Current activities: The imported fire ant control project at the USDA-ARS-SABCL in Argentina has involved the survey, evaluation, and shipment of natural enemies of local fire ant species such as Solenopsis richteri, S. invicta, S. quinquecuspis, S. macdonaghi, and others (Hymenoptera: Formicidae). The research has been focused mainly in the pathogens Thelohania solenopsae (Microsporidia: Thelohaniidae) and Vairimorpha invictae (Microsporidia: Burenellidae), the parasitoid flies Pseudacteon spp. (Diptera: Phoridae) and the parasitic ant Solenopsis daguerrei (Hymenoptera: Formicidae). More recently, research on several new candidate organisms has been initiated.
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Thelohania solenopsae was found to be the most common microorganism infecting fire ants in Argentina. It produces a chronic infection in the entire colony and it is vertically transmitted to the progeny. Long-term studies of diseased populations and individual colonies of S. richteri showed also that (1) diseased colonies are 67% smaller than healthy colonies, (2) immature stages of sexual ants (reproductive stages) are less frequent in infected colonies, (3) fire ant population densities within infested areas decline over time, and (4) infected laboratory colonies die quicker than healthy ones. Field host range of this microsporidium is restricted to closely related species within the genus Solenopsis. In 1996, T. solenopsae was found in southern United States infecting multiple-queens colonies of the red imported fire ant, S. invicta, and is being evaluated within the Area Wide Suppression of Fire Ant Program (http://fireant.ifas.ufl.edu) conducted by researchers at the CMAVE. The lack of infection in the single-queen form of the host in the United States is in contrast with the South American populations and is being investigated.
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Vairimorpha invictae has been considered a potential candidate for biological control of imported fire ants in the United States since the mid 1980's. However, its deleterious effects in the field had never been documented because of its low prevalence and discontinuous occurrence. The need for extensive surveys to find fire ant populations infected with V. invictae was a top priority. In May 2004, long-term ecological studies were concluded in Santa Fe Province. Results revealed that local populations of S. invicta infected with V. invictae (and T. solenopsae) showed a substantial decrease in most study plots with an overall reduction of the mean population index per plot of 69%. Practically, all large colonies were healthy. These results, combined with additional evidence reported previously, suggest that these infections have deleterious effects on native populations of S. invicta. Field host range of V. invictae is also restricted to closely related species within the genus Solenopsis. This microorganism has not been found in the United States and was introduced into quarantine at the CMAVE during 2003 and 2004 for transmission and specificity studies. Periodically, infected colonies are being collected in the field (Santa Fe and Corrientes Provinces) and shipped to CMAVE for further testing. V. invictae is expected to be field released in late 2006 or 2007.
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Surveys of parasitoid phorid flies, Pseudacteon spp., against local fire ants have been conducted in Argentina since 1995. Most frequent species found are P. litoralis, P. curvatus, P. tricuspis, P. nocens, and P. obtusus. Recently, explorations were extended to northwestern Argentina where P. disneyi was found. Also, Chile, western Argentina and northern Patagonia were explored for the presence of Solenopsis ants and their phorid flies. Host preference tests are being conducted. Adult flies and parasitized fire ant workers are being shipped to the CMAVE for mass rearing. P. tricuspis (biotypes Brazil and Argentina) and P. curvatus (biotypes Buenos Aires and Formosa) were released in the United States and are being used in the Area Wide Suppression of Fire Ant Program (http://fireant.ifas.ufl.edu) conducted by the CMAVE. More recently, P. litoralis from northern Argentina was also released. Further surveys and studies are being conducted to find and evaluate new species for introduction. Petition for field release of P. obtusus has been requested by CMAVE and it will be probably released in mid 2006.
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The presence of the workerless parasitic ant, Solenopsis daguerrei, has been surveyed in Argentina, Brazil, and Uruguay since 1989 and more intensively since 1995. The areas of San Eladio, Buenos Aires Province, Argentina, and Dourados, Mato Grosso do Sul, Brazil, showed the highest abundance of this parasite. S. daguerrei was found only in fire ant colonies. Laboratory and field studies revealed that parasitized colonies have fewer queens than non parasitized ones. Because all attempts to artificially propagate the parasite to non parasitized colonies were unsuccessful, genetic studies are being conducted in the United States to match local populations of the parasite with their fire ant hosts.
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Studies on the wasp Orasema xanthopus (Hymenoptera: Eucharitidae) were initiated in February 2005. Surveys were conducted in north-central Argentina and high occurrence of wasps was detected in Corrientes and Entre Ríos Provinces. The field and laboratory biology, host range, and effect on the fire ant hosts are being investigated.
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A mermithid nematode (Allomermis sp. ?) was recently found infecting Solenopsis invicta in northern Argentina and Solenopsis richteri in central Argentina. Identification and description are in progress in the United States and Argentina. Field biology and ecology are being investigated.
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A virus was recently found infecting Solenopsis invicta in northern Argentina. Identification and molecular biology is being conducted in the United States. Field occurrence and ecology are being investigated in Argentina.
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