PSEUDACTEON OBTUSUS (DIPTERA: PHORIDAE) 
ATTACKING SOLENOPSIS INVICTA 
(HYMENOPTERA: FORMICIDAE) IN BRAZIL* 
By David F. Williams and William A. Banks 
USDA-ARS, 
Insects Affecting Man and Animals Research Laboratory, 
P.O. Box 14565, 
Gainesville, Florida 32604, U.S.A. 
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; Donis- 
thorpe, 1927; Steyskal, 1944). Recently Williams et al. (1973), Wil- 
liams 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 observa- 
tions of Pseudacteon attack on foraging S. invicta workers and 
document parasite oviposition on a host ant. Our observations in- 
dicate 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 
(1 1:00-15:30) 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 (n = 3) were made by phor- 
ids to attack minor workers, and in these cases two of the minor 
workers escaped by crawling under debris, leaves or grass, while the 
* Manuscript received by the editor October 8, 1986. 
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