Range Expansion of Two Phorid Flies (Diptera: Phoridae) In Imported Fire Ant Populations (Hymenoptera: Formicidae) in Alabama

L.C. ‘Fudd’ Graham1, R.K. Vander Meer2, V.E. Bertagnolli1, K Ward 3and R. Ward3

Department of Entomology and Plant Pathology, Auburn University, Auburn, Alabama [1]USDA-ARS, CMAVE, Imported Fire Ants and Household Insects, Gainesville, Florida [2]Department of Plant and Soil Science, Alabama A&M University, Normal, Alabama [3]

When fire ants were introduced into Alabama in the early 1900's, almost all of their natural enemies were left behind in South America (Jouvenaz 1990). As a result, fire ant densities are much higher in Alabama than they are in South America (Porter et al. 1997). Two species of imported fire ant occur in Alabama. The red imported fire ant, Solenopsis invicta, is located in the southern portion of the state and the black imported fire ant, Solenopsis richteri, is located in northwest Alabama.

Although the black imported fire ant was introduced into the United States before the red imported fire ant, its current range is thought to be northeastern Mississippi and northwestern Alabama. Vander Meer et. al. (1985) first detected a hybrid between the two species in Mississippi. The hybrid is thought to populate the northern tier of Alabama, Mississippi and Georgia .

One group of natural enemies that have shown some promise in the battle against fire ants are phorid flies (Figs. 1 & 2) in the genus Pseudacteon (Porter 2000). Currently, eleven populations of phorids have been successfully established in Alabama (Fig. 3). Pseudacteon tricuspis shows a strong preference for S. invicta and is established at seven sites in South Alabama. Pseudacteon curvatus shows a strong preference for S. richteri and the hybrid fire ant. It is established at four sites in North Alabama (releases in Madison and Lauderdale Counties by K. Ward).

Introduction

Methods and Materials

Releases of P. tricuspis and P. curvatus were conducted as described by Graham et al. (2004 and 2003). The first successful release was in 1999 and new releases have been conducted yearly in different counties(Fig 3).

A release site and a corresponding control site approximately 9.5 km apart were selected in Macon and Talladega. In Houston, Lowndes, Walker and Cullman, these sites were ca. 32 km apart. Two plots were established at each site for bait stations and pit fall traps. Sampling areas for population data were set up in conjunction with each plot. Data collected in each sampling area were total number of mounds and mound size. At the Baldwin, Barbour, Marengo, and Tuscaloosa sites, Solenopsis spp. populations are monitored by counting mounds in three 0.1 ha circles near the control site.

A grid was superimposed on a map of Alabama. The grid is an extension of the one used by Diffie et al. (2002) in Georgia. Worker ants were collected from three mounds at or near each intersection on the grid. These sites were surveyed during 2003-2004. Ants were collected from each site by inserting a 30 x 80 mm plastic tube into a mound and capping it once at least 25 ants were collected. The ants were prepared as describe by Vander Meer et al. (1985) and were shipped to CMAVE in Gainesville, FL for species determination by R. Vander Meer.

Fig. 4

Field-reared P. tricuspis were found at the Macon County release site in fall of 1999. Flies reached the control site by fall of 2000. Once field-reared flies were established, the number of mounds decreased at both sites each sampling period until Spring 2002.

P. curvatus were first found at the Talladega County release site in fall of 2000. They were found over 50 km south of the control site in 2004. As in Macon County, the number of mounds decreased each sampling period once field-reared flies were present.

The white arrow on a graph indicates the approximate time field-reared phorids were first located at the site.

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Diffie. S., R. K. Vander Meer, and W. Gardner. In Proceedings of the 2002 Imported Fire Ant Conference. Graham, L. C., S. D. Porter, and V. E. Bertagnolli, 2003. J. Agric. Urban Entomol. 20(3): in press.Graham, L. C., S. D. Porter,R. M. Pereira, H. D. Dorough, A. T. Kelley, 2003. Fla. Entomol. 86: 334-339Jouvenaz, D. P. 1990. In “Applied Myrmecology: World Perspective” pp. 620-627. Porter, S. D., Williams, D. F., Patterson, R. S., and Fowler, H. G. 1997. Environ. Entomol. 26 :373-84.Porter, S. D., 2000. Biol. Control 19: 35-47.Vander Meer, R. K., C. S. Lofgren, and F. M. Alvarez. 1985. Florida Entomol. 68:501-506.

References

Results

P. tricuspis reached the Macon control site in mid- to late-summer of 2000. P. curvatus were found approximately 1.6 kilometers north of the control site in Talladega in August 2002, but were not found south of the site until 2004. Flies from the Lowndes, Houston, Walker and Cullman sites have not reached the control sites.

Population data are presented from the two oldest sites (Fig. 4). Droughts occurred in Alabama in 1999, 2000, and 2001. In addition, the coldest November and December on record were recorded in 2000. These environmental factors have influenced fire ant populations, as evidenced by the low number of mounds in May 2001 and reduction in mound areas in 2001 and 2002 at the Talladega control site, where no phorids had been found. Even though it appears that the phorid flies may have affected the fire ant populations at these sites, further study will be required to determine if these population reductions are permanent and due to the flies and not other factors.

The two Pseudacteon spp. released in Alabama have been recovered at eleven of twelve release sites and are spreading rapidly across the state and into Georgia (Fig. 5). These populations were mapped extensively during the 2004 field season. The ovals estimate the currently mapped ranges of the eleven populations (see Fig. 3). The ranges of the Macon and Talladega are underestimates. We have not found the leading edges of these two populations.

P. tricuspis and P. curvatus have expanded their ranges to cover over 35,000 sq. miles in Alabama and Georgia (Fig 5). P. curvatus, which prefer S. richteri and the hybrid fire ant, are now moving into populations of S. invicta (see green squares) and we expect to find P. tricuspis on hybrid fire ant populations soon.

We hope to obtain Pseudacteon litoralis and the Formosa biotype of P. curvatus that is host specific to S. invicta when these two species are available for release. We are collecting ants along the grid (Fig. 5) to determine the location of each imported fire ant species in Alabama. This will allow us to release new species of phorid flies on their preferred host.

Sites in the area where P. curvatus and P. tricuspis now coexist have been monitored since 2003, when no flies were present. Hopefully, the presence of more than one species of these parasitoids in an area will reduce fire ant numbers more dramatically and permanently than the reductions seen in Fig. 4 above.

Acknowledgements

The authors wish to thank S. D. Porter, USDA-ARS, CMAVE, J. T. Vogt, USDA-ARS, BCMRRU, A. Callcott and D. Roberts, USDA-APHIS, H. Dorough, C. Mason, D. Daniels, M. Faver, R. Hudson, D. Cain, K. Tucker, C. Pinkston, M. Mobley and K. Flanders, Ala. Coop. Ext. Sys.

Fig. 2Fig. 1

Fig. 3. Pseudacteon spp.releases in Alabama

Fig. 5 Range of Pseudacteon and Solenopsis spp. in Alabama

Photo courtesy of S. Bauer Photo courtesy of S. Porter