suggested that sibling larva eating together exhibit "coaxing" behavior where each member is encouraged to eat more. The higher intake of food allows these social groups of larvae develop faster than their solitary counterparts, and complete early stadia more rapidly as compared to solitary larvae.

Anti-Predation

Grouping has also been seen to provide anti-predatory measures. In some species such as H. Oliviae, larvae will group together to form highly visibly colored masses. This, coupled with their urticating spines deter predators from consuming the larvae due to the stinging or toxic nature of the spines. Such defenses work best against inadvertent predators like cattle, bison, or large herbivores, who are more likely to recognize and be deterred by the mass of brightly-colored hairy caterpillars

Resistance to Viruses

There is recent evidence suggesting that gregarious behavior in Lepidoptera is correlated with development of age-dependent resistance to certain viruses and parasites. Such a phenomena has been observed to develop independently in several phylogenetic lineages.

Costs

Increased Predation

Feeding in groups renders the larvae much more visible to any would-be predators.

Increased Susceptibility to Viral/Parasitic Spreading

Another cost of staying within close proximity of other larvae is an increased susceptibility to virus or parasite spreading. Despite having developed age-dependent viral resistance, species like H. Zea are still very susceptible to the spreading of viruses or parasites due to their close proximity to each other. Their development of resistance can be viewed as a response to such viral/parasitic concerns. Also, H.Zea have been shown to exhibit gregarious feeding when young, but as they age, begin to change their behavior to become more solitary, perhaps to offset viral susceptibility. The mechanism to such a phenomenom is still yet to be understood.

Mechanisms of Aggregation

Such group behavior sometimes begin before birth, when a mother decides where to deposit her eggs. Factors that seem to influence ovipositioning include potential nutritious value of plant, proximity to potentially helpful ant populations [found in Ogyris amaryllis in Australia], and plant chemistry, among others.

It has also been shown that in H. Oliviae, methylene chloride-extractable chemical(s) from range caterpillar larval silk facilitates trail-following and aggregation by early-instar larvae, but late-instar larvae are less responsive to the pheromone.