Apocephalus borealis is a species of North American parasitoid phorid fly that parasitizes bumblebees, honey bees and wasps (Brues 1924, Ennik 1973, Brown, 1993, Otterstatter et al. 2002). The association with honey bees has so far only been documented from California and South Dakota (Core et al. 2012); elsewhere, they are primarily associated with bumblebees (Otterstatter et al. 2002).
Female flies land on the abdomen of their host and pierce the cuticula with a sharp, swordlike ovipositor. As the larvae develop, they attack the host's brains and cause it to become disoriented, fly at night and exhibit other unusual behaviors. Whether these changes in host behavior increase parasite fitness remains to be established (Core et al. 2012). Mature larvae emerge from the junction of the head and thorax, often decapitating the host (the generic name Apocephalus refers to this grim result). A. borealis has been suggested as a possible vector promoting the spread of the pathogens responsible for colony collapse disorder in bees (Coghlan 2012, Core et al. 2012).