Testing for population-level impacts of introduced Pseudacteon tricuspis flies, phorid parasitoids of Solenopsis invicta fire ants

Parasitoid phorid flies in the genus Pseudacteon are potential biocontrol agents of invasive Solenopsis fire ants in the US. One Pseudacteon species, P. tricuspis Borgmeier, has been released and is now established over a large region of north central Florida. We conducted a multi-year, large-scale field experiment to investigate potential population-level impacts on host Solenopsis invicta Buren fire ants and associated arthropods. We established and monitored 67 field sites over a 3.5 year period, documenting S. invicta density and activity by three complementary sampling methods: (1) mound counts and measurements, (2) attraction to baits, and (3) pitfall trap catches. P. tricuspis abundance was determined by censusing flies at disturbed S. invicta mounds. P. tricuspis populations increased in abundance over the first two years following colonization and then oscillated seasonally, reaching higher abundances in the fall than in the preceding spring. S. invicta populations varied greatly over time, and repeated-measures ANCOVA revealed no effect of phorid parasitism pressure on S. invicta density or activity. P. tricuspis likely has a relatively small effect on S. invicta populations, which may not be consistent over space and time and is difficult to document against the background of high variability in fire ant populations due to various environmental factors (i.e., rainfall, disturbance). P. tricuspis parasitism pressure (averaged over the course of the experiment) was positively associated with changes in S. invicta density. Additionally, P. tricuspis abundance was positively correlated with S. invicta density after P. tricuspis populations had been established for >1 year. Introduced phorid populations may require many years before a measurable impact is apparent. Alternatively, parasitism pressure of a single phorid species may not be a major factor regulating fire ant populations.