By Meredith Swett Walker
Imagine a parasite about the size of a grapefruit, and it’s latched onto your back where you just can’t reach it. Now imagine that parasite is sucking your blood and that its cronies are reproducing rapidly in your home and attacking your family. This horrifying scenario is essentially what the mite Varroa destructor inflicts on honey bees. The aptly named destructive parasite is a leading cause of colony collapse disorder in honey bees and responsible for significant economic losses in the beekeeping industry.
A study published in March in the Journal of Economic Entomology examines whether honey bees specially bred to be “mite-resistant” might be the solution to Varroa infestations. Researchers at the USDA’s Carl Hayden Bee Research Center compared Varroa mite populations in hives of the Russian honey bee, a stock of the European honey bee (Apis mellifera) that has been selected (or bred) for its resistance to Varroa, with mite populations in hives of honey bees that have not been selected for mite-resistance. Previous studies have shown that Russian bees have lower levels of Varroa infestation than do unselected lines of European honey bees. This study measured mite populations in the hives, but it also measured the numbers of foraging worker bees with mites on them that went in and out of hives—a variable that proved to be crucial.
Varroa mite infestations wreak havoc on honey bee colonies. The mites feed on the bee’s hemolymph—a bodily fluid analogous to blood—weakening the bee. Their bites leave open wounds on bees and can transmit serious diseases. Female Varroa mites lay their eggs on developing bee larvae and young mites feed on the larvae before they emerge from their brood cells. Bees who have been parasitized as larvae may have deformities and shortened life spans.
European honey bees have limited defenses against Varroa mites, and infestations frequently kill colonies. Beekeepers typically control Varroa by treating hives with mite-specific pesticides, or “miticides.” These can be effective, but Varroa is developing resistance to some of these chemicals. Also, miticides can contaminate honey and wax harvested from treated hives. This has led some beekeepers to turn to mite-resistant stocks of honey bees to avoid using miticides.
The mite-resistant “Russian honey bee” is a strain of A. mellifera that originated in the Primorsky Krai region of Russia where A. mellifera had been exposed to Varroa since around 1900. Some strains of A. mellifera in the region appeared to have developed a degree of genetic resistance to Varroa, so the USDA Agricultural Research Service evaluated these strains, and, in 1997, mite-resistant Russian strains were imported into the United States.
Russian honey bees resist Varroa infestations because they exhibit hygienic behaviors such as increased grooming and removal of parasitized larvae. These bee behaviors make it harder for Varroa to reproduce within the hive and thus reduce mite populations. But recent research suggests that the mite population in a bee colony is not solely dependent on mite reproduction within the colony; it also depends on mites being introduced from outside the colony. Gloria DeGrandi-Hoffman, Ph.D., a research leader at the Carl Hayden Bee Research Center and first author on the Russian honey bee study says mites can enter the colony by hitching rides on foraging workers.
There are two ways this can happen, she says. When a colony is badly infested with Varroa, it begins to collapse. Workers from the failing colony may sometimes drift into neighboring hives, bringing their mites along with them. Alternatively, foraging workers from a healthy colony may enter a collapsing, mite-infested hive to steal honey, and they can inadvertently bring mites back to their home hive along with their loot.
The number of mites hitching rides into to a colony on foraging bees is key because the hygienic behaviors that suppress Varroa infestations in Russian honey bee hives may not be effective against these hitchhiking mites. Indeed, DeGrandi-Hoffman and colleagues found that at a study site where similar numbers of foragers with mites were observed at Russian and European honey bee colonies, the two types of colonies showed similar levels of mite infestations. At another study site, where fewer foragers with mites were collected at Russian hives than at European hives, the Russian colonies had smaller Varroa populations.
This suggests that when there are few foraging workers transferring mites between colonies, the Russian honey bee’s mite-resistant behaviors are able to suppress Varroa populations relative to mite populations in European colonies. But at locations or times when there are greater numbers of foragers with mites, Russian honey bees may be just as susceptible to Varroa infestations as non-mite resistant bees.
Sadly, climate change may exacerbate the Varroa problem. Mite populations within a colony increase in the fall, as do the numbers of foraging bees carrying mites. “As fall temperatures get warmer and periods of flight weather continue into November, not only do bees fly late in the fall when they should be in the hive in winter cluster, but mites can continue to migrate into colonies on foragers,” says DeGrandi-Hoffman.
Breeding stocks of honey bees that do not admit foragers from other hives or that exclude foragers with mites could help control Varroa infestations. But in the meantime, Russian honey bees may only be able to resist Varroa in areas in which mites are already well-controlled—or, in other words, neighborhoods where bees are unlikely to pick up hitchhikers.
Journal of Economic Entomology
Meredith Swett Walker is a former avian endocrinologist who now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. You can read her work on her blogs Pica Hudsonia and The Citizen Biologist or follow her on Twitter at @mswettwalker.