Parasitoids and competitors influence colony-level responses in the red imported fire ant, Solenopsis invicta.

Social insect colonies respond to challenges set by a variable environment by re-allocating work among colony members. In many social insects, such colony-level task allocation strategies are achieved through individual decisions that produce a self-organized adapting group. We investigated colony responses to parasitoid and native ant competitors in the red imported fire ant ( Solenopsis invicta). Parasitoid flies affected fire ants by decreasing the proportion of workers engaged in foraging. Competitors also altered colony-level behaviours by reducing the proportion of foraging ants and by increasing the proportion of roaming majors, whose role is colony defence. Interestingly, the presence of both parasitism and competition almost always had similar effects on task allocation in comparison to each of the biotic factors on its own. Thus, our study uniquely demonstrates that the interactive effect of both parasitism and competition is not necessarily additive, implying that these biotic factors alter colony behaviour in distinct ways. More generally, our work demonstrates the importance of studying the dynamics of species interactions in a broader context.