Foraging ecology and patterns of diversification in dipteran parasitoids of fire ants in south Brazil

1. At least sixteen species of parasitoid flies in the genus Pseudacteon (family Phoridae) attack fire ants in the Solenopsis saevissima subcomplex in South America. Little is known of behavioural or ecological differences among Pseudacteon parasitoids of fire ants, although their coexistence in multispecies communities would suggest that important differences exist. Seven Pseudacteon species in two separate communities were studied in south-east Brazil. The way in which hosts detect and respond to the presence of parasitoids, attack rates of the parasitoids, and host location behaviour of the parasitoids were examined. 2. Reductions in fire ant recruitment were more closely related to the number of ants attacked along a foraging trail than to the amount of time that a phorid was present. 3. Pseudacteon solenopsidis differed from other phorid species by flying backwards while pursuing ants, by attacking at lower rates than other phorids, and by spending longer around fire ant foraging trails than other phorids before departing. Fire ant recruitment to food often rebounded in the continued presence of P. solenopsidis. 4. In each of the two communities, certain Pseudacteon species appeared frequently at Solenopsis foraging trails, whereas others appeared predominantly at mound disturbances. Two distinct size classes of phorids were present in each community, and the community with the larger ant host species also had a third and larger phorid species. No phorid species from the same community had similar body sizes and similar host location behaviours, although numerous species from different communities shared both of these traits. 5. Heterogeneity in host size and in the ecological circumstances under which hosts are vulnerable to attack appears to have influenced the evolution and perhaps maintenance of diverse Pseudacteon communities.