A Pseudacteon fly attacking Solenopsis geminata. PHOTO: Dr. Larry Gilbert

Pseudacteon spp. (Diptera: Phoridae) by Lloyd W. Morrison, Department of Biology, Southwest Missouri State University, 901 S National Ave, Springfield, MO 65804 Phorid flies in the genus Pseudacteon are parasitoids of ants. Several ant genera serve as hosts, including Crematogaster, Lasius, Linepithema, and Solenopsis. Here I focus on Pseudacteon species that parasitize Solenopsis workers, as this is the group on which most research has been done. Solenopsis fire ants are hosts to at least 27 species of Pseudacteon flies in the New World. At least 18 species of Pseudacteon attack S. saevissima complex ants in South America, and at least 9 species of Pseudacteon attack S. geminata complex ants in North and Central America. 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 (only S. invicta and S. richteri are present in the U.S.) and ignore native fire ants in the S. geminata complex represent host-specific potential biocontrol agents for imported fire ants in the United States. Pseudacteon species native to North and Central America that parasitize S. geminata complex ants represent potential biocontrol agents for regions of the world where S. geminata is introduced and has become a pest (i.e., India, Africa and Pacific islands).

Habitat

Many Pseudacteon species that parasitize Solenopsis ants are widely distributed in the natural range of their hosts. In tropical areas, Pseudacteon species are active all year; in temperate regions, Pseudacteon adults are active in all except the winter months. Pseudacteon phorids are not known to be attracted in large numbers to anything other than host ants. Many phorids appear to be generalists, feeding on honeydew, plant sap, nectar, and dead insects, and Pseudacteon adults are also likely to be generalist feeders.

Pests Attacked (Host Range)

Individual Pseudacteon spp. are almost always restricted to a single ant genus. Pseudacteon spp. that attack Solenopsis may attack multiple Solenopsis spp., but are usually restricted to a single species complex within that genus.

Life Cycle

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, and the larva 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).

Head capsule of S. invicta with Pseudacteon species emerging. Head of fly is visible in the oral cavity (arrow) of the ant head capsule. PHOTO: Dr. Lloyd Morrison.

Sex in most Pseudacteon species is apparently determined environmentally, as males are produced from smaller workers whereas females are produced from larger workers.

Females are attracted to disturbed mounds, mating flights, or foraging trails of their hosts to oviposit on worker ants. 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.

Relative Effectiveness

Pseudacteon parasitism rates have been documented to be very low (<3%). In the presence of some Pseudacteon species, however, colony-level Solenopsis foraging is disrupted, leading to as much as a 50% decrease in resource retrieval. Other Pseudacteon species may be attracted to alarm pheromones of their hosts, and mediate the outcome of interspecific interactions such as fighting for territory. Thus the direct effect of mortality of this parasitoid is relatively small, yet there may be a relatively large indirect effect (i.e., other competing ant species may be able to obtain food or territory not claimed by Solenopsis workers in the presence of Pseudacteon flies).

It is unclear whether such short-term behavioral effects may translate into long-term population-level impacts. Thus far, monitoring of released Pseudacteon species has revealed no detectable changes in host Solenopsis population densities.

Conservation

Specific information on how to conserve Pseudacteon is not known. For general information about conservation of natural enemies, see Conservation in the Tutorial section on this site, or the Volume II, No. 1 Feature Article on conservation in the Midwest Biological Control News Online.

Pesticide Susceptibility

Not known.

Commercial Availability

Pseudacteon species are not commercially available. However, several Pseudacteon species are being mass-reared and released in many southeastern states. These species are dispersing, and in several years will be established across much of the southeastern U.S.

References

Disney, R. H. L. 1994. Scuttle flies: The Phoridae. Chapman & Hall, London.

Feener, D. H., Jr. and B. V. Brown. 1992. Reduced foraging of Solenopsis geminata (Hymenoptera: Formicidae) in the presence of parasitic Pseudacteon spp. (Diptera: Phoridae). Ann. of the Ent. Soc. of Amer. 85: 80-84.

Gilbert, L.E. and L.W. Morrison. 1997. Patterns of host specificity in Pseudacteon parasitoid flies (Diptera, Phoridae) that attack Solenopsis fire ants (Hymenoptera, Formicidae). Environmental Entomology, 26: 1149-1154.


Morrison, L. W. 2000. Mechanisms of Pseudacteon parasitoid (Diptera: Phoridae) effects on exploitative and interference competition in host Solenopsis ants (Hymenoptera: Formicidae). Annals of the Entomological Society of America, 93: 841-849.

Morrison, L. W. 2000. Biology of Pseudacteon (Diptera: Phoridae) ant parasitoids and their potential to control imported Solenopsis fire ants (Hymenoptera: Formicidae). In S. G. Pandalai (ed.), Recent Research Developments in Entomology, 3: 1-13.

Morrison, L.W. 1999. Indirect effects of phorid fly parasitoids on interspecific competition between fire ants. Oecologia, 121: 113-122.

Morrison, L. W., C. G. Dall'Aglio-Holvorcem and L. E. Gilbert. 1997. Oviposition behavior and development of Pseudacteon flies (Diptera: Phoridae), parasitoids of Solenopsis fire ants (Hymenoptera: Formicidae). Env. Ent. 26: 716-724.

Morrison, L. W. and J. R. King. 2004. Host location behavior in a parasitoid of imported fire ants. Journal of Insect Behavior, 17: 367-383.

Morrison, L. W., S. D. Porter, and L. E. Gilbert. 1999. Sex ratio variation as function of host size in Pseudacteon flies (Diptera: Phoridae), parasitoids of Solenopsis fire ants (Hymenoptera: Formicidae). Biological Journal of the Linnean Society, 66: 257-267.

Orr, M. R., Dahlsten, D. L., Benson, W. W., 2003. Ecological interactions among ants in the genus Linepithema, their phorid parasitoids, and ant competitors. Ecological Entomology 2003, 203-210.

Orr, M. R., S. H. Seike, W. W. Benson and L. E. Gilbert. 1995. Flies suppress fire ants. Nature 373: 292.

Porter, S. D. and M. A. Pesquero. 2001. Illustrated key to Pseudacteon decapitating flies (Diptera: Phoridae) that attack Solenopsis saevissima complex fire ants in South America. Florida Entomologist 84: 691-699.

Porter, S. D. 1998. Biology and behavior of Pseudacteon decapitating flies (Diptera: Phoridae) that parasitize Solenopsis fire ants (Hymenoptera: Formicidae). Florida Entomologist 81: 292-309.

Porter, S.D., L.W. Morrison, and L. A. Nogueira de Sa. 2004. Establishment and dispersal of the fire ant decapitating fly, Pseudacteon tricuspis, in North Florida. Biological Control, 29:179-188.

Porter, S. D., M. A. Pesquero, S. Campiolo and H. G. Fowler. 1995. Growth and development of Pseudacteon phorid fly maggots (Diptera: Phoridae) in the heads of Solenopsis fire ant workers (Hymenoptera: Formicidae). Env. Ent. 24: 475-479.

Porter, S. D., R. K. Vander Meer, M. A. Pesquero, S. Campiolo and H. G. Fowler. 1995b. Solenopsis (Hymenoptera: Formicidae) fire ant reactions to attacks of Pseudacteon flies (Diptera: Phoridae) in Southeastern Brazil. Ann. of the Ent. Soc. of Amer. 88: 570-575.

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