A Potential Collective Defense ofDrosophilaLarvae Against the Invasion of a Harmful Fungus

The establishment of a collective defense is an important means of controlling the spread of harmful microbes in group-living animals. Collective defenses are associated with costs resulting from the investment in resources and the risk taking of infections or the exposure to microbial toxins for the performing individual and are often assumed to have evolved in (eu)social insects, like bees and ants, as a result of close contact and pathogen transmission between nestmates. We hypothesize that collective antimicrobial defense mechanisms are potentially also found in insects that exhibit simpler forms of sociality or even mere aggregation behavior. The larvae of the saprophagous fruit flyDrosophila melanogasterdevelop in high-density aggregations on rotting fruits, which are often colonized by insecticidal filamentous fungi. Here we show that fruit fly larvae suppress the invasion of a harmful fungus not only by the summative effect of individuals at high densities but also because larger groups of larvae at the same density can control fungal growth more efficiently. We achieved the necessary manipulation of the group size by increasing the number of larvae in proportion to an increase in habitat size, thereby excluding the effect of density changes on fungal growth as a confounding factor. We found evidence that part of the variation in the ability to suppress the fungus in this group size-dependent manner can be explained by genetic variation at the insects'foraging(for) locus. Group size therefore influences the extent to which the larval aggregates suppress the spread of a harmful fungus. This indicates a potential collective defense against habitat invasion by pathogenic fungi. The selection pressure on the efficiency of this potential defense strategy may contribute to the evolution of aggregation behavior in non-(eu)social insects.