Atta laevigata (Smith) (Hymenoptera: Formicidae) foragers collected in the field and parasitized by Apocephalus attophilus Borgmeier (Diptera: Phoridae) (1.2% of sampled ants) were larger and had lower survival rates than similar non‐parasitized foragers. Moreover, the size of parasitized ants was significantly correlated with the number of A. attophilus puparia and percentage of adult emergence. These results suggest that host size is important for the reproductive success of A. attophilus. Different from most ant‐parasitizing phorids, A. attophilus exhibits a pre‐oviposition behavior that involves walking towards the host and inspecting it, and this careful approach may be responsible for a relatively low percentage of parasitoid detection by A. laevigata. When an exotic resource (Acalypha sp. leaves) was placed on ants' foraging trails, more foragers were recruited, which resulted in the attraction of A. attophilus. The number and recruiting rates of small workers (minima) were significantly higher on leaves visited by A. attophilus, but larger foragers showed no response to phorids. These results demonstrated that minima react to the presence of A. attophilus and suggest a defensive role of these ants against phorid parasitism.

The parasitic phorid Myrmosicarius grandicornis Borgmeier is commonly found around nest entrances of the leaf-cutting ant Atta sexdens (L.) in Brazil, but there is no information about the importance of this fly for A. sexdens. We evaluated the parasitic capability of female M. grandicornis collected in the field and released in laboratory nests of A. sexdens and compared ants' foraging rhythm before and after M. grandicornis were released. We also determined biological characteristics of the parasitoid. Presence of M. grandicornis elicited foragers to abandon their loads and return to the nest, an effect previously described for the phorid Neodohrniphora sp., which is a more abundant A. sexdens parasitoid. Both phorid species occur at the same foraging trails during part of the year and attack ants of different size classes. Therefore, they may have a combined effect on the reduction of A. sexdens foraging. Myrmosicarius grandicornis pupariated inside hosts' head capsules, which are detached from their bodies and deposited in the nest's refuse pile. Adults emerged through the hosts' mouth cavity about a month after oviposition. The characteristics of M. grandicornis biology suggest that this parasitoid completes its development inside A. sexdens nests.

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