Varroa destructor, the Varroa mite, is an external parasitic mite that attacks and feeds on honey bees and is one of the most damaging honey bee pests in the world. A significant mite infestation leads to the death of a honey bee colony, usually in the late autumn through early spring. Without management for Varroa mite, honey bee colonies typically collapse within 2 to 3 years in temperate climates. These mites can infest Apis mellifera, the western honey bee, and Apis cerana, the Asian honey bee. Since it is very similar physically to the closely related Varroa jacobsoni, these species were thought to be one prior to 2000, but they were found to be two separate species by DNA analysis.
Parasitism of bees by mites in the genus Varroa is called varroosis. The Varroa mite can reproduce only in a honey bee colony. It attaches to the body of the bee and weakens the bee. The species is a vector for at least five debilitating bee viruses, including RNA viruses such as the deformed wing virus (DWV). The Varroa mite is the parasite with possibly the most pronounced economic impact on the beekeeping industry and is one of multiple stress factors contributing to the higher levels of bee losses around the world. Varroa mite has also been implicated as one of the multiple causes of colony collapse disorder.
Management of this pest focuses on reducing mite numbers through monitoring to avoid significant hive losses or death. 3% of bees infested in a hive is considered an economic threshold where damage is high enough to warrant additional management. Miticides are available, though some are difficult to time correctly while avoiding harm to the hive, and resistance has occurred for others. Screened bottom boards on hives can be used for both monitoring and mite removal, and drone comb, which mites prefer, can be used as a trap to remove mites from the hive. Honey bee lines in breeding programs also show partial resistance to Varroa mite through increased hygienic behavior that is being incorporated as an additional management strategy.
Description and taxonomy The adult female mite is reddish-brown in color, while the male is white. Varroa mites are flat, having a button shape. They are 1–1.8 mm long and 1.5–2 mm wide, and have eight legs. Varroa mites lack eyes. These mites have curved bodies that allow them to fit between the abdominal segments of adult bees.
Host bee species can help differentiate mite species in the genus Varroa; both V. destructor and Varroa jacobsoni parasitize Apis cerana, the Asian honey bee, but the closely related mite species originally described as V. jacobsoni by Anthonie Cornelis Oudemans in 1904 does not attack Apis mellifera, the western honey bee, unlike V. destructor. Until 2000, V. destructor was thought to be V. jacobsoni and resulted in some mislabeling in the scientific literature. The two species cannot be easily distinguished with physical traits and have 99.7% similar genomes, so DNA analysis is required instead. Because the more virulent and damaging species V. destructor could not be distinguished at the time, most pre-2000 research on western honey bees that refers to V. jacobsoni was actually research on V. destructor.
Other Varroa species V. underwoodi and V. rindereri can also parasitize honey bee species and can be distinguished from V. destructor and V. jacobsoni with slight differences in body size and setae characteristics, though each of the four species within the Varroa genus have similar physical characteristics. If a Varroa species is found on a western honey bee, it will typically be V. destructor except where V. underwoodi is present, such as in Papua New Guinea.
The name "Varroa mite" is typically used as the common name for V. destructor after the species was considered separate from V. jacobsoni.
+ Bee hosts of Varroa species Mite species Bee host Varroa destructor western honey bee, Asian honey bee Varroa jacobsoni Asian honey bee Varroa rindereri Apis koschevnikovi Varroa underwoodi western honey bee, Asian honey bee, Apis nigrocincta, Apis nuluensis
Varroa mite has two distinct genetic strains from when it switched hosts from the Asian honey bee to the western honey bee: Korean and Japanese. The Korean strain that emerged in 1952 is now found worldwide in high frequencies, while the Japanese strain that started around 1957 occurs in similar areas at much lower frequencies. Varroa mite has low genetic diversity, which is typical for an invasive species undergoing a range or host expansion.
Range Varroa mites originally occurred only in Asia on the Asian honey bee, but this species has been introduced to many other countries on several continents, resulting in disastrous infestations of European honey bees.Invasion Biology Introduction: Varroa mites University of Columbia. Accessed 26 April 2017
Introduction data prior to 2000 is unclear because of confusion with V. jacobsoni. By 2020, V. destructor was confirmed to be present throughout North America (excluding Greenland), South America, most of Europe and Asia, and portions of Africa. The species was not present in Australia as well as Oman, Congo, Democratic Republic of Congo, and Malawi. It was suspected to not be present in Sudan and Somalia. Mites were found in 2022 in New South Wales in Australia.
Life cycle Female mites enter brood cells to lay eggs on the comb wall just before the cell is capped. Eggs are approximately 0.2 to 0.3 mm in diameter and cannot be seen without magnification. These eggs hatch into male and female protonymphs that are both transparent white. Immature mites can only feed on capped brood, so the life cycle cannot be completed during broodless periods. Protonymphs molt into deuteronymphs that more closely resemble the curved body of adults before they molt into adults. Development time from egg to adult is 67 days. Males will not leave brood cells and only mate with females present in the brood cell.
Adult females can be found feeding both on brood and adult bees. After reaching the adult stage, females will leave the brood cell and enter a phoretic stage where mites attach to adult bees in order to disperse. Mites will feed on adult bees at this time and can be transmitted from bee to bee during this stage. Nurse bees are preferred hosts in order to be moved to new brood cells. Because the nurse bee spends more time around the drone brood (i.e., male bees) rather than the worker brood, many more drones are infected with the mites. These phoretic females can also be transmitted to other hives through bee contact or hive equipment transfer. The phoretic stage can last for 4.511 days during brood production periods or up to five to six months when no brood is present in winter months. Female mites have a life expectancy of 27 days when brood is present.
After the phoretic stage, female mites leave the adult bee and enter brood cells with bee larvae. Drone cells are preferred over workers. These females are called foundress mites, and they bury themselves in brood food provided by worker bees before the cell is capped. Brood cell capping begins egg cell activation for a foundress mite while she emerges to feed on the larva. She will lay a single unfertilized egg after feeding to produce a male mite. After laying this egg, fertilized eggs to produce females are laid approximately once a day. Both the mother and nymphs will feed on the developing pupa. Unless multiple foundress mites are present in a cell, mating occurs between siblings when they reach the adult stage. Once females mate, they are unable to receive additional sperm. Varroa mite's genetic bottleneck is also likely due to its habit of sibling mating.
Host interactions
Adult mites feed on both adult bees and bee larvae by sucking on the fat body, an insect organ that stores glycogen and triglycerides with tissue abundant under epidermis and the surrounding internal body cavity. As the fat body is crucial for many bodily functions such as hormone and energy regulation, immunity, and pesticide detoxification, the mite's consumption of the fat body weakens both the adult bee and the larva. Feeding on fat body cells significantly decreases the weight of both the immature and adult bee. Infested adult worker bees have a shorter lifespan than ordinary worker bees, and they furthermore tend to be absent from the colony far more than ordinary bees, which could be due to their reduced ability to navigate or regulate their energy for flight. Infested bees are more likely to wander into other hives and further increase spread. Bees will occasionally drift into other nearby hives, but this rate is higher for Varroa infested bees.
Adult mites live and feed under the abdominal plates of adult bees primarily on the underside of the abdominal region on the left side of the bee. Adult mites are more often identified as present in the hive when on top of the adult bee on the thorax, but mites in this location are likely not feeding, but rather attempting to transfer to another bee.
Varroa mites have been found on flowers visited by worker bees, which may be a means by which phoretic mites spread short distances when other bees, including from other hives, visit. They have also been found on larvae of some wasp species, such as Vespula vulgaris, and flower-feeding insects such as the bumblebee, Bombus pensylvanicus, the scarab beetle, Phanaeus vindex, and the flower-fly, Palpada vinetorum. There have not been any indications that Varroa mites are able to complete their life cycle on these insects, but instead they become distributed to other areas while a mite is still alive on these insects.
Virus transmission
Open wounds left by the feeding become sites for disease and virus infections. The mites are vectors for at least five and possibly up to 18 debilitating bee viruses, including RNA viruses such as the deformed wing virus.
Prior to the widespread introduction of Varroa mite, honey bee viruses were typically considered a minor issue. Virus particles are directly injected into the bee's body cavity and mites can also cause immunosuppression that increases infection in host bees. Varroa mites can transmit the following viruses: Kashmir bee virus Sacbrood virus Acute bee paralysis virus Israeli acute paralysis virus Deformed wing virus
Deformed wing virus is one of the most prominent and damaging honey bee viruses transmitted by Varroa mites. It causes crumpled deformed wings that resemble sticks and also causes shortened abdomens.
Colony collapse disorder
There is some evidence that harm from both Varroa mite and associated viruses they transmit may be a contributing factor that leads to colony collapse disorder (CCD). While the exact causes of CCD are not known, infection of colonies from multiple pathogens and interaction of those pathogens with environmental stresses is considered by entomologists to be one of the likely causes of CCD. Most scientists agree there is not a single cause of CCD.
Management Mite populations undergo exponential growth when bee broods are available, and exponential decline when no brood is available. In 12 weeks, the number of mites in a western honey bee hive can multiply by roughly 12. Mites often invade colonies in the summer, leading to high mite populations in autumn. High mite populations in the autumn can cause a crisis when drone rearing ceases and the mites switch to worker larvae, causing a quick population crash and often hive death. Various management methods are used for Varroa mite integrated pest management to monitor and manage damage to hives.
Monitoring Beekeepers use several methods for monitoring levels of Varroa mites in a colony. They involve either estimating the total number of mites in a hive by using a sticky board under a screen bottom board to capture mites falling from the hive or estimating the number of mites per bee with powdered sugar or an ethanol wash.