Fire Ant Decapitating Fly (Pseudacteon spp.)

Biocontrol Agent Factsheet

Pseudacteon decapitating flies parasitize and kill fire ant workers and inhibit colony activities like foraging and mound building. 

Common Names

Decapitating flies

Relative effectiveness

Unfortunately, decapitating flies released to date have not been very effective. The impact of decapitating flies on fire ant colonies is sufficient to have caused the evolution and maintenance of a series of defensive behaviors affecting fire ant foraging and mound building. However, a multi-year field experiment in Florida did not find measurable effects of introduced P. tricuspis flies on imported fire ant abundance (Morrison and Porter 2005a). The release of additional decapitating fly species may have improved impacts (Callcott et al. 2011), but field parasitism rates usually do not exceed 1-3% (Morrison et al. 1997, Morrison and Porter 2005b, Calcaterra et al. 2008).

Where to use

Anywhere fire ants are found, from agricultural settings, to yards and other managed landscapes and structures

About Fire Ant Decapitating Flies

Pseudacteon decapitating flies parasitize and kill fire ant workers and inhibit colony activities like foraging and mound building. They also have the potential to vector pathogens. More information about this biocontrol agent is available on University of Florida’s Featured Creatures: Fire Decapitating Flies.

Six species of Pseudacteon flies have been successfully released and established in the United States and Puerto Rico (Chen and Morrison 2021). It is estimated that P. tricuspis now occurs in at least 65% of the imported fire ant range, while P. curvatus probably occurs in nearly 100% of S. invicta/S. richteri range in the US (Callcott et al. 2011). Pseudacteon obtusus occurs in large areas of Florida, Texas, and is likely found in adjoining states. Three other species P. litoralisP. nocens, and P. cultelatus have been established in localized areas of Alabama, Texas, and Florida, respectively (Callcott et al. 2011, Plowes et al. 2012, Porter et al. 2013).

  • Native/Non-native: Non-Native
  • Preferred climate: humid, wet, temperate, subtropical
  • Region: Southeast and Southwest US, Puerto Rico, US Virgin Islands
  • Established: Yes
  • Where established: Virginia, North Carolina, South Carolina, Georgia, Florida, Tennessee, Alabama, Mississippi, Louisiana, Arkansas, Texas, Oklahoma, California, Puerto Rico, US Virgin Islands

Fire Ant Decapitating Flies Appearance

Pseudacteon flies are very small, about the size of their host ant's head (1-2 mm). Seen under a microscope, adult Pseudacteon flies have relatively large eyes, a sort of humped back and, in females, an elaborately shaped ovipositor that varies among species. In the field, they appear as minute, fuzzy specks as they hover over host ants.

Close-up photo of a fire ant worker with a small fly maybe a quarter of its size hovering nearby

Female decapitating fly preparing to lay an egg in the body of a fire ant worker.

Close-up of a fire ant decapitating fly (Pseudacteon nocens)

Side view of a fire ant decapitating fly (Pseudacteon nocens)

How to Use Fire Ant Decapitating Flies

Biocontrol category: Classical - released once and persists 

Pest stage: Adult worker ants

Mode of action: Larva kills ant workers and adult flies inhibit ant foraging 

Compatibility: Little is known. It has been found that exposure of parasitized ants to methoprene and pyriproxyfen (active ingredients in some fire ant baits) can reduce emergence of one species of decapitating fly (P. tricuspis; Farnum and Loftin 2010). (Any time you use a pesticide, you must read and follow the label directions and comply with all applicable laws and regulations related to pesticide use. Also be sure that any pesticide used is approved for use in your country and state/province. )

Risk: The Pseudacteon decapitating flies released in the United States are highly host-specific. They will only attack fire ants in the genus Solenopsis and will only successfully develop in imported red and black fire ants. Pre- and post-release studies confirm that they are not attracted to other ants, other arthropods, other animals, people, fruits, garbage, carrion, or feces. They do not harm crops, ornamentals, or native plants. Because of their specialized life history, these flies are only a risk to other ants and then only on evolutionary time scales of thousands to millions of years (Porter and Gilbert 2004 (pdf)). 

Commercially available: No

About fire ants targeted by decapitating flies

  • Red imported fire ant (Solenopsis invicta)
  • Black imported fire ant (Solenopsis richteri)
  • Hybrid imported fire ant (Solenopsis invicta x richteri)

Fire ants in the genus Solenopsis often have large earthen mounds with tens of thousands to hundreds of thousands of workers of different sizes, each of which can sting quickly when disturbed. Solenopsis fire ants are myrmicine ants with 2 waist segments, a thorax that lacks spines, and antennae with 10 segments ending in a 2-segmented club.

Fire ant damage

Fire ants are pests of many kinds of agricultural crops. For example, they damage soybeans, potatoes, sprouting corn seeds, and citrus saplings. Fire ant stings are often a human health problem, especially to young children and anyone with allergies to their venom. Fire ants also damage electrical equipment including transformers, air conditioning compressors, and airplane landing lights. Fire ants also cause widespread problems for wildlife and native species.

A ring of different sized fire ant workers ordered from large to small with a queen shown to the right. A scale indicates worker ant sizes range from 2 to 5 mm, while the queen is nearly 10 mm long.

Fire ant workers range in length from about 2 mm to almost 5 mm. A queen is shown on the right. Different species of Pseudacteon flies parasitize different sizes of ants but none are known to parasitize winged male and female ants.

Close-up of rusty brown fire ant workers tending what look like small white grubs (brood, or immature fire ants).

Fire ant workers tending brood (immature ants).

Fire ant mound with tunnels exposed.

Fire ant mounds are used to regulate temperature so the brood (immature ants) can develop faster. The left side of the mound has been sliced open to expose the tunnel system.

Learn more about fire ant decapitating flies

Phorid flies in the genus Pseudacteon are parasitoids of ants. Several ant genera serve as hosts, including CrematogasterLasiusLinepithema, and Solenopsis (Disney 1994). Here we focus on Pseudacteon species that parasitize Solenopsis fire ants, as this is the group on which most research has been done. Over 20 Pseudacteon species are known to parasitize Solenopsis saevissima complex fire ants in South America (Patrock et al. 2009). Similarly, more than 20 Pseudacteon species parasitize Solenopsis geminata complex fire ants from North America to northern South America (Plowes et al. 2009). None of the North or Central American Pseudacteon species parasitize the red imported fire ant, S. invicta, or the black imported fire ant, S. richteri, both accidentally introduced from South America and now major pests in the United States. Pseudacteon species from South America that parasitize S. saevissima complex ants represent classical biological control agents for imported Solenopsis fire ants in the United States. 

In their native range, different Pseudacteon species divide host resources by species, host size, time of day, season, and activities of the ants.  The biology and natural history of Pseudacteon phorids that parasitize Solenopsis fire ants have been summarized by Porter (1998a) and Morrison (2000a). 

Pseudacteon phorid flies are very small, about the size of their host ant's head. Seen under a microscope, adult Pseudacteon flies have relatively large eyes, a sort of humped back ('humpbacked' flies are one of the common names for this family) and in females, an elaborately shaped ovipositor that varies among species. In the field, they appear as minute, fuzzy specks as they hover over host ants.

Pseudacteon species that parasitize Solenopsis fire ants are widely distributed in the natural range of their hosts. In tropical and subtropical areas, Pseudacteon species are active all year (Folgarait et al. 2003, Morrison and Porter 2005a, Calcaterra et al. 2008). In temperate regions, Pseudacteon adults are active in all except the winter months (Morrison et al. 1999a). Pseudacteon phorids are not known to be attracted in large numbers to anything other than host ants. Adult phorids appear to be generalists, feeding on honeydew, plant sap and nectar (Fadamiro and Chen 2005).

Individual Pseudacteon spp. are almost always restricted to a single ant genus (Disney 1994, Weissflog et al. 2008). A given Pseudacteon species may attack multiple Solenopsis spp., but is usually restricted to a single species complex, and often reveals affinities for a single species. The host specificity of Pseudacteon that attack Solenopsis has been documented in tests conducted: (1) in the field in South America (Porter et al. 1995a, Porter 1998b), (2) in the lab prior to the release of South American Pseudacteon species to North America (Gilbert and Morrison 1997, Porter and Alonso 1999), and (3) in the field in North America after the establishment of introduced populations (Vazquez and Porter 2005, Morrison and Porter 2006). Because of their specialized life history, these flies are only a risk to other ants and even then only on evolutionary time scales of thousands to millions of years (Porter and Gilbert 2004).

Pseudacteon are solitary parasitoids. Each female Pseudacteon may produce 100 - 300 eggs (Zacaro and Porter 2003). The adult female inserts a single egg into a worker ant with a hypodermic-style ovipositor in a rapid aerial attack. The egg is inserted into the thorax region using highly specialized ovipositors. The larva hatches in several days and migrates to the head capsule of the worker. There are three instars, during which time worker ants appear to behave normally until just before pupariation.

At pupariation, the tissue inside the ant's head capsule is consumed, killing the ant in the process. The ant's head usually falls off, and the mouth parts are pushed away so that the puparium is visible inside the oral cavity. The pupa completes development in the head capsule, and the adult fly emerges from the oral cavity.

Larval development takes 2-3 weeks and pupal development requires an additional 2-3 weeks, depending upon temperature and the Pseudacteon species (larger species have longer developmental times) (Porter et al. 1995b, Morrison et al. 1997). Development in Pseudacteon has been studied most thoroughly in P. litoralis (Porter et al. 1995b) and P. tricuspis (Consoli et al. 2001). Sex in most Pseudacteon species is apparently determined environmentally, with males produced from smaller host workers and females produced from larger workers (Morrison and Gilbert 1998, Morrison et al. 1999b).

Female Pseudacteon flies are attracted to worker ants at disturbed mounds, mating flights, or foraging trails, although host location behavior varies among species (Orr et al. 1997). Introduced P. tricuspis in Florida were primarily attracted to interspecific interactions involving their hosts (Morrison and King 2004). In some Pseudacteon species, males are also attracted to host ants where Pseudacteon matings occur. In other Pseudacteon species, however, males have not been collected in the vicinity of their host ants (although females may be abundant) and mating apparently occurs elsewhere (Morrison et al. 2000, Wuellner et al. 2002).

Pseudacteon parasitism rates have been documented to be very low (<3%) (Morrison et al. 1997, Morrison and Porter 2005a, Calcaterra et al. 2008). In the presence of Pseudacteon species, however, Solenopsis worker behavior is affected and foraging is disrupted, leading to dramatic short-term decreases in resource retrieval (Feener and Brown 1992, Porter et al. 1995c, Orr et al. 1995, Folgarait and Gilbert 1999, Morrison 1999, 2000b). Although forager numbers may decrease, Solenopsis workers do not usually completely abandon food resources in the presence of Pseudacteon, but some workers remain behind to guard the resource (Porter et al. 1995c, Orr et al. 1997, Morrison et al. 2000). Host Solenopsis workers have sometimes been observed to lose control of food resources to competing ant species in the presence of Pseudacteon flies (Orr et al. 1995, Feener et al. 2008), although most of the time they appear to remain in control of the resource (Morrison 1999, Morrison et al. 2000, Feener et al. 2008).

It is unclear to what degree the observed short-term behavioral effects may translate into long-term population-level impacts (Morrison 2012). A multi-year field experiment in Florida did not find any measurable effect of a single introduced Pseudacteon species (P. tricuspis) on S. invicta abundance (Morrison and Porter 2005b). Pseudacteon flies may also vector diseases among fire ant colonies, although tests with the microsporidian Kneallhazia and Vairimorpha were negative (Oi et al. 2009).

Little is known. It has been found that exposure of parasitized ants to methoprene and pyriproxyfen-insect growth regulators that are the active ingredient in some fire ant baits-can reduce P. tricuspis emergence (Farnum and Loftin 2010).

Six species of Pseudacteon flies have been successfully released and established in the United States and Puerto Rico (Chen and Morrison 2021). It is estimated that P. tricuspis now occurs in at least 65% of the imported fire ant range, while P. curvatus probably occurs in nearly 100% of S. invicta/S. richteri range in the US (Callcott et al. 2011). Pseudacteon obtusus occurs in large areas of Florida, Texas, and is likely found in adjoining states. Three other species P. litoralisP. nocens, and P. cultelatus have been established in localized areas of Alabama, Texas, and Florida, respectively (Callcott et al. 2011, Plowes et al. 2012, Porter et al. 2013).

Authors

  • Sanford D. Porter
    Retired from USDA-ARS, CMAVE, 1600 SW 23rd Dr, Gainesville, FL 32608

  • Lloyd W. Morrison
    Department of Biology, Missouri State University, 901 S National Ave, Springfield, MO 65897

Date: November 2021

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  • Callcott, A.-M., S. D. Porter, R. D. Weeks, Jr., L. C. Graham, S. J. Johnson and L. E. Gilbert. 2011. Fire ant decapitating fly cooperative release programs (1994-2008): Two Pseudacteon species, P. tricuspis and P. curvatus, rapidly expand across imported fire ant populations in the southeastern United States. Journal of Insect Science 11:19 Available online: https://www.ars.usda.gov/ARSUserFiles/60360510/publications/Callcott_et_al-2011(M-5095).pdf  
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  • Zacaro, A. A., and S. D. Porter. 2003. Female reproductive system of the decapitating fly Pseudacteon wasmanni Schmitz (Diptera: Phoridae). Arthropod Structure and Development 31:329-337.
  • Female decapitating fly preparing to lay an egg in the body of a fire ant worker. Photo credit: SD Porter, USDA-ARS
  • Female (left) and male (right) decapitating flies (Pseudacteon curvatus). Female flies have a hardened ovipositor used to inject eggs into the body of ants while male flies have a straplike or triangular anal tube at the end of their abdomen.  Photo credit: SD Porter, USDA-ARS 
  • Female decapitating fly preparing to lay an egg in the body of a fire ant worker. Photo credit: SD Porter, USDA-ARS
  • Figure 1: Side view of a fire ant decapitating fly (Pseudacteon nocens).
  • Ovipositors of 19 Pseudacteon species that attack Solenopsis saevissima complex fire ants in South America. The last image shows a male abdomen.
  • Decapitating fly life stages, from left to right: eggs, larva (within an ant head), pupa, emerging fly. Photo credit: SD Porter, USDA-ARS
  • Fire ant workers range in length from about 2 mm to almost 5 mm. A queen is shown on the right. Different species of Pseudacteon flies parasitize different sizes of ants but none are known to parasitize winged male and female ants. Photo credit: SD Porter, USDA-ARS
  • Fire ant workers tending brood (immature ants). Photo credit: SD Porter, USDA-ARS
  • Fire ant mounds are used to regulate temperature so the brood (immature ants) can develop faster. The left side of the mound has been sliced open to expose the tunnel system. Photo credit: SD Porter, USDA-ARS
Portrait of Amara Dunn
Amara Dunn-Silver

Senior Extension Associate

NYS Integrated Pest Management

Amara Dunn-Silver