I’ve been receiving emails from beekeepers asking about media reports concerning a fly that is attacking honey bees. These media reports are the result of recently published research from San Francisco State University. This research actually began in 2008 when Biology Professor John Hafernik picked up some dead honey bees from underneath lights outside his biology building at San Francisco State. He wasn’t really interested in the bees; he was just looking for some food for a praying mantis that he had found on a recent field trip. (Praying mantises, as you probably know, are carnivorous and feed on a variety of insects.) He left the dead bees sealed in a small bottle in his lab, and was surprised a few days later to find fly pupae along with the dead bees. These pupae were identified as Apocephalus borealis, a species of parasitic fly that commonly lays its eggs inside an insect host – usually bumble bees. This finding was the beginning of a trail that would lead to the discovery of a new honey bee parasite.
Until Dr. Haferniks picked up those dead bees, A. borealis was not known to parasitize honey bees. The mechanism of the parasitism involves a female A. borealis fly’s piercing the exoskeleton of a honey bee or bumble bee with her ovipositor, and laying her eggs in the abdomen of the host bee. The eggs then hatch inside the bee and, about a week later, the fly larvae emerge. The larval and adult stages of the fly occur outside the host. This strategy of using other insect species as incubators for their offspring is common not only to some species of fly, but to many wasp species as well. Until the discovery at San Francisco State, it was not known that this fly would choose honey bees as hosts as well as bumblebees. Usually, the parasite-host relationship is very specific, with a single species of fly or wasp making use of a single host species. As a result, many parasitic wasps are very useful as biological controls for agricultural insect pests. We now know, however, that A. borealis is less picky about its victims.
Dr. Hafernik found those bees under exterior lights because the infected bees, it was found, will often fly from the hive at night and are attracted to lights. Beekeepers know that flying at night is not common. Observations indicate that the bees will remain at the lights, and die there the next day. Before dying, the stranded bees may exhibit abnormal behavior, including crawling in small circles and not being able to stand. This behavior has caused some to refer to the infected bees as zombie bees. The reason the bees are flying from the hive at night is unclear, but it could be as simple as the infected bees’ being perceived as impaired by the colony and being evicted from the hive by the healthy bees. It is also possible that this expulsion from the hive occurs in the daylight as well, and is yet to be observed. Laboratory testing of the infected bees and the hives from which they originated indicate a high rate of Nosema ceranae and deformed wing virus infection. Both diseases are thought to be associated with Colony Collapse Disorder (CCD). The A. borealis flies which were sampled also tested positive for these diseases.
Bottom line – does this study prove that A. borealis’ attacking honey bees is the cause of CCD? It does not. However, the abandonment of the hives by infected honey bees and the high rates of infection from the afore mentioned diseases are conditions thought to be associated with CCD. This study does not indicate that high numbers of abandoned hives were found – only that parasitized bees were found to have abandoned their hives, flown to lights at night and subsequently died. This behavior is similar to that which occurs in CCD, but it is possible that the similarity is coincidental and not from the same cause. In fact, the primary hive that was used in the study still contained an active colony of honey bees at the time the study was published.
Are these flies a problem for beekeepers all over the United states? That is not known. They have been found in beekeepers’ hives in the San Francesco area, and also in hives of a commercial beekeeping operation in California and South Dakota. Testing of honey bees in other areas, possibly by trapping them around outside lights near apiaries, is needed to answer that question. It is also not clear whether the use of honey bees as an alternative host by the A. borealis fly is a recent change or has been occurring for many years. To answer these questions, more research is required and the answers lie in the future. Stay tuned.