Twenty-two species of Pseudacteon flies (Diptera: Phoridae) are known to attack fire ants (Solenopsis spp.) in Argentina, Brazil, Bolivia, and Paraguay, but none are known in Chile. Surveys were conducted in central Chile and at similar latitudes in western Argentina to detect the presence of fire ants and parasitoid flies and to determine their relationship. Flies and fire ants were much more common and abundant in Argentina. In total, 100 colonies of four fire ant species were found at 63.6% of the sites surveyed. In contrast, only six colonies of one species, Solenopsis gayi (Spinola), were found at 4.7% of the sites surveyed in Chile. Our survey includes the first record of five parasitic fly species in central western Argentina and a new host, Solenopsis quinquecuspis Forel. The large form of Pseudacteon obtusus Borgmeier was found attacking S. gayi in Chile, which is the first record in that country, and the first record on this host species. The southern-most and western-most records were established for fire ant-decapitating flies. Mitochondrial DNA (mtDNA) indicates that S. gayi is genetically closer to the Argentine fire ants (saevissima-group) than the North American S. geminata-group. However, S. gayi venom alkaloid composition is similar to S. geminata-group, whereas cuticular hydrocarbon composition has characteristics of both groups. Analysis of mtDNA from the collected flies supports the monophyly of P. obtusus and suggests that the Chilean population is similar to populations in eastern Argentina. The presence of P. obtusus in Chile could be explained by immigrant parasitized fire ant species from Argentina or by an accidental introduction.
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