Review article Biological control of fire ants: an update on new techniques David F. Williams, PhD,* and Richard D. deShazo, MD† Objective: To review the present understanding of biological control methods for imported fire ants (IFAs). Data Sources: We searched MEDLINE, Biological Abstracts, and the US Department of Agriculture Formis Ant Literature database. Study Selection: All articles published in the last 10 years on biological control of fire ants were selected. Results: The decapitating flies Pseudacteon tricuspis, Pseudacteon curvatus, and Pseudacteon litoralis have been successfully released in the United States. The continued releases of multiple species of decapitating flies will expand the area of impact, applying greater pressure on IFA populations throughout the southern United States. The microsporidium Thelohania solenopsae causes the slow demise of a fire ant colony. The advantages of T solenopsae as a biological control agent include debilitation of queens, specificity for IFAs, self-sustaining infections, and lower relative tolerance to chemical pesticides. Solenopsis daguerrei has also been shown to have detrimental effects on IFA colony growth, the number of sexual reproductives produced, and the number of host queens in multiple queen colonies; however, this parasite is difficult to rear in the laboratory and to introduce into IFA colonies. Conclusions: It is unlikely that IFAs can be completely eradicated from the United States. However, technology using chemicals and/or natural control agents could eventually maintain populations at low levels if an integrated approach is used for control. Ann Allergy Asthma Immunol. 2004;93:15–22. INTRODUCTION Trinidad.8 Continued expansion of the IFA range is expected During the last several years, we have detailed the medical northward along both US coasts and southward into Mexico consequences of stings from imported fire ants (IFAs).1–4 and other areas of the Caribbean. Recently, IFAs were de- Control measures to halt the rapid expansion of their range tected in Australia (Brisbane), where eradication efforts are have failed. Recently, the US Department of Agriculture, under way,9 and in New Zealand, where they were eliminated Agricultural Research Service (USDA-ARS) instituted sev- (Ministry of Agriculture and Forestry, New Zealand, www. eral innovative fire ant control programs, which may be maf.govt.nz/biosecutiry, 2002). The USDA regularly updates useful in controlling these insects and decreasing the use of quarantines for transport of nursery stock and other agricul- insecticides. tural materials in the area of infestation (USDA, Animal and Plant Health Inspection Service, 2003)10 (Fig 1). An ultimate FIRE ANTS IN NORTH AMERICA potential range has been proposed, but the final range of the Imported fire ants, Solenopsis richteri and Solenopsis invicta, IFAs is unknown.11 were accidentally introduced into the United States between In infested areas, fire ants are the dominant arthropod,12 1918 and 1930 from South America. They currently infest resulting in a major impact on humans, animals, agriculture, more than 320 million acres in 14 states (Alabama, Arkansas, and wildlife.13–17 During the past 2 decades, expansion of the California, Florida, Georgia, Louisiana, Mississippi, New polygynous form, characterized by multiple queens rather Mexico, North Carolina, Oklahoma, South Carolina, Tennes- than the traditional single queen per colony, has resulted in see, Texas, and Virginia), with limited infestations in Ari- even greater population densities per unit area, worsening the zona, Maryland, and Delaware.5–7 Infestations also occur in a impact and the difficulties in managing this pest. number of Caribbean islands, including Puerto Rico, the Bahamas, the British and US Virgin Islands, Antigua, and EFFECTS OF FIRE ANT INFESTATION Although the exact economic costs of fire ant damage and control are unknown, estimates for the southeastern United * US Department of Agriculture, Agricultural Research Service, Center for States have been a half billion to several billion dollars per Medical, Agricultural, and Veterinary Entomology, Gainesville, Florida. 18–20 † University of Mississippi Medical Center, Jackson, Mississippi. year. This includes the IFA’s impact in urban areas, Received for publication November 6, 2003. medical costs, pesticide costs, crop and livestock losses, and Accepted for publication in revised form December 8, 2003. damage to farm implements. In addition, the cost of produc- VOLUME 93, JULY, 2004 15 Figure 1. Current imported fire ant (Solenopsis invicta) quarantine areas in the United States (www.aphis.usda.gov/ppq/maps/fireant.pdf). ing agricultural commodities, such as nursery and sod, has predigested food by other castes of ants. The chemical insec- increased in infested areas, because quarantine regulations ticides available for use are only effective for short-term mandate treatment of materials to be shipped to noninfested control (3–12 months), require periodic reapplication, and areas. can be detrimental to nontarget organisms.26 Furthermore, Fire ants are highly aggressive when their nests are dis- chemicals are not economical for large-acreage situations, turbed and cause painful stings to humans, pets, domestic such as farms, ranches, roadsides, and natural areas.27 Their animals, and wildlife. A variety of medical consequences of utility is also limited because most are not approved for use IFA stings have been reported. More than 50% of the people where food-bearing crops are grown or in environmentally in the infested areas are stung each year, with anaphylaxis sensitive areas, including wetlands, habitats of endangered 4 occurring in 0.6% to 16% of those people. More than 80 species, and animal care facilities. Because untreated areas 1–4 deaths from fire ants have been reported. serve as a reservoir of populations that reinfest adjacent Fire ants occur across a wide variety of ecological habitats treated areas, reapplication of chemicals is necessary. Re- distributed broadly across season, geography, and climate. gardless, chemicals still are one of the most important tools They are most commonly found in open, sunny habitats for rapid fire ant control, especially in those areas considered where soil has been disturbed and are ever present in parks, as significant risks for humans, such as infested yards, school pastures, yards, and cultivated fields. Like weeds, the aggres- grounds, recreational areas, or health care facilities. sive and abundant IFAs reduce biodiversity, particularly ants and other ground nesting species, including birds.21–24 Under The absence of natural enemies for IFAs in the United certain conditions, such as extreme weather and/or food States appears to have allowed the IFAs to reach much higher shortages, IFAs move indoors, where they can come in con- population densities (5- to 7-fold greater) than in South tact with and sting humans. This now presents a major America, where as many as 40 parasites, pathogens, preda- problem in health care facilities, where increasing numbers of tors, and competitors are believed to be the major controls on 28–32 debilitated individuals are being stung.3,4 density. The successful establishment of biological con- trol agents could narrow the ecological gap between IFAs and CONTROL OF FIRE ANT INFESTATION: native ants, allowing native ants to better compete. If this INTEGRATED PEST MANAGEMENT happens, fire ant populations in the United States could be Until recently, attempts to control IFAs centered on the use of reduced to the low-density levels found in South America and insecticides.2,25 To be successful in killing IFA colonies, an contact with humans and animals would be diminished.33 insecticide needs to reach the queen or queens of a colony. Thus, an integrated management strategy of biological con- Queens are sequestered deep within the colony and are fed trol agents and the judicious use of chemicals could not only 16 ANNALS OF ALLERGY, ASTHMA & IMMUNOLOGY offer longer control but also reduce the risk of pesticides to humans and the environment.33,34 Research on biological control of fire ants is currently being conducted on natural enemies by the USDA-ARS and in some universities in the United States. We will review some of the more promising techniques under study. An in-depth review of biological control agents used against fire ants has been previously published.33 We searched MED- LINE, Biological Abstracts, and the US Department of Ag- riculture Formis Ant Literature database. All articles pub- lished in the last 10 years on biological control of fire ants were selected. BIOLOGICAL CONTROL OF FIRE ANTS Two biological control agents now being used against IFAs are the small decapitating (phorid) flies belonging to the genus Pseudacteon (Diptera: Phoridae)35 and a protozoan pathogen, Thelohania solenopsae.36 Successful laboratory rearing and production techniques have made experimental field releases of decapitating flies possible.37,38 The discovery of the microsporidium T solenopsae (Knell, Allen, and Haz- ard) in fire ant populations in the United States36 and devel- opment of methods to spread the disease to other colonies39 have allowed investigation of the use of this protozoan as a biological control agent. An ant species parasitic for IFAs is also under study. DECAPITATING FLIES There are approximately 20 species of Pseudacteon decapi- tating flies that attack fire ants in South America.40,41 Some of these parasitic flies attack large workers,