Individual differences influence collective behaviour in social caterpillars

The expression of individual differences within a population often depends on environmental conditions. We investigated, first, whether there are differences between individual group-living forest tent caterpillars, Malacosoma disstria, that are expressed only in nutritionally unbalanced environments, and second, to what extent these individual behavioural differences influence the strategies used by the group to exploit food resources. We offered groups of caterpillars a binary choice between two equal food sources, either containing a balanced ratio of protein and carbohydrate or lacking digestible carbohydrate. Individual caterpillars responded to the diet treatment by becoming either inactive or active. The existence of these two behavioural categories was evident under dietary imbalance but not when foods were nutritionally balanced. At a collective level, the individual differences in behaviour led to colony decisions that were dependent upon the ratio of the two behavioural categories present in the group. Colonies comprising a majority of active caterpillars (‘active biased’) were less cohesive than inactive-biased colonies. Under dietary imbalance, active-biased groups did not focus their activity on one food source but split and exploited two sources at the same time. Since both food sources were nutritionally unbalanced, these groups grow less well than inactive-biased groups that remained on one food source. The coexistence of two foraging strategies may provide a compromise between maintaining colony cohesion and optimizing food location and diet balancing.