Varroa destructor and Varroa jacobsoni are parasitic mites that feed off the bodily fluids of adult, pupal and larval bees. Varroa mites can be seen with the naked eye as a small red or brown spot on the bee's thorax. Varroa is a carrier for a virus that is particularly damaging to the bees. Bees that are infected with this virus during their development will often have a visible "K-wing" deformity.

Varroa has led to the virtual elimination of feral bee colonies in many areas and is a major problem for kept bees in apiaries. Some feral populations are now recovering — it appears that they have been naturally selected for varroa resistance.

Varroa was first discovered in Southeast Asia in about 1904, but has now spread virtually worldwide. Varroa was discovered in the United States in 1987, in New Zealand in 2000.

Varroa is generally not a problem for a hive that is growing strongly. When the hive population growth reduced in preparation for winter or due to poor late summer forage the mite population growth can overtake that of the bees and can then destroy the hive. Often a colony will simply abscond (leave as in a swarm, but leaving no population behind) under such conditions.


Life Cycle

The life cycle of the varroa mite is very much synchronized with that of its honey bee host; it may be that hormones or pheromones of honey bees are necessary for the mite to complete its development. The female lays eggs in bee brood cells. Developing mites feed on developing honey bee larvae. Males and females copulate in the cell. The male dies, but pregnant females emerge from the cell along with their bee host and seek another cell to repeat the cycle. It is thought the length of the postcapping period in honey bees is an important indicator of eventual infestation. The longer the postcapping time, the more time there is for more female mites to develop.

Synthetic chemicals

• pyrethroid insecticide (Apistan) as strips
• organophosphate insecticide (Coumaphos (Check-mite)) as strips

Natural occurring chemicals

• Sugar esters (Sucrocide) in spray application
• Oxalic acid trickling method or applied as vapor (Dany's BienenWohl or VARROX-Vaporiser)
• Formic acid as vapor or pads
• Food grade mineral oil as vapor and in direct application on paper or cords.
• Essential oil especially lemon, mint, and thyme oil

Physical

• Screened bottom board with sticky board is a purely physical method. It separates mites that fall through the screen and the sticky board prevents them from crawling back up.
• Dusting with powdered sugar (Dowda Method)
  freezing or heating of capped drone brood method

Behavioral

• swarming or queen arrest method. When the honeybee brood cycle is interrupted the mites cannot multiply either.

Preventive measures and treatment

Varroa mites can be treated with commercially available miticides. Miticides must be applied strictly according to the label in order to minimize the risk of contamination of honey that might be consumed by humans. Proper use of miticides will also help to slow the development of resistance among the mites.

Varroa mites can also be controlled through non-chemical means. Most of these controls are intended to reduce the mite population to a manageable level, not to eliminate the mites completely.

Many beekeepers use a screened bottom board on their hives. When mites occasionally fall off a bee, they must climb back up to parasitize a new bee. If the beehive has a screened floor with mesh the right size, the mite will fall through and can not return to the beehive. The screened bottom board is also being credited with increased circulation of air which reduces condensation in a hive during the winter. (Studies at Cornell University done over several years found that screened bottoms have no measurable effect at all. Northeast Beekeeper Vol 1 #1 Jan 2004)

Powdered sugar (Dowda Method), talc or other "safe" powders with a grain size between 5 and 15 micrometers can be sprinkled on the bees. The powder does not harm the bees (and, if you use sugar, can even become a small source of feed), but does interfere with the mite's ability to maintain its hold on the bee. It is also believed to increase the bees' grooming behavior. This causes a certain percentage of mites to become dislodged. Powdered sugar works best as an amplifier of the effects of a screened bottom board.

Freezing drone brood takes advantage of varroa mites' preference for longer living drone brood. The beekeeper will put a frame in the hive that is sized to encourage the queen to lay primarily drone brood. Once the brood is capped, the beekeeper removes the frame and puts it in the freezer. This kills the varroa mites that are parasitizing those bees. It also kills the drone brood, but most hives produce an excess of drone bees so it is not generally considered a loss. After freezing, the frame can be returned to the hive. The nurse bees will clean out the dead brood (and dead mites) and the cycle continues.

Drone brood excision is a variation applicable to top bar hives. Honeybees tend to place comb suitable for drone brood along the bottom and outer margins of the comb. Cutting this off at a late stage of development ("purple eye stage") and discarding it reduces the mite load on the colony. It also allows for inspection and counting of varroa on the brood.

Small cell foundation (4.9 mm across - about 0.3 mm smaller than standard) is believed to limit the space in each cell that varroa mites have in which to inhabit and also to enhance the difference in size between worker and drone brood with the intention of making the drone comb traps more effective in trapping varroa mites. Small cell foundation has staunch advocates though controlled studies have been generally inconclusive.

The Konya revolving or rotating hive design is a patented invention of Lajos Konya, a beekeeper in Otteveny, Hungary. The hive has a cylindrical brood chamber, circular frames and an apparatus to rotate the frames according to a specific schedule. The rotation is believed to disrupt the varroa mite reproduction cycle with this rotation thereby reducing fecundity of the parasite.

Several attempts have been made (and are continuing) to breed bees with an increased "resistance" to varroa mites. In fact, the Africanized honeybee was originally an experiment to cross-breed mite resistance into the European honeybees common in the Americas.