Carbohydrate supply drives colony size, aggression, and impacts of an invasive ant

Abstract

Worldwide, the most ecologically damaging invasive ants are associated with introduced honeydew-producing insects, yet the mechanisms by which this mutualism may support invasive ant populations and drive impacts remain unclear. On Christmas Island, the invasive yellow crazy ant (Anoplolepis gracilipes; YCA), found in association with introduced honeydew-producing scale insects, can reach extraordinary densities, triggering an ecological meltdown. In this study, we utilized stable isotope analysis to determine resource use of YCAs in naturally occurring low- and high-density populations, and we conducted laboratory experiments mimicking availability of honeydew to measure effects of carbohydrate subsidies on colony growth and worker behavior. High-density YCA populations in nature incorporated a larger proportion of plant-based resources in their diet than low-density populations, and laboratory colonies with access to greater sugar resources had larger colony sizes, higher reproduction rates and worker survival, and higher per capita activity levels and showed higher aggression and potency toward opponents. These results support the positive feedback mechanism between honeydew-producing insects and YCAs that underlies the ecological meltdown on Christmas Island, and add to the growing body of literature illustrating the importance of mutualistic interactions in the success of invasive species.