Ecological interactions of Pseudacteon parasitoids and Solenopsis ant hosts: environmental correlates of activity and effects on competitive hierarchies

1. Solenopsis (Hymenoptera: Formicidae) fire ants are host to Pseudacteon ( Diptera: Phoridae) parasitoids. The activity of S. geminata (F.) hosts and relative abundance of Pseudacteon phorids, along with five environmental va riables, were measured at weekly intervals over an 8-month period at two si tes. 2. Pseudacteon relative abundances often varied greatly from week to week, and were only weakly positively correlated with S. geminata activity. 3. A quadratic function of soil temperature was the single best predictor o f ant activity at both sites, explaining 32 and 73% of the variation in ant activity. A linear function of soil moisture was the single best predictor of phorid relative abundance at one site (r(2) = 0.23) whereas no measured variables were significant predictors of phorid relative abundance at the other site. 4. Interspecific interactions at 600 baits were monitored at a third site t o document dominance hierarchies and determine whether the presence of Pseu dacteon phorids mediated interspecific interactions in their host, S. gemin ata. 5. Solenopsis geminata was near the top of dominance hierarchies, which did not diverge greatly from a linear pattern. Three species (S. geminata, S. invicta Buren, and Crematogaster laeviuscula Mayr) won the majority of thei r interspecific interactions and appear to be co-dominants at this microhab itat-rich site. 6. Overall, the presence of phorids had no significant effect on the outcom e of interspecific contests involving S. geminata and all other ant species grouped together. Phorids may have contributed to some of the S. geminata losses against other co-dominant species.