Evaluating biological control of fire ants using phorid flies: effects on competitive interactions

Abstract

Our first objective was to assess the host specificity of the parasitic phorid fly, Pseudacteon tricuspis Borgmeier, in the laboratory against two ant species native to the United States, Solenopsis geminata (F.) þ S. xyloni McCook and Forelius pruinosus (Roger) as well as the invasive Solenopsis invicta Buren. Parasitic phorid flies are thought to suppress fire ant populations by shifting worker ant behaviors from foraging to defense. Reduced foraging would result in decreased nutrient intake for the ant colony and, consequently, a decrease in competitive ability with other ant species. Therefore, host specificity was tested in terms of fly attack preference and also ant behavioral response to the presence of the flies, as these behaviors may influence the ability of P. tricuspis to mediate interspecific competition. Pseudacteon tricuspis was tested under choice and no-choice conditions. Choice tests were included to examine the possibility that P. tricuspis will inadvertently attack native ants if stimulated into attack behavior by the presence of S. invicta. The flies were never observed attacking either native ant species in choice or no-choice tests, whereas flies readily attacked S. invicta in both test types. Furthermore, only S. invicta responded to the presence of the flies via characteristic defensive postures. Pseudacteon tricuspis, therefore, was determined to be highly host specific to S. invicta relative to Solenopsis geminata [x] xyloni and F. pruinosus. Our second objective was to investigate the effects of P. tricuspis on the competitive interactions between S. invicta and F. pruinosus. We tested the effect of P. tricuspis on the colony growth rates of S. invicta and F. pruinosus when the two ant species were competing for protein in laboratory competition arenas. We also quantified the effect of the phorid flies on the foraging rates of S. invicta and F. pruinosus. Though S. invicta significantly reduced its foraging rate in the presence of the phorid flies, we did not detect an effect of the flies on colony growth rates of either species. We conclude that P. tricuspis is unlikely to have a measurable effect on S. invicta populations in the field.