Varroa destructor

Varroa destructor
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Arachnida
Subclass:	Acari
Order:	Parasitiformes
Suborder:	Mesostigmata
Family:	Varroidae
Genus:	Varroa
Species:	V. destructor
Binomial name
Varroa destructor Anderson & Trueman, 2000

Varroa destructor is an external parasitic mite that attacks the honey bees Apis cerana and Apis mellifera. The disease caused by the mites is called varroosis.

Varroa destructor can only reproduce in a honey bee colony. It attaches to the body of the bee and weakens the bee by sucking hemolymph. In this process, RNA viruses such as the deformed wing virus (DWV) spread to bees. A significant mite infestation will lead to the death of a honey bee colony, usually in the late autumn through early spring. The Varroa mite is the parasite with the most pronounced economic impact on the beekeeping industry. It may be a contributing factor to colony collapse disorder, as research shows it is the main factor for collapsed colonies in Ontario, Canada and Hawaii, USA.

Physical description

The adult mite is reddish-brown in color; has a flat, button shape; is 1–1.8 mm long and 1.5–2 mm wide; and has eight legs.

Reproduction, infection and hive mortality

Mites reproduce on a 10-day cycle. The female mite enters a honey bee brood cell. As soon as the cell is capped, the Varroa mite lays eggs on the larva. The young mites, typically several females and one male, hatch in about the same time as the young bee develops and leave the cell with the host. When the young bee emerges from the cell after pupation, the Varroa mites also leave and spread to other bees and larvae. The mite preferentially infests drone cells.

The adults suck the "blood" (hemolymph) of adult honey bees for sustenance, leaving open wounds. The compromised adult bees are more prone to infections. With the exception of some resistance in the Russian strains and bees with varroa-sensitive hygiene genes developed by the USDA, the European Apis mellifera bees are almost completely defenseless against these parasites (Russian honey bees are one-third to one-half less susceptible to mite reproduction).

The model for the population dynamics is exponential growth when bee brood 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. 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.

Low temperature scanning electron micrograph of V. destructor on a honey bee host

Varroa mites have been found on Tricia larvae of some wasp species, such as Vespula vulgaris, and flower-feeding insects such as the bumblebee, Bombus pennsylvanicus, the scarab beetle, Phanaeus vindex and the flower-fly, Palpada vinetorum. It parasitizes on the young larvae and feeds on the internal organs of the hosts. Although the Varroa mite cannot reproduce on these insects, its presence on them may be a means by which it spreads short distances (phoresy).

Varroa mites on pupa

Varroa mites on pupae

Varroa destructor on bee larva

Introduction around the world

• Early 1960s Japan, USSR
• 1960s-1970s Eastern Europe
• 1971 Brazil
• Late 1970s South America
• 1980 Poland
• 1982 France
• 1984 Switzerland, Spain, Italy
• 1987 Portugal
• 1987 USA
• 1989 Canada
• 1992 United Kingdom
• 2000 New Zealand (North Island)
• 2006 New Zealand (South Island)
• 2007 Hawaii (Oahu, Hawaii Island)

As of mid-2012, Australia was thought to be free of the mite. In early 2010, an isolated subspecies of bee was discovered in Kufra (southeastern Libya) that appears to be free of the mite.

Identification

Until recently, V. destructor was thought to be a closely related mite species called Varroa jacobsoni. Both species parasitize the Asian honey bee, Apis cerana. However, the species originally described as V. jacobsoni by Anthonie Cornelis Oudemans in 1904 is not the same species that also attacks Apis mellifera. The jump to A. mellifera probably first took place in the Philippines in the early 1960s where imported A. mellifera came into close contact with infected A. cerana. Until 2000, scientists had not identified V. destructor as a separate species. This late identification in 2000 by Anderson and Trueman corrected some previous confusion and mislabeling in the scientific literature.

Varroosis

The infection and subsequent parasitic disease caused by varroa mites is called varroosis. Sometimes, the incorrect names varroatosis or varroasis are used. A parasitic disease name must be formed from the taxonomic name of the parasite and the suffix -osis as provided in the Standardised Nomenclature by the World Association for the Advancement of Veterinary Parasitology. For example, the World Organisation for Animal Health (OIE) uses the name varroosis in the OIE Terrestrial Manual.

Treatments have met with limited success. First, the bees were medicated with fluvalinate, which had about 95% mite falls. However, the last 5% became resistant to it, and later, almost immune. Fluvalinate was followed by coumaphos.

Control or preventive measures and treatment

Honeybee coated with oxalic acid to protect from mites

Chemical measures

Varroa mites can be treated with commercially available miticides. Miticides must be applied carefully to minimize the contamination of honey that might be consumed by humans. Proper use of miticides also slows the development of resistance by the mites.

Synthetic chemicals

• Pyrethroid insecticide (Fluvalinate) as strips
• Organophosphate insecticide (Coumaphos or Check-mite) as strips
• Manley's Thymol Crystal and surgical spirit recipe with sugar as food

Naturally occurring chemicals

• Formic acid as vapor or pads (Mite-Away)
• Powdered sugar (Dowda method), talc, or other "safe" powders with a grain size between 5 and 15 µm (0.20 and 0.59 mils) can be sprinkled on the bees.
• Essential oils, especially lemon, mint and thyme oil
• Sugar esters (Sucrocide) in spray application
• Oxalic acid trickling method or applied as vapor
• Mineral oil (food grade) as vapor and in direct application on paper or cords
• Natural hops compounds in strip application (Hopguard)

Physical, mechanical, behavioral methods

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

• Perforated bottom board method is used by many beekeepers on their hives. When mites occasionally fall off a bee, they must climb back up to parasitize another bee. If the beehive has a screened floor with mesh the right size, the mite will fall through and cannot 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 two years found that screened bottoms have no measurable effect at all. Screened bottom boards with sticky boards (glue traps) separate mites that fall through the screen and the sticky board prevents them from crawling back up. Insecticide may also be applied to the sticky boards to help kill the mites.

• Heating method, first used by beekeepers in Eastern Europe in the 1970s, later became a global method. In this method, hive frames are heated to at least 104 deg.F (40 deg.C) for several hours at a time, which causes the mites to drop from the bees. When combined with the perforated bottom board method, this can control varroa sufficiently to aid colony survival. In Germany, anti-varroa heaters are manufactured for use by professional bee keepers. A thermosolar hive has been patented and manufactured in the Czech Republic.

• Limited drone brood cell method limits the brood space cell for Varroa mites to inhabit (4.9 mm across — about 0.5 mm smaller than standard), and also enhances 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 foundations have staunch advocates, though controlled studies have been generally inconclusive.

• Comb trapping method (also known as the swarming method) is based on interrupting the honey bee brood cycle. It is an advanced method that removes capped brood from the hive, where the Varroa mites breed. The queen is confined to a comb using a comb cage. At 9-day intervals, the queen is confined to a new comb, and the brood in the old comb is left to be reared. The brood in the previous comb, now capped and infested with Varroa mites, is removed. The cycle is repeated. This complex method can remove up to 80% of Varroa mites in the hive.

• Freezing drone brood method takes advantage of the 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 feeding on 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 method is a variation applicable to top bar hives. Honey bees tend to place combs 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 mites on the brood.

Genetic engineering

Researchers have been able to use RNA interference to knock out genes in the Varroa mite. There have also been efforts to breed for changes in the honey bees. Two strains have been developed in the United States that can detect damaged pupae under cappings and remove them before the infestation spreads further. Another strain is under development that can more easily recognize adult phoretic varroa so they can be groomed and removed from the hive.

See also

• Colony collapse disorder (CCD)

Further reading

• Tracheal and Varroa Mite Controls Apiculture Factsheet #221, Ministry of Agriculture, Food and Fisheries, Government of British Columbia; April 2004
• Oils for Varroa Control Botanicals For Mite Control, Canadian Honey Council, 3/16/2003

External links

• Varroa mite on the University of Florida / Institute of Food and Agricultural Sciences Featured Creatures website
• [1]
• Beediseases