Host-parasitoid interactions are complex. A parasitoid can change its hosts behavior by direct infection or simply by its presence in the shared environment. In the red imported fire ant (RIFA,
Solenopsis invicta), workers display defensive postures to avoid potential parasitism when fire ant decapitating flies ( Pseudacteonspp.) hover above them. In addition to changes in the behavior of individual ants, RIFA colonies limit their foraging when decapitating flies are present. To further understand these interactions, we investigated the molecular basis of changes in foraging behavior at the colony level. By quantifying expression of the S. invicta foraginggene ( Sifor), odorant binding protein 11, and immune genes of workers of unknown infection status in colonies exposed to decapitating flies and control colonies over a 48-h period, we aimed to provide information on the molecular processes related to change in foraging behavior of RIFA. We found upregulation of an immune gene, defensin-2, in colonies exposed to decapitating fly parasitoids. However, we did not detect significant differences in expression levels of three other genes related to immune function: abaecin, cytochrome P450 4C1-like, and hymenoptaecin.Although colonies exposed to parasitoids foraged less than control colonies (flies absent), they did not experience significant changes in Sifor, a gene correlated with worker caste performance, or odorant binding protein 11, a gene upregulated in foragers. These results show that molecular processes associated with diminished foraging activity are set into place by the mere presence of the flies or by early stages of phorid fly infection.