Mutualisms between ants (Hymenoptera: Formicidae) and honeydew-producing insects: Are they important in ant invasions?

Recent studies suggest that the extraordinary success of invasive ants may be facilitated by facultative mutualisms with honeydew-producing insects. I evaluate this possibility in a review of the literature, focusing on five invasive ants that exhibit exceptionally large populations and whose impacts are considered to be most severe: Anoplolepis gracilipes (FR. SMITH, 1857), Linepithema humile (MAYR, 1868), Pheidole megacephala (FABRICIUS, 1793), Solenopsis invicta BUREN, 1972, and Wasmannia auropunctata (ROGER, 1863). For each, I consider whether they are strongly associated with honeydew-producing insects in their introduced range, whether honeydew or its constituent nutritional components can promote large population sizes, whether honeydew is utilized to a greater extent in the introduced than native range, and whether the ants promote larger populations of honeydew-producing insects, thereby further increasing honeydew availability. While each of these questions could not all be answered for all species, the evidence is consistent overall with the importance of honeydew-producing insects to invasive ants. Nevertheless, definitive studies remain to be conducted. Important questions to address include whether invasive ants are strongly associated with honeydew-producing insects throughout the broad geographic ranges they often occupy, and if the degree of association varies among locations, whether those locations also exhibit concordant differences in the size and impacts of invasive ant populations. In addition, it remains unclear whether associating with honeydew-producing insects may only be important in facilitating large populations of invasive ants, or whether those associations may also be important in determining if an introduced species actually becomes invasive. Finally, much of the work to date has focused on the characteristics of invasive ants, and little is known about the honeydew-producing insects that may promote them. What is known suggests that those most important to invasive ants may often be introduced or invasive themselves, and if so they may also exhibit important traits that facilitate their abundance at introduced locations.