varroa mite

varroa mite, (Varroa destructor), species of parasitic mite that is a serious pest of the western honeybee (Apis mellifera). Capable of devastating entire western honeybee colonies, the parasite is considered a factor in colony collapse disorder (CCD) and is one of the most significant threats to both feral and managed honeybees. Though native to Asia, the varroa mite is now found nearly worldwide; it is considered an invasive species in Europe, North America, Australia, and other areas outside its original range. Although most commonly associated with V. destructor, varroa mite can also refer to other mites of the genus Varroa.

Physical description

The varroa mite is an ectoparasite: it attaches to the outside of the host’s body. Adult female varroa mites are visible to the unaided eye, appearing as flat reddish brown ovals on the abdomen or thorax of infested adult honeybees. They are about 1.1 mm (0.04 inch) in width and 1.6 mm (0.06 inch) in length. Adult males are smaller, with light tan legs and spherical yellowish bodies; they are rarely seen. Female mites are covered in small bristlelike structures, called setae, that help hold them against their hosts.

Life cycle

In Asia the varroa mite is a parasite of the eastern honeybee (Apis cerana), but it does not typically cause serious damage to their colonies. This is because the eastern honeybee coevolved with the varroa mite and thus has some natural defenses against the mite. In these colonies, varroa mites tend to reproduce on the drone (male) brood, which is less than 5 percent of the colony’s population. If the mites attack the worker (female) brood, adult bees carry the infested pupae outside the colony to limit the spread of the parasite. Adult workers regularly groom one another, picking off the mites and killing them. When western honeybee colonies were introduced in Asia, in the mid-20th century, the varroa mite was quickly able to exploit the novel bee as a defenseless host, and the pest was soon spread to vulnerable western honeybees worldwide.

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Depending on the stage of the mites’ life cycle, female varroa mites can attach to and feed on both brood and adult honeybees. (Male mites live only on the brood and die after mating.) During their phoretic, or traveling, life stage, female mites feed on adult honeybees, typically hiding in the insects’ abdominal folds or between the wings on the thorax. Newly emerged bees and workers that tend the brood are preferred hosts, and the arachnids frequently change host individuals. The mites will also attach to foraging worker bees, and it is during the phoretic stage that the mites can be spread to other colonies by “drifter bees,” which inadvertently wander from their own colony to another, and during honeybee raids of another colony’s honey. Human action, such as combing hives or moving food stores between hives, can also spread the mites. Phoretic mites are vulnerable to bee grooming (particularly in eastern honeybee colonies) and experience high fatality rates in this life stage.

During the mites’ reproductive phase, the female mites make their way from their adult hosts to the brood areas of the hive. They each then enter a honeybee egg compartment before worker bees have sealed it with a waxy cap and hide in the food store. The mites are generally more attracted to the drone bee brood, but they will also parasitize the worker bee brood. The gravid female mite is sealed into the egg compartment by unwitting worker bees, and she quickly lays her own tiny white eggs on the bee larva. She lays fertilized eggs, which develop into female mites, and unfertilized eggs, which are males. The white transparent mite nymphs feed on the growing bee as they undergo several molts until they reach adulthood. An infected bee larva often makes it to adulthood—often in a weakened and underweight state—but may die at the pupal stage. Female mites mate with males before leaving the brood cell with their mother, and males die shortly after mating. A single female mite may reproduce five to seven times in her life.

Colony collapse

Colony collapse disorder (CCD)—in which a honeybee colony experiences sudden death—may be caused by a combination of factors, including infestation by varroa mites and other parasites, viruses, and pesticides and agrochemicals, particularly neonicotinoid insecticides. A study published in the Journal of Insect Science in 2024 found that pollen contaminated with two types of neonicotinoids, clothianidin and thiamethoxam, was associated with increased rates of varroa mite parasitism in western honeybee colonies. It is thought that neonicotinoids may suppress the bees’ immune system, increasing their colony’s vulnerability to varroa mites and the viruses they introduce. Indeed, the viruses transmitted by feeding mites—including deformed wing virus and black queen cell virus—can cause malformations in the bees’ wings, bodies, and legs, resulting in more damage to the bees than the feeding of the mites alone. All these factors, coupled with stressors including habitat loss and climate change, seem to contribute to the sudden die-off of adult honeybees that characterizes CCD.

Control

It is impossible to eradicate varroa mites from an infected honeybee colony completely. If the colony is left untreated, an infestation of the mites can wipe it out in a matter of months, and unmanaged or feral colonies are typically dead within two or three years. Treatment usually focuses on keeping the number of mites below the threshold at which they begin to damage the colony seriously. Some beekeepers focus on just one treatment method, such as the application of chemicals, but integrated pest management—which uses a combination of methods—has been found to be most effective. Beekeepers continuously monitor the population levels of the mites, employing a variety of sampling strategies and rotating treatment methods as needed.

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Treatment methods include using genetic bee stocks that show resistance to mites (including Africanized honeybees), applying fumigants such as formic-acid- or thymol-based products to kill mites, isolating colonies to keep bees from transporting mites between the colonies, keeping the bee populations of hives at reduced densities, and using screened, sticky bottom boards on hives to capture mites that fall off bees as they groom one another.

Disturbingly, some mite populations have shown increasing resistance to miticides and other chemical controls, and the chemicals themselves may play a role in CCD. In addition, climate change is expected to exacerbate varroa mite infestations. Milder winters mean that honeybees have longer foraging seasons—and thus more time to potentially spread mites from hive to hive—and winter temperatures in many places no longer get low enough for long enough to limit mite populations within colonies significantly. Higher mite populations over winter exact a heavy toll on overwintering bee colonies, often resulting in high losses.