Towards a nutritional ecology of invasive establishment: aphid mutualists provide better fuel for incipient Argentine ant colonies than insect prey

Many potential species invasions fail before establishment. This is likely especially true for invasive Argentine ants that must overcome a severe founding bottleneck and transition from propagules that rely on protein-rich prey to massive supercolonies that dominate by consuming carbohydrate-rich honeydew from hemipteran mutualists. While this dietary shift supports the classic idea that protein fuels early colony development and carbohydrates maintain adult workers, recent evidence suggests that carbohydrates can govern initial colony establishment. In this study, we use lab experiments to show that resources from aphid mutualists had greater benefits for Argentine ant propagule survival, maintenance, growth, and worker activity rates than did prey items. These effects persisted at low aphid densities, and when colonies were otherwise starved. Moreover, prey-starved colonies did not appear to consume aphids, suggesting that carbohydrate-rich honeydew is a mechanism that facilitates colony establishment. Combined, these results support a hypothesis that the dietary shift from prey to honeydew is driven more by increased access to hemipterans after establishment, than by specific benefits of prey early in colony development. The results highlight the important role of nutritional ecology for studying invasive establishment, linking propagule success not only to the supply of food resources, but also to their quality.