The dacetine ant Strumigenys arizonica, an apparent obligate commensal of the fungus-growing ant Trachymyrmex arizonensis in southwestern North America

Over 40 years ago, the dacetine ant Strumigenys arizonica was discovered in a nest of the fungus-growing ant Trachymyrmex arizonensis at Madera Canyon in the Santa Rita Mountains of the southwestern United States. This discovery suggested that the two species form compound nests, but this hypothesis has not been investigated. Here, we characterize this symbiosis through an analysis of collection records supplemented by recent field and laboratory observations. Our observations show that S. arizonica and T. arizonensis form compound nests that are a type of commensalistic symbiosis. Individuals of S. arizonica forage in galleries and tunnels of T. arizonensis nests but do not steal fungus or brood. Instead, individuals of S. arizonica hunt collembolans in the internal refuse piles of T. arizonensis nests. Interestingly, S. arizonica was never found independent of its host T. arizonensis over a significant portion of the geographic range of T. arizonensis. These results suggest a tight but asymmetric association where compound nesting is obligate for S. arizonica and facultative for T. arizonensis.