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Varroa destructor is an external parasitic mite that attacks honey bees Apis cerana and Apis mellifera. It was until recently mislabeled as Varroa jacobsoni.

Varroa mites have been found on flower feeding insects such as the bumblebee Bombus pennsylvanicus, the scarab beetle Palpada vinetorum and the flower-fly Phanaeus vindex (Kevan et al. 1990). Although the varroa mite cannot reproduce on other insects, its presence on them may be a means by which it spreads short distances.

Varroa can only replicate in a honey bee colony. It attaches at the body of the bee and weakens the bee by sucking haemolymph. In this process the mite may also spread RNA viral agents to the bee. A significant mite infestation will lead to the death of a honeybee colony, usually in the late fall through early spring. The varroa mite has been the parasite with the most pronounced economic impact on the beekeeping industry.

Origin

Varroa destructor was until recently thought to be a closely related mite species called Varroa jacobsoni . Both species parasitize the Asian honey bee, Apis cerana. The mite species originally described as V. jacobsoni by Oudemans in 1904 is part of the same species complex, but not the same species that made the jump to Apis mellifera. That jump probably first took place in the Philippines in the early 1960’s. Only after Apis mellifera were imported to the Philippines, it came into close contact with Apis cerana. Varroa as a parasite of Apis cerana, also became a parasite of Apis mellifera. Up until 2000, scientists did not positively identify Varroa destructor as a separate species. In 2005, we know that the only varroa mites that can reproduce in colonies of Apis mellifera (Western honeybee) are the Korea and Japan/Thailand genotypes of Varroa destructor. Varroa jacobsoni is a fairly benign parasite of Apis cerana. This late identification in 2000 by Anderson and Trueman led to some confusion and mislabeling in the scientific literature.

Anatomy

  • Reddish-brown in color
  • 1.00-1.77 mm long and 1.50-1.99 mm wide
  • Flat, button shape
  • Eight legs

Life cycle

The female mite enters a honeybee brood cell. As soon as the cell is capped, the mite lays eggs. The young mites hatch in about the same time as the young bee develops and leave the cell with the host. The Swiss Bee Research Centre scientifically investigated the life cycle of the varroa in the capped cell and described it in a paper (A Look under the cap).

The model for the population dynamics is that there is exponential growth when bee brood is available and exponential decline when no brood is available. High mite populations in the fall can cause a crisis when drone rearing ceases and the mites switch to worker larvae, causing a quick population crash and often hive death.

Reproduction

Mites reproduce on a 10-day cycle. In 12 weeks the number of mites in a Western honeybee hive can roughly multiply by 12. Russian honey bees are one third to one half less susceptible to mite reproduction.

Source: Russian Queens Bee-little Mites' Impact by J. Raloff, Science News, Aug. 8, 1998

Varrioatosis

The parasitic disease caused by varroa mites is called varroatosis.

Its treatment has been of limited success. First the bees were medicated with fluvinate which had about 95% mite falls. It was a good product but, the last five percent became resistant to it and later, almost immune. Fluvinate was followed by coumophose; an active ingredient in VX nerve gas. Apis cerana has developed a tolerance to these parasites and has learned to thrive with them.

The European Apis mellifera bees are almost completely defenseless against these parasites, with the exeption of the Russian Honey Bee. What makes Varroa mites so bad for beekeeping is the way which they survive. The adults suck the "blood" of adult honey bees for sustenance and reproduce on the honey bee larvae. The adult honeybees are at this point, due to their open wounds, more prone to BPV and DWV. The varroa mite reproduces inside a cell with a honey bee larva. The varroa mite lays eggs on the larva which hatch into females and finally lays one male. The bee larva, in the pupal stage, dies and as the other bees open the cell to remove the dead larva the varroa mites spread to other bees and larva.

Worldwide expansion

Control methods in beehive

Synthetic chemicals Natural occurring chemicals 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 measureable 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 micrometres 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.

References

  1. D. Anderson & J. W. H. Trueman (2000). "Varroa jacobsoni (Acari: Varroidae) is more than one species." Experimental & Applied Acarology, 24, 165-189.
  2. Notes on Varroa destructor (Acari: Varroidae) parasitic on honeybees in New Zealand ZHI-QIANG ZHANG, Systematic & Applied Acarology Special Publications (2000) 5, 9-14

Kevan, P., et al. 1990 Association of Varroa jacobsoni with organisms other than honeybees and implications for its dispersal. Bee World 71: 3, 119-121.

Acari | Beekeeping | Parasites

Varroa-mide | Varroamilbe | Varroa | Varroa destructor | Varroamijt | Warroza

 

This article is licensed under the GNU Free Documentation License. It uses material from the "Varroa destructor".

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