Cooperation vs. competition: Factors influencing the success of group-founding queens in the red imported fire ant

Social insects represent a unique opportunity to study the evolution of group behaviors. Reproductives that establish colonies in groups must balance benefits gained from cooperation (more rapid colony growth; increased genetic diversity creating herd immunity and behavioral flexibility) and the costs due to competition (queen and worker-mediated effects on survival and reproductive output). This cost-benefit tradeoff is expected to be further modified by queen relatedness and differences in competitive ability. Red imported fire ants (Solenopsis invicta) are a highly invasive pest species in the southern US. Their invasive success has been accompanied by a shift to genetically diverse colonies which accept many unrelated queens. In this study we looked at the effect of queen number, weight, and place of origin (a measure of genetic distance) on queen productivity, worker development, and overall colony growth for founding groups of queens. Groups with more queens produced more workers (p=.0376) in a non-additive fashion, indicating the presence of other queens reduced individual performance. Larger queens significantly reduced worker development time (p=.0008), while groups with greater queen size differences exhibited decreased effects of competition on worker development (p=.0383). Finally, groups of queens collected in the same area significantly out-performed both single queens and mixed groups of queens collected from different areas (p=.0487), indicating that queen relatedness may play an important role in mitigating queen competition. These results provide insight into the invasive success and ecological effects of fire ants and may assist in the development of management techniques for these and other social insects.