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United States Department of Agriculture

Agricultural Research Service

Title: Testing for Population-Level Impacts of Introduced Pseudacteon Tricuspis Flies, Phorid Parasitoids of Solenopsis Invicta Fire Ants

Authors
item Morrison, Lloyd
item Porter, Sanford

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 5, 2005
Publication Date: February 12, 2005
Citation: Morrison, L.W., Porter, S.D. 2005. Testing for population-level impacts of introduced Pseudacteon tricuspis flies, phorid parasitoids of Solenopsis invicta fire ants. Biological Control. 33: 9-19.

Interpretive Summary: Parasitoid phorid flies in the genus Pseudacteon are potential biocontrol agents of invasive Solenopsis fire ants in the U.S. One Pseudacteon species, P. tricuspis Borgmeier, has been released and is now established over a large region of northern Florida. Scientists working at the USDA-ARS, Center for Medical, Agricultural, and Veterinary Entomology, in Gainesville, FL, conducted a multi-year, large-scale field experiment to investigate the potential population-level impacts on host S. invicta Buren ants and associated arthropods. We established and monitored 67 field sites over a 3 1/2 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, although no detrimental effect of phorids was found on S. invicta populations. P. tricuspis may have a very small effect on host P. tricuspis 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). Introduced phorid populations may require many years before a measurable impact is apparent. Alternatively, parasitism pressure of a single species may not be a major factor regulating fire ant populations.

Technical Abstract: Parasitoid phorid flies in the genus Pseudacteon are potential biocontrol agents of invasive Solenopsis fire ants in the U.S. One Pseudacteon species, P. tricuspis Borgmeier, has been released and is now established over a large region of north Florida. We conducted a multi-year, large-scale field experiment to investigate potential population-level impacts on host S. invicta Buren fire ants and associated arthropods. We established and monitored 67 field sites over a 3 1/2 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 host 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 change in S. invicta density. Additionally, P. tricuspis abundance was positively correlated with S. invicta density after P. tricuspis populations had reached an apparent equilibrium. 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.

Last Modified: 12/21/2014
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