Does fly larval cooperative behavior protect against parasitic wasps?

Larval Drosophila exhibit cooperative foraging behavior in which groups of animals synchronize digging movements to form deep feeding cavities. Such clustering has been hypothesized to provide protection from predators and parasitoids by reducing surface exposure. Here, we tested whether clustering by Drosophila melanogaster larvae protects against parasitism by two Leptopilina species—L. boulardi (Lb), a specialist that primarily targets D. melanogaster, and L. heterotoma (Lh), a broadly infective generalist. Using controlled manipulations of larval number, sensory disruption, and environmental conditions, we examined how clustering affects wasp behavior, wasp mortality, infectivity, and the behavior of infected larvae. Contrary to expectation, larval clusters did not repel wasps, increase wasp mortality, or reduce parasitism. Instead, clustering increased the probability of successful infection, and Lb—but not Lh—infection altered host behavior in ways that promoted prolonged cluster residence. Lb infected larvae exhibited reduced coordination yet remained within clusters significantly longer than uninfected animals. Finally, the survival of infected larvae decreased in the presence of increasing numbers of uninfected siblings, an effect dependent on clustering. These results potentially reveal a bidirectional interaction in which parasitism enhances larval clustering and clustering enhances parasitism. Our findings challenge the assumption that cooperative digging evolved as an anti-parasitoid defense and instead suggest that parasitoid exploitation of social microenvironments may shape larval group behavior.