Zombie Flies: A Potential New Threat to Beekeeping?

By Jon Zawislak, EAS Master Beekeeper and Program Associate – Apiculture, University of Arkansas, Division of Agriculture, Cooperative Extension Service

As though parasitic mites, hive beetles, a new strain of Nosema, and a legion of microbes weren’t enough for beekeepers to contend with, scientists recently announced a potentially devastating new honeybee parasite in North America. The so-called “zombie” fly parasite, Apocephalus borealis, is a native species of phorid fly known to attack bumble bees and paper wasps, but not honeybees. Researchers in California caused a bit of a buzz when they suggested the case is changing.

Dr. John Hafernik, a biology professor at San Francisco State University, had collected a few honeybees to feed a captive praying mantis. After few days in a jar, fly larvae began to emerge from the bees’ bodies. The discovery prompted him to identify the fly, which led to further investigations with other scientists.

Genetic tests confirmed that the flies emerging from captured bumble bees and honeybees were the same species of parasite. Its ability to attack honeybees was previously unknown, and is believed to be a recent adaptation to a new host. Normally, dying honeybees remain in one place with little or no movement. Bees attacked by the fly parasite remained alive, but disoriented. They walked in circles or were unable stand. Researchers who observed the bees compared their uncoordinated movements to those of movie zombies.

The researchers noted that bees which left their hive at night, attracted to nearby lights, were more likely to contain the parasites than those foraging during daylight hours. The scientists studying the fly-bee interactions are hoping to shed light on similar hive-abandonment behaviors associated with Colony Collapse Disorder. While some have been quick to associate this discovery as a significant culprit behind CCD, there is no evidence that the fly is a major contributing factor.

So what does it all mean for beekeepers? Is this the dawn of the next major catastrophe for a struggling industry? Will it reach the epidemic scale of Varroa mites, or remain a minor pest like the bee louse, Braula coeca? None of us can predict the future, but the problem is far from epidemic. So far.

The presence of A. borealis is nothing new. Since the 1920s, specimens have been collected by entomologists from diverse habitats across the United States and Canada. It was not until 2008 that it was found killing honeybees in the San Francisco area. Some parasitized bees were also confirmed in samples from South Dakota, but so far, no other data concerning honeybee attacks have been confirmed. The fly itself is a widely distributed native species whose natural hosts include bumble bees and paper wasps. While the fly has presumably been here for ages, honeybees are relative newcomers in the new world. As Apis mellifera is among the most closely studied animals on the planet, if this relationship with the fly was not novel, it’s reasonable to expect it would have been observed in hives before now.

Beekeepers may view the discovery of another hive pest with trepidation. After all, many are still learning how to cope with small hive beetles. This time, though, a native species threatens our little honeybees, who are really the invaders in this part of the world.

Among insects, parasitism is a common strategy. Cuckoo bees, for instance, are cleptoparasites that lay their eggs in the nest of other solitary bees. Their larvae emerge quickly, kill the egg of the host bee, and consume its food provisions. Many insects among the wasps and flies are endoparasitoids, whose young develop inside another insect, feeding on and killing the host before they emerge. Many are considered beneficial in agriculture. These armies of tiny raiders go virtually unnoticed by most people, but play an effective role at reducing numbers of aphids, caterpillars, and other pests in our gardens and on our farms.

The genus Apocephalus is perhaps best known for a group of “decapitating flies” that parasitize many ant species. Other members of the genus may attack beetles, stingless bees, bumble bees, wasps and spiders, and can be found throughout the new world. A. borealis is far from the only species that preys on honeybees. In South and Central America, many species of bee-killing flies will attack honeybees, although none are generally considered great threats to beekeeping.

Endoparasitoids generally have a limited host range because they must be specially adapted to developing inside of their hosts while overcoming their internal defenses. The tendency to change from one host to another is uncommon for such specialized organisms. A. borealis already has the ability to successfully attack multiple species of hymenoptera, while honeybees do not have especially complicated immune systems. Individual honeybees are short-lived, with behaviors that compensate for their lack of immunities. A bee hive is a meticulously clean environment, and the bees fly, rather than crawl along the ground, among any number of soil-borne pathogens. If honeybees become sick, they often leave the hive voluntarily or may be forcibly expelled by nest mates. This behavior sounds harsh, but effectively protects the rest of the hive from contamination.

Bumble bees live in relatively small nests with just a few hundred members. Colonies die out each winter except for new queens, which hibernate and start new colonies in the spring. Paper wasps, the other known host group for A. borealis, follow a similar season pattern. Perhaps this is why the researchers in California detected higher numbers of parasitic flies in the fall. Population growth of parasites necessarily follows that of their hosts. We see the same trend with Varroa mites, which are more numerous in the hive after a long period of brood rearing than they are in the early spring. As bumble bee and paper wasp populations decline in the fall, the fly parasites may be looking harder for suitable hosts, such as honeybees. Perhaps their meeting was inevitable.

Setting beekeeper concerns aside for a moment, this apparent host-switching is a fascinating phenomenon from a biologist’s perspective. It’s truly example of evolution while-you-wait. Many species of bees are in decline, both in terms of population numbers and in diversity. A. borealis, as a species, depends on locating suitable hosts. Perhaps a few individuals have begun expanding their host range to ensure the survival of their own kind. Who can blame them?

The “zombie” flies have captured the spotlight for a moment, but should not be a cause for alarm. These flies have been among us all along without our noticing. Beekeepers should continue caring for their colonies as before, but should be aware and remain vigilant for strange bee behavior. A team of researchers behind the website www.phoridproject.org is gathering the latest information on new fly-bee interactions. They hope to expand our knowledge of Apocephalus borealis parasitism on bees by enlisting citizen scientists to collect data on its distribution. They plan to publish information and sampling protocols on their website as soon as it becomes available.

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