Eight releases of the South American fire ant decapitating fly, Pseudacteon tricuspis Borgmeier, were conducted in Louisiana from 1999 to 2006. Although P. tricuspis was initially recaptured at 88% of the release sites, four of the releases ultimately failed, so long-term establishment was only 43%. Fly populations from two releases have expanded out 50-80 km from their release sites (Fall 2006).
The Red Imported Fire Ant, Solenopsis invicta Buren (Hymenoptera: Formicidae) was first discovered in Louisiana in the early 1950s (Callcott & Collins 1996) and is now ubiquitous over the entire state. Early efforts to control S. invicta with widespread insecticide applications were met with mixed success. Concerns regarding the negative effects of these pesticides on non-target organisms and the environment led to the exploration of alternative, less ecologically disruptive, methods of S. invicta control. One such method is biological control with imported natural enemies that attack S. invicta in South America, including flies of the genus Pseudacteon Coquillet (Diptera: Phoridae).
Pseudacteon flies have been demonstrated to be effective in disrupting foraging activity of ants in the genus Solenopsis (e.g., Feener & Brown 1992; Orr et al. 1995), thereby mediating inter- and intraspecific competitive interactions among ants (Feener 2000). This phorid parasitoid could theoretically reduce the ant community dominance currently in favor of S. invicta in the United States (Feener 2000) and allow native ant species to better compete with S. invicta for resources (Porter 1998).
In 1998, the Southern Legislative Conference and the USDA-ARS created the National Fire Ant Strategy whose mandate was to rear and distribute P. tricuspis Borgmeier to cooperators in S. invicta infested areas of the southeastern United States. This effort combined with considerable support from the State of Louisiana and a supplement of flies provided by the USDA-APHIS mass-rearing program provided the foundation for decapitating fly releases in Louisiana. The aim of this paper is to describe the methodology employed in releasing P. tricuspis in Louisiana, report on the status of releases made in Louisiana thus far, and provide information on current range expansion of established populations of P. tricuspis in Louisiana.
Materials and Methods
Selection of Release Sites
In association with planned evaluations of P. tricuspis effects on S. invicta populations in Louisiana, paired release and control study areas were scouted and evaluated for their similarity in habitat and ecotones. Release and control study areas were usually unmaintained pastures that were 10 km to 30 km apart. At least 2 study plots at each site were selected and marked, except for the 2005 and 2006 releases.
Handling of P. tricuspis Puparia
Pseudacteon tricuspis were initially mass-reared at the USDA-ARS facilities located in Gainesville, FL. Puparia were shipped to Louisiana State University Department of Entomology for incubation until adult emergence. Pseudacteon tricuspis maggots develop and eventually pupariate inside individual S. invicta head capsules (Porter 1998). Head capsules were placed in small plastic cups [Solo® 59 mL Soufflé cup (B200)] containing plaster that was saturated with distilled water. These cups were held inside small plastic emergence cages (22 × 12 × 14 cm) (Fig. 1), and emergence cages were then placed within a larger environmental chamber set at 28°C with a 14:10 L:D photoperiod. Plaster blocks were moistened daily to maintain humidity (approximately 80%) inside emergence cages and minimize desiccation of puparia. We found that most flies eclosed and were capable of flight by 10:00 am (CDT). To collect flies for releases, emergence cages were placed inside a larger holding cage and the flies were allowed to escape into the larger cage where they flew toward a light source. Flies were then aspirated and counted into 20 cm × 2.5-cm tubes, one end of which contained a slightly moistened cotton wad and the other a small opening over which a mesh screen was glued. Excess moisture was squeezed from the cotton wad to minimize condensation forming on the insides of the tubes. Enough flies to treat at least 10 S. invicta mounds per day (usually 300-400) were transported in a cooler from the laboratory for field releases. Releases were done daily in the afternoons over a 7-14 d period, depending on weather conditions and availability of flies. Pseudacteon tricuspis were released at seven locations in Louisiana, with two releases done at the same location (Table 1, Fig. 2).
Flies released at disturbed S. invicta mounds. Four releases (Covington, the 2 Natchitoches releases, and Pitkin) involved simply liberating P. tricuspis adults near disturbed S. invicta mounds. Mounds were disturbed by removing approximately a 100-cm3 portion of the mound with a small spade, inverting, and flattening this portion with the spade for several seconds. Usually 20-50 flies were released at each mound by removing a cap on one end of the tubes, with the open mouth facing the mound within 30 cm and allowing the flies to escape and orient to the disturbed ants. The treatment period was 2 h, during which time S. invicta mounds were frequently disturbed to maintain ant availability to attack by P. tricuspis.
Styrofoam plate sun shields were employed to shade S. invicta mounds from full sun (Fig. 3). This was important to maintain S. invicta activity at the soil surface during phorid attacks, particularly when the weather was hot and/or the soil was dry. Soil surface temperatures achieved in full sun would otherwise increase the likelihood that S. invicta would retreat below ground to escape heat and desiccation. The upper critical thermal limit of S. invicta is reported to be around 40°C (Cokendolpher & Phillips 1990). Applying some water (0.5 liter) to the disturbed mounds during very warm (28-35°C) and dry conditions before releases caused S. invicta to remain at the surface and available to attacking P. tricuspis females for longer periods (see also Porter et al. 2004). However, when ambient temperatures exceeded 36°C ants were reluctant to remain at the surface, even with the mounds shaded and moistened (see Discussion).
Flies released in closed containers. At Norwood, Montpelier, Lake Arthur and Waverly, Louisiana we released P. tricuspis into large plastic storage containers (approx. 86 liters) containing S. invicta (Fig. 4), and also at a few disturbed mounds. Sides of containers were lined with fluon to prevent S. invicta from escaping. Solenopsis invicta from individual mounds were dug up with a shovel and deposited inside 3.8 liter talc-lined pails and the contents then carefully poured into the containers, along with brood and enough moistened mound substrate to cover the bottom (approximately 2 kg). Fire ant mounds were marked by number with wire stake flags, with the pails and storage containers marked by the same number as the source mounds to ensure correct identification of colonies when they were returned to the source mounds at the end of the treatment period. A wooden block was placed inside the storage containers to enable S. invicta workers to relocate brood to a protected area. A ventilated lid was placed over the containers and P. tricuspis were introduced through a hole of the same width as the tube containing the P. tricuspis. The exposure period was 2 h. The lid was slowly opened every 20 min and wooden blocks were repositioned to stimulate ant activity and maintain phorid attack. Treatment containers were kept in a partially shaded area throughout the 2-h treatment period to prevent overheating. After the treatment period had elapsed, treated S. invicta workers (and brood) were returned to the same mound from which they were collected and adult P. tricuspis were allowed to disperse into the surrounding environment. Treated mounds were watered (0.5 L/mound) after return of ants to reduce stress from desiccation and disturbance. When possible, releases of P. tricuspis were limited to warm, sunny days when air temperatures were >21°C, as this has been demonstrated to be the flight threshold for P. tricuspis (Morrison et al. 1999).
Determining Status of Releases
Release sites were visited at 3-4-month intervals to determine the status of P. tricuspis at release sites. Normally, 10-40 S. invicta mounds were disturbed in the late morning and afternoon. Mound disturbances involved removing one-half of the exposed mounds with a shovel, inverting, and crushing this removed portion plus ants that appeared for 5-10 s. Disturbed mounds were visited 3 or more times in rotation and all flies seen were counted. Multiple locations at release sites were sampled during each visit to obtain information about the status of P. tricuspis populations.
Evaluating Population Expansion
During the fall months (Sep-Nov) we attempted to locate the approximate limits to the populations of P. tricuspis in 4 cardinal directions (i.e., north, south, east, and west) of the release sites. At this time of year, P. tricuspis population densities are at their peak in Louisiana. Two observers each disturbed 5 S. invicta mounds and awaited arrival of P. tricuspis. Mound disturbances were the same as above for determining status of P. tricuspis releases. If no flies appeared within 30 min of mound disturbance, then the observers moved 1-2 km towards the release area. If flies were observed attacking S. invicta at disturbed mounds, then the observers selected another location 1-2 km farther away from the release area. This procedure was continued until the approximate population limits were located. Expansion surveys of the Covington release were conducted from 1999 to 2005 and from 2003 to 2006 for the Norwood release. Flies were not observed at the Norwood release location during 2001 and very few (n = 2 flies at 74 combined mounds surveyed May, Jun and Aug) during 2002.
Results and Discussion
Pseudacteon tricuspis Establishment and Population Expansion
Covington release. As of Oct 2005 P. tricuspis were found at least 60 km north, and 50 km west of the release site. Population spread to the south reached Lake Pontchartrain by Oct 2004 (29 km). We were unable to determine the eastern limit of spread into Mississippi, as populations from the Covington release and a Mississippi release had fused in 2005. However, we did observe declining, then increasing, numbers of flies as we moved from west to east in southwestern Mississippi between the cities of Picayune and Harrison.
Norwood release. As of Oct 2006, flies from the Norwood release site were recovered at least 82 km north, 55 km south and 62 km west of the release site. The eastward expanding Norwood population fused with the westward expanding Covington population in 2006.
Montpelier release. Pseudacteon tricuspis initially established at Montpelier. Flies were observed at disturbed S. invicta mounds at least 1 km south and west of the release site during Oct 2001. However, despite subsequent repeated intensive searching from 2002-2005, we failed to observe any P. tricuspis at, or near, the Montpelier release site. However, spreading populations of P. tricuspis from the Covington release to the east and the Norwood release from the west reached this location in 2006.
Lake Arthur release. Pseudacteon tricuspis were never recovered from the Lake Arthur release site after repeated visits (May, Jul and Sep 2001; Apr, Jul and Aug 2002; May 2003).
Waverly release. A single fly was observed at one mound in 15 in Jun 2002. No flies were observed on subsequent visits in Jul and Sep 2002, Aug 2003, and Aug 2005.
Natchitoches releases. A single fly appeared at 2 of 10 mounds in Jul 2002, one of 10 mounds in Sep 2002 at the Natchitoches release site and none thereafter (Jun and Sep 2003). We made a second release in Jun 2005 at the same Natchitoches location as the 2002 release. In Oct 2005, one to 3 flies were collected at 7 of 10 disturbed mounds. Successful overwintering and establishment was confirmed in Sep 2006 when flies were collected at 10 of 20 disturbed mounds. It is unknown at this time how far P. tricuspis has spread from this location.
Pitkin release. Initial establishment of P. tricuspis at the Pitkin release site was confirmed on 31 Oct 2006, when a total of 34 flies were observed at 8 of 10 mounds.
Initial confirmed establishment of our releases was 88% (7 out of 8 releases). However, our long-term establishment rate of P. tricuspis in Louisiana (43% for releases up to 2005) is lower compared to Texas releases (60%) (Gilbert & Patrock 2002) and Florida releases (66%) (Porter et al. 2004). The failure of several Louisiana releases to establish, even when first generation flies had been recovered, may be attributed to environmental events. Extreme flooding to the extent that the release sites were almost completely submerged occurred at the Montpelier (Spring 2002) and Natchitoches (Fall 2002) release sites. Prolonged drought in north Louisiana during 2002 could explain the failure of the Waverly release, but weather conditions during the time of the release at this location were often cool, with temperatures frequently near the lower flight threshold temperature for P. tricuspis. The Lake Arthur release was often hampered by high winds (>30 km/h) during the entire duration of the release. Additionally, the use of closed containers as a release method was eventually abandoned. Phorid attack rates within the containers did not appear as high as those at disturbed mounds. Each time the lid was lifted to reposition the wooden block, flies would escape.
During our 2005 release at Natchitoches, there were occasions during very hot weather (>36°C) when efforts to maintain S. invicta at the surface were ineffective, despite the use of shade and water. We observed that the majority of P. tricuspis disappeared soon after release, and S. invicta workers immediately retreated into the ground. An infrared thermometer registered unshaded bare soil surface temperatures on these occasions exceeding 55°C. These temperatures are almost certainly lethal to both S. invicta and P. tricuspis and should be considered when doing releases under extremely hot conditions that can be expected from Jun-Aug in Louisiana.
No P. tricuspis releases were made in Louisiana from fall 2002 through 2004. An effort to produce a laboratory colony of P. tricuspis at Louisiana State University (LSU) was initiated during the fall of 2002. Sufficient fly production for release was not attained until 2005. The 2005 release of P. tricuspis at Natchitoches and the 2006 release near Pitkin, Louisiana used flies produced by our rearing facility at LSU.
The authors gratefully acknowledge the following individuals for allowing access to private property: Mr. Gant Boswell, Mr. Leonard Broussard, Mr. Philip Hooper, and Dr. Jack Jones. Thanks are extended to Drs. Abner Hammond and Linda Hooper-Bui (Louisiana State University) for comments and criticisms of this manuscript. Approved for publication by the Director, Louisiana Agricultural Experiment Station as Manuscript number 06-26-0658.