Invasive ants and honeydew-producing Hemiptera: Partners in crime?

Background/Question/Methods: Mutualistic interactions encourage the establishment and spread of introduced species to a degree that is currently underappreciated and that may well alter how ecologists view the relationship between biotic interactions and invasion success. Ants participate in an astonishing array of mutualistic associations with other organisms, but the commonly formed partnerships between invasive ants and honeydew-producing Hemiptera (e.g., aphids, scales, membracids, and mealybugs) seem especially likely to contribute to ecological success. Results/Conclusions: Control of carbohydrate-rich hemipteran honeydew may enhance the ecological success of invasive ants in a variety of ways. Compared to native populations of the Argentine ant (Linepithema humile), for example, introduced populations appear to rely more on plant-based resources. Stable isotope (N) data on ants from Argentina (native range) and California (introduced range) indicate that relative to native populations, where L. humile is among the most carnivorous of ants, Argentine ants from California occupy lower trophic positions. Consistent with this pattern, observational surveys in Argentina suggest limited opportunities for the monopolization of honeydew-producing Hemiptera. Access to carbohydrate-rich resources, such as honeydew, can alter the behavior of invasive ants in ways that directly contribute to behavioral dominance. Recent research on L. humile, for example, demonstrates that carbohydrates fuel worker activity and stimulate aggressive behavior. Exploitation of honeydew (and other plant-based resources) might also subsidize ants at higher densities than would be possible if the ants were primarily acting as predators. In agreement with this hypothesis are data from greenhouse experiments showing that worker survival is enhanced in Argentine ant colonies that have access to honeydew-producing aphids. Given the high densities often achieved by invasive ants, their interactions with honeydew-producing insects may represent a major form of herbivory and disease transmission, which may lead in certain cases to invasional meltdown.