I conducted a series of laboratory experiments to quantify the effects and elucidate the mechanisms by which Pseudacteon tricuspis Borgmeier phorid flies affect the exploitative and interference components of interspecific competition in host Solenopsis invicta Buren ants. In manipulative experiments, workers retrieved 50% less food in a foraging tray with phorids present, compared with an equidistant foraging tray with the same food resource but without phorids. The average number of workers actively foraging in the tray with phorids was significantly less than in the tray without phorids. There were no significant differences in either average worker size or average number of workers present at the food resource at the end of the trials in trays with phorids versus trays without phorids. In a control set of trials in which no phorids were added to either foraging tray, the size of foraging workers averaged over both foraging trays was significantly larger than in either the phorid or no-phorid tray of the manipulative experiments. This suggests that colonies can communicate the generalized presence of phorids in an area, which leads to a decrease in foraging by major workers. S. invicta workers retrieved an intermediate amount of food in the foraging trays of the control experiment compared with the phorid and no-phorid trays of the manipulative experiment. Moreover, the overall amount of food obtained in both foraging trays was similar for the manipulative and control experiments, suggesting that the S. invicta colonies were able to compensate for harassment by phorids by altering their foraging strategy, which resulted in no net loss of food retrieved. When S. invicta was paired with S. geminata (F.) in interference competition experiments, phorid flies had no effect on the outcome of interspecific interactions. Phorids did not appear to be attracted to host workers once they were engaged in combat with enemy workers, and the spatial distribution of fighting was significantly different than the spatial distribution of parasitization attempts.
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