Pseudacteon Obtusus (Diptera: Phoridae) Attacking Solenopsis Invicta (Hymenoptera: Formicidae) in Brazil

Although parasitism of ants (Hymenoptera: Formicidae) by phorid flies (Diptera: Phoridae) has been reported by a number of investigators, their behavior is still poorly understood (Pergande, 1901; Wasmann, 1918; Borgmeier, 1925, 1931, 1963, 1969; Donisthorpe, 1927; Steyskal, 1944). Recently Williams et al. (1973), Williams and Whitcomb (1974) and Williams (1980) reported on two genera of phorids, Pseudacteon and Apodicrania, that attack fire ants in South America. They noted that Apodicrania larvae were endoparasites of fire ant larvae (Solenopsis invicta) while sixteen species of the genus Pseudacteon seem to parasitize workers of the Solenopsis saevissima complex. Although attacks by Pseudacteon on fire ants have been observed, evidence of actual parasitism (i.e. oviposition by the parasite) is lacking. Here, we report field observations of Pseudacteon attack on foraging S. invicta workers and document parasite oviposition on a host ant. Our observations indicate that the parasite preferentially attacks major workers of the host species. Possible reasons for this caste-bias are discussed. Data were collected on April 18 (16:00-16:30), April 19 (11:00-15:30) and on April 24 (15:00-17:00), 1985 in Caceres, Mato Grosso, Brazil during studies of ant species competing for baits. At this time the phorid flies, Pseudacteon obtusus Borgmeier were observed hovering over workers of the fire ant Solenopsis invicta Buren as they fed at bait stations or moved along the foraging trails. Between one and four P. obtusus were observed during any one observation period. Only a few attempts (n3) were made by phorids to attack minor workers, and in these cases two of the minor workers escaped by crawling under debris, leaves or grass, while the