Varroa Mites and Honey Bees

Varroa Mites and Honey Bees

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Varroa mites, sometimes referred to as varroa destructors, are parasitic mites that attack certain species of honey bees. Varroa mites are button shaped, 1 to 1.8 millimeters long and up to 2 millimeters wide; they are reddish brown in color. These pernicious mites can only replicate in a honey bee colony; they latch onto a bee’s body and suck hemolymph, which for bees serves as both blood and intestinal fluid. Hemolymph is not used for oxygen circulation, as blood is in mammals; bees respirate directly through their body surfaces. However, hemolymph is crucial in circulating nutrients such as proteins and sugars to the cells.

The varroa mites weaken bees by sucking hemolymph. In their weakened condition, the stricken bees become susceptible to RNA viruses (viruses with ribonucleic acid as their genetic material) such as deformed wing virus. With this condition, bees suffer from abdominal deformities, damaged appendages, stubby and useless wings, and eventually paralysis. Bees with deformed wing virus generally do not live longer than 48 hours and are often expelled from the hive before perishing. Additionally, if deformed wing virus spreads through a hive, healthy bees become susceptible to other pathogens. This sequence of destructive events is seen as a factor in colony collapse disorder among honey bee populations.

Varroa mites reproduce on a ten-day cycle; they lay their eggs on bee larvae, and new mites hatch as the young bees develop. As the young bee emerges from her brood cell, the mites are carried into the general hive as well, spreading to other bees and larvae. Mite populations can multiply exponentially. Most species of honey bee are completely defenseless against these mites, although some honey bees in Russia have become partially resistant. The mites have spread to most parts of the world, affecting honey bee colonies from the Americas to Europe, Russia, New Zealand, and Hawaii.

Varroa Mites
Varroa Mites and Honey Bees

The mites can be controlled in a number of ways. There are various commercially available miticides that kill varroa mites, but these must be used carefully, to minimize any contamination of honey intended for commercial sale. Synthetic miticides include pyrethroid insecticides as well as organophosphate insecticides. Some naturally occurring chemicals can be effective as well, such as essential oils (lemon, mint, and thyme), sugar esters (in spray applications), oxalic acid (applied via trickling or as a vapor), and formic acid (applied as a vapor).

Some physical measures can control varroa mite populations without completely eliminating them. Some beekeepers employ a screened bottom board on their hives. Mites will fall off bees on occasion; with a solid bottom on the hive, the mite can easily climb onto another bee. However, if the bottom is a meshed screen, the mite will fall through, and out of the hive. Such a screened bottom can also improve air circulation in a hive, preventing condensation within the hive during the winter. And by placing a sticky board underneath the screened bottom board, the mites will be further hindered.

More advanced techniques, such as comb trapping and small cell foundation, have been shown to be more effective than installing a screened bottom. Comb trapping, a complex sequence of isolating combs in turn and remove infected combs, can remove up to 80 percent of varroa mites from a hive.

Some powders with a grain size between 5 and 15 micrometers, such as powdered sugar or talc, can be sprinkled over bees. The powders do not harm the bees — powdered sugar can even be a food source — but they make it more difficult for mites to attach themselves to bees. Bees also tend groom each other if they are dusted with powder, and the process of grooming also dislodges the mites.

If you are purchasing bees from an apiary, make sure that your new bees are free of varroa mites; you should ask the dealer how he controls mites. Some apiaries have stopped using chemical methods to kill mites, arguing logically that mites gradually build up resistance to chemical insecticides. These chemical-free apiaries attempt to breed stronger strains of honey bees, using only bee colonies that survive varroa mite infestations as breeder stock. Some long-standing apiaries have found success with such genetically based methods. Be sure to discuss varroa mites with any dealer you’re thinking of purchasing from.

Varroa mites are destructive and can kill off a hive. However, if you can detect an infestation early, you stand a good chance of fighting off the mites and averting disaster.