In the search for new avenues to protect honey bees (Apis mellifera) from Varroa mites, researchers have identified breeds of A. mellifera that show signs of “social apoptosis,” a defense mechanism common in Asian bees that are resistant to Varroa infestation. Here, a honey bee (not part of the research summarized here) carries two Varroa destructor mites, one feeding on the side of its abdomen and one hitching a ride on the top of its abdomen. (Public domain photo via USGS Bee Inventory and Monitoring Lab on Flickr)

By Andrew Porterfield

Andrew Porterfield

The mite Varroa destructor has been a devastating parasite and disease vector to honey bees worldwide. After being introduced to the honey bee (Apis mellifera), it has resulted in enormous bee colony deaths over the past two decades.

However, the mite’s original host, the Asian bee Apis cerana, has been able to survive mite infestations and avoid the colony collapses seen in western honey bees.

One significant difference appeared to be a social response among A. cerana called “social apoptosis.” Bee colonies exhibiting this behavior involve delayed development and eventual “intentional” death by female (worker) bees. On the colony level, social apoptosis produces resistance to Varroa. Researchers have found some resistance among stocks of western honey bees recently. Could “social apoptotic” behavior be behind this resistance and develop in western honey bees, too?

A team of researchers from the U.S. Department of Agriculture’s Honey Bee Breeding, Genetics. and Physiology Laboratory in Baton Rouge, Louisiana, led by research molecular biologist Kate Ihle, Ph.D., compared Varroa infestations in two stocks of A. mellifera known to show resistance to the mite, with a third, Varroa-susceptible Italian stock of A. mellifera bees. Their findings—that A. mellifera did show some social apoptosis behavior that conferred resistance—were published in January the open-access Journal of Insect Science, part of a special collection featuring research in the United States on fundamental and applied aspects of honey bee biology, published in collaboration with the American Association of Professional Apiculturists.

Varroa mites do their damage by invading honey bee cells just before adult workers seal those cells with wax, which allows for development of bees in the hive. The founding mite (a female) punctures the developing bee inside the cell, and she and her offspring feed on the larva. This opening also allows pathogens (such as deformed wing virus) into the colony, contributing to its mortality and morbidity. Untreated infestations can kill an entire honey bee colony in one to three years.

Because Varroa mites and A. cerana honey bees share a longer evolutionary history, the bee species has developed a number of defenses against the mite. Mites only reproduce in drone (male) broods of A. cerana, which are usually outnumbered by workers (females). In A. mellifera, the mites reproduce in both broods. When Varroa invades A. cerana worker broods, the bees uncap infested cells and kill and discard the infested larvae. Social apoptosis expands on this behavior to produce social immunity against the mite, sacrificing individual young (pupae) to prevent infection.

A diagram illustrates the experiment design in a research study exploring social apoptosis behavior in varying strains of honey bees (Apis mellifera): “””For each experimental replicate, a frame from each stock containing newly laid eggs was placed in a single host colony with either a high or low Varroa mite population. Frames of brood were removed from the host colonies within 12 hr of capping of the cells to receive treatment. Cell caps were carefully removed, and brood was assigned to either control, experimental wounding, or experimental infestation with a foundress mite collected from several mite source colonies. In brood reared in highly infested colonies, we also included cells which contained a mite when opened in a fourth treatment group: naturally infested cells. Cells were then sealed with a clear gelatin cap, which could be used as a viewing window for the remainder of the study. Finally, brood was allowed to mature in an incubator to prevent removal by workers.””” (Image originally published in Ihle et al 2022, Journal of Insect Science)

The USDA team tested two Varroa-resistant honey bee stocks: a “Pol line” bee raised by the USDA laboratory, and a Russian honey bee imported from far eastern Russia, which is known for its long association with Varroa. Each stock had six colonies of bees. The team looked for social apoptosis by comparing survival and bee development after Varroa inoculation and micro-punctures. Mortality and delayed development were then compared to the Italian honeybees, which are susceptible to Varroa.

New broods of bees were placed in three groups: a control, a group wounded with a needle, and a third group, infested with a single mite. Nine days after capping and treatment of cells and larvae, the researchers removed the broods from the cells and evaluated pupae development and mite reproduction.

The researchers found social apoptosis behavior in the Russian stock of A. mellifera, which appeared to be tied directly to Varroa infestation. The experimental Pol line also experienced increased mortality when housed in lightly infested colonies. The researchers warned, however, that the study’s small sample size prevented more definitive findings, and they called for further studies.

“Host colony infestation status had a very large effect on brood survival,” the scientists write. “Survival was much lower in brood housed in colonies with high versus low levels of Varroa infestation.” High infestations “seem to overwhelm the effects of additional stress induced by our treatments.”

While European honeybee populations have recovered from colony collapse disorders in the mid-2000s, the risk of bee losses from Varroa mites remains high. Chemical treatments against the mites have been used to some beneficial effect, but Varroa resistance to insecticides has grown. “Breeding Varroa-resistant honey bee stocks may be a more sustainable alternative to developing new treatments,” the researchers write.

Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.