Varroa destructor

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Varroa destructor
Varroa destructor, Photo by Scott Bauer
Varroa destructor, Photo by Scott Bauer
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Arachnida
Order: Acari
Family: Parasitidae
Genus: Varroa
Species: V. destructor
Binomial name
Varroa destructor
Anderson & Trueman, 2000
Low Temperature Scanning Electron Microscope (LTSEM) image of Varroa destructor on a honey bee host
File:Vorroa Mite on pupa.JPG
Varroa mites on pupa
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Varroa destructor on bee larva.

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

Varroa destructor can only replicate in a honey bee colony. It attaches at the body of the bee and weakens the bee by sucking hemolymph. In this process the mite spreads RNA viruses like Deformed Wing Virus to the bee. 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 has been the parasite with the most pronounced economic impact on the beekeeping industry. It may be a contributing factor to Colony Collapse Disorder (CCD) which is threatening hives throughout North America.

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 which hatch into several females and typically one male. The young mites 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" 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 Honey Bee, 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 [1].) Apis cerana has developed grooming procedures that remove these parasites so they are not a threat to these hives.

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 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.

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

Introduction around the world

Identification

Varroa destructor was, until recently, thought to be a closely related mite species called Varroa jacobsoni[2] [3] [4]. Both species parasitize the Asian honey bee, Apis cerana. However, the species originally described as V. jacobsoni by Oudemans in 1904 is not the same species that also attacks Apis mellifera. The jump to mellifera probably first took place in the Philippines in the early 1960’s where imported Apis mellifera came into close contact with infected Apis cerana. Up until 2000, scientists had not identified Varroa destructor as a separate species. This late identification in 2000 by Anderson and Trueman corrected some previous confusion and mislabeling in the scientific literature. As of 2005, the only Varroa mites that can reproduce in colonies of Apis mellifera (Western honeybee) are the Korea and Japan/Thailand genotypes of Varroa destructor.

Control or preventive measures and treatment

Chemical measures

Varroa mites can be treated with commercially-available miticides. Miticides must be applied strictly according to the label in order to minimize the 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.

Synthetic chemicals

Natural occurring chemicals

Physical or mechanical methods

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)
  • Screened bottom board with sticky board. It separates mites that fall through the screen and the sticky board prevents them from crawling back up.
  • Small cell foundation (4.9 mm across - about 0.5 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 Öttevény, 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.

Behavioral methods

  • 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. Honey bees 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.
  • Swarming or queen-arrest method. By interrupting the honey bee brood cycle, mites reproduction is also blocked.
  • Hygienic Behavior. Hygienic behavior is biological behavior with genetic traits that can be bred into bees. This behavior causes bees to smell infected brood and remove them before the infestation spreads further. http://www.sare.org/publications/factsheet/0305_01.htm

Varroatosis

The infection and subsequent 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. However the last five percent became resistant to it and later, almost immune. Fluvinate was followed by coumophose, an active ingredient in VX nerve gas.

References

  1. ^ Anderson, D & Trueman, J. W. H. (2000). "Varroa jacobsoni (Acari: Varroidae) is more than one species." Experimental & Applied Acarology, 24, 165-189.
  2. ^ ZHANG, ZHI-QIANG Notes on Varroa destructor (Acari: Varroidae) parasitic on honeybees in New Zealand Systematic & Applied Acarology Special Publications (2000) 5, 9-14
  3. ^ Delaplane, Keith S. Varroa destructor: Revolution in the Making University of Georgia; Bee World; 2001; 82(4): 157-159
  • British Ministry of Agriculture, Fisheries and Food Managing varroa , 2005
  • 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.

Notes

External links

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).


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