This work describes the annual and daily activity patterns of two parasitoid fly communities of the fire ant Solenopsis invicta (Hymenoptera: Formicidae) in their native range in Argentina. Pseudacteon (Diptera: Phoridae) flies were censused monthly for 1 yr at two sites in northwestern Corrientes province. Additionally, parasitism rates of S. invicta colonies naturally attacked in the field by Pseudacteon flies are reported for the first time from its native habitat. In total, 4,528 flies (86.3% females) of eight Pseudacteon species were collected attacking ants in the field. Pseudacteon litoralis Borgmeier and Pseudacteon nocens Borgmeier represented 71–79% of female flies censused in both sites. Most species were active throughout the year, although abundances were variable over time and between sites. The highest occurrence peaks of flies were recorded in spring, whereas the lowest occurrence was in summer. Fly abundance was higher close to dusk, whereas species diversity was highest at midday. Relationships among species were established based on their activity patterns and genetic proximity. The presence and abundance of four species were explained by climatic variables, whereas two species may have inherited similar circadian rhythms from a common ancestor. Overall, the parasitism rate by the nine species recorded was very low (0.24%). The highest percentage of parasitized workers was found in spring (0.5%) and occurred within the most complex habitat (gallery forest). The highest parasitism rate per site and colony was also for this habitat in spring (1.16 and 2.81%, respectively). The highest rates of emergence were recorded for Pseudacteon nudicornis Borgmeier.
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