Apocephalus borealis is a species of North American parasitoid phorid fly that attacks bumblebees, honey bees, and paper wasps.

[2] Association with honey bees has so far only been documented from California, South Dakota, Oregon, Washington, British Columbia (Vancouver Island), and Vermont.

[1] The infection of European honey bees in North America by A. borealis is a recent development that was first discovered by Dr. John Hafernik, who collected some dead specimens near a light source at San Francisco State University's campus.

However, some regions have a greater population of honeybees, which increases the chances of a successful A. borealis infection[6] Information is insufficient to explain why the parasitic fly jumped to its new host, but concern exists that this new host provides an opportunity for the fly to thrive and further threaten the decreasing honey bee population.

[7] To identify this fly, DNA barcoding was used, demonstrating that the phorids that emerged from Apis and Bombus had no more than 0.2% (1 bp) divergence among samples.

No adults of the fly were found within hives, indicating that phorids do not survive in large numbers in the late winter when foraging bees are inactive.

[1] A citizen science project, "Zombee Watch", uses a social media framework for people to report sightings of potentially parasitized bees.