Species abundances in natural systems are usually close to some equilibrium making the mechanisms that maintain or prevent species co-existence and that determine relative abundance difficult to discern. Biological control projects provide an opportunity to observe systems transition between equilibriums as a result of the influence of the newly introduced species. In the southeastern United States and Texas, species of phorid fly parasitoids are being sequentially introduced as biological control agents for imported fire ants. Two species are currently widespread in Texas: Pseudacteon tricuspis introduced first and currently distributed through most of central Texas, and P. curvatus, introduced second and currently restricted in distribution. These species partition the host niche based upon body size, and co-exist broadly in their native range in Argentina. The a priori expectation was that they would form a co-existing and complementary suite of parasitoids in their introduced North American range. Using trap based sampling, this study examines the population level interaction between these two parasitoids.
Results/Conclusions

Both samples within sites across years and simultaneous samples of sites varying in time since colonization by P. curvatus reveal a strong reduction in P. tricuspis densities as P. curvatus establishes at a site. However, the speed of this reduction appears to depend upon habitat. At the release site where P. curvatus has been established the longest, this population interaction approaches complete displacement. This result is surprising given the apparent complementary nature of these parasitoids natural history and the widespread co-existence of these species in their South American native range. A re-examination of published data from their native range reveals that a negative relationship across sites between the densities of these two species also exists there, suggesting that the same processes as those reported here from Texas also operate in South America.