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The Fire Ant Laboratory at the University of Texas at Austin's Brackenridge Field Laboratory pursues several lines of investigation at Brazos Bend State Park. All of our research at Brazos Bend State Park centers on our ongoing efforts to implement a self-sustaining biological control program for imported fire ants (Solenopsis invicta) using phorid fly parasitoids in the genus Pseudacteon. Pseudacteon flies are parasitoids of adult fire ant workers. They locate imported fire ants by eavesdropping on the chemical odors fire ants use to regulate foraging, nest and resource defense. 23 known species of Pseudacteon flies attack fire ants in South America, and differ from each other in activity periods, environmental tolerances, and in the contexts in which they exploit fire ants: foraging or nest defense. Those flies that attack ants while foraging disrupt food collection and harm the ability of the fire ant colony to harvest the resources it needs to grow. Our laboratory's goal is to continue to establish in Texas a suite of Pseudacteon fly species that attack imported fire ants in ways that do not overlap the Pseudacteon species already introduced and naturalized. Pseudacteon fly parasitoids of imported fire ants are extremely species-specific, attacking only imported fire ants to the exclusion of even the relatively closely related native fire ant species (Solenopsis geminata and S. xyloni). Because of this extreme environmental specificity no non-target effects of this biological control program have been observed or are anticipated.
Release of Pseudacteon obtusus and P. curvatus
The first effort our laboratory undertook at Brazos Bend State Park was the release and establishment of 2 Pseudacteon species. In 2006, the first population of P. obtusus in North America was released and subsequently became established in the park. This is the first species of phorid fly that attacks imported fire ants at foraging trails successfully established in North America. These efforts are detailed in Plowes, LeBrun, and Gilbert 2011 . Subsequent to that effort a second species P. curvatus was also released and established in the park. From these releases, both of these species have spread widely through the region (Gilbert et al. 2008). Following on these efforts, several ongoing research projects are being conducted in the park investigating various aspects of the biology and impacts of these flies. These efforts are outlined below.
Relative effects of currently established Pseudacteon species on the Foraging Ecology of S. invicta
Studies of the direct mortality induced by phorid flies have uniformly shown low prevalence of infected workers within colonies that range from 0 - 3%. However, studies uniformly report large impacts on the foraging efficiency of S. invicta subject to attack by phorids and also impacts on the ability of S. invicta to retain food resources in the face of competition (Orr et al. 1995; Porter et al. 1995; Feener et al. 2008). Thus, the degree to which an individual Pseudacteon species impacts fire ant populations is determined by how commonly this species attacks ants on foraging trails and how strongly it impacts the recruitment and foraging efficiency of the ants when it is present. Pseudacteon species fall along a continuum with respect to how commonly they attack fire ants along foraging trails (Orr et al. 1997), and the strength of the ant's behavioral responses to parasitism also varies (Wuellner et al. 2002). However, studies from the native range on Pseudacteon species impacts on S. invicta's foraging and competitive capabilities lack detail on the individual species involved (Feener et al. 2008; Orr et al. 1997), and very little has been done to assess how these species impact foraging in the novel context of the southeastern United States. We are quantifying two aspects of the relationship between each Pseudacteon species and its host: the relative propensity of each species to attack its host at foraging trails versus mound disturbances, and the degree to which each species impacts the resource harvesting capability of S. invicta when it attacks at foraging events. These factors will be evaluated for the three species established in Texas to date: P. curvatus, P. tricuspis, and P. obtusus.
Effects of Pseudacteon on above ground homoptera tending by S. invicta
An early observation of the fire ant invasion in central Texas was the widespread occurrence of S. invicta foraging in tree canopies (Kaspari 2000). Arboreal foraging allows imported fire ants in North America increased access to carbohydrate resources in the form of homopteran honeydew and extrafloral nectar. However, arboreal foraging trails are extremely exposed to attack by phorid fly parasitoids. The importance of not being exposed while foraging is evident from S. invicta's extensive underground foraging tunnel network and the effort it invests in burying any large or persistent food resource. In addition, the tendency of S. invicta to engage in above ground tending of Homoptera has important agricultural consequences as it leads to higher densities of aphid and scale insect populations in row crops (Coppler et al. 2007). Results of investigations into whether above ground foraging by S. invicta also reduces herbivore populations in these same row crops are mixed with impacts on herbivores varying by herbivore taxa and crop (Coppler et al. 2007; Eubanks 2001). We are evaluating whether introduced Pseudacteon populations reduce the prevalence of S. invicta foraging in trees. If it exists, this behavioral impact of phorids has direct importance to population level control of fire ants because increased access to sugar resources is known to fuel rapid colony growth in fire ants (Helms and Vinson 2008).
Effects of multiple species of Pseudacteon parasitoids on the population densities of S. invicta
The ultimate goal of the release of phorid flies as biological control agents is the reduction in the population density of S. invicta. Because phorids attack worker ants, and reduce foraging activity, Pseudacteon will primarily impact colony growth rates. As colonies are long-lived entities, these impacts will take years to noticeably affect densities of fire ant populations. However, eventually reduced growth rates will translate into reduced budding by polygyne colonies and fewer alates produced by monogyne colonies: factors that should lead to a reduction in colony densities. In an experiment to assess the impact of introduced complexes of Pseudacteon parasitoids on S. invicta populations, Brazos Bend State Park served as an introduction site amongst pairs of control and phorid introduction sites in a variety of Texas Ecoregions. To quantify impacts, control sites needed to remain Pseudacteon free for sufficient time to provide meaningful comparisons with our areas of introduction. However, introduced Pseudacteon populations spread more rapidly than anticipated with populations expanding at a rate of 50 km per year (LeBrun et al. 2008). Established control sites were all colonized by Pseudacteon before impacts of phorid flies on imported fire ant populations were anticipated. We continue to monitor these sites in to see if long-term reductions in fire ant population densities are evident.
Publications resulting in part from our work done at Brazos Bend State Park
LeBrun EG, Plowes RM, Gilbert LE (2008) Dynamic expansion in recently introduced populations of fire ant parasitoids (Diptera: Phoridae). Biol Invas 10:989-999
Gilbert LE, Barr CL, Calixto AA, Cook JL, Drees BM, Lebrun EG, Patrock RJW, Plowes RM, Porter SD, Puckett RT (2008) Introducing phorid fly parasitoids of red imported fire ant workers from South America to Texas: Outcomes vary by region and by pseudacteon species released. Southwestern Entomologist 33 (1):15-29
Bibliography
Coppler LB, Murphy JF, Eubanks MD (2007) Red imported fire ants (Hymenoptera: Formicidae) increase the abundance of aphids in tomato. Fla Entomol 90 (3):419-425
Eubanks MD (2001) Estimates of the direct and indirect effects of red imported fire ants on biological control in field crops. Biol Cont 21 (1):35-43
Feener DH, Orr MR, Wackford K, Longo JM, Benson WW, Gilbert LE (2008) Geographic variation in resource dominance, discovery, and parasitoid-mediated competition in the Brazilian range of the red imported fire ant (Solenopsis invicta). Ecology In press
Gilbert LE, Barr CL, Calixto AA, Cook JL, Drees BM, Lebrun EG, Patrock RJW, Plowes RM, Porter SD, Puckett RT (2008) Introducing phorid fly parasitoids of red imported fire ant workers from South America to Texas: Outcomes vary by region and by pseudacteon species released. Southwestern Entomologist 33 (1):15-29
Helms KR, Vinson SB (2008) Plant resources and colony growth in an invasive ant: The importance of honeydew-producing Hemiptera in carbohydrate transfer across trophic levels. Environ Entomol 37 (2):487-493
Kaspari M (2000) Do imported fire ants impact canopy arthropods? Evidence from simple arboreal pitfall traps. Southwest Nat 45 (2):118-122
LeBrun EG, Plowes RM, Gilbert LE (2008) Dynamic expansion in recently introduced populations of fire ant parasitoids (Diptera: Phoridae). Biol Invas 10:989-999
Orr MR, Seike SH, Benson WW, Gilbert LE (1995) Flies suppress fire ants. Nature 373:292
Orr MR, Seike SH, Gilbert LE (1997) Foraging ecology and patterns of diversification in dipteran parasitoids of fire ants in south Brazil. Ecol Entomol 22:305-314
Plowes RM, LeBrun EG, Gilbert LE (2011) Introduction of the fire ant decapitating fly Pseudacteon obtusus in the United States: factors influencing establishment in Texas. Biocontrol 56 (3):295-304
Porter SD, Vander Meer RK, Pesquero MA, Campiolo S, Fowler HG (1995) Solenopsis (Hymenoptera: Formicidae) fire ant reactions to attacks of Pseudacteon flies (Diptera: Phoridae) in southeastern Brazil. Ann Entomol Soc Am 88:570-575
Wuellner CT, Porter SD, Gilbert LE (2002) Eclosion, mating, and grooming behavior of the parasitoid fly Pseudacteon curvatus (Diptera: Phoridae). Fla Entomol 85 (4):563-566
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