The Varroa bee mite (Varroa jacobsoni) was first discovered by A.C. Oudemans in 1904, as a parasite of the Asian honey bee, Apis cerana. In the late 1940s, Through movement of the western honey bee, Apis mellifera, colonies into and out of Asia, Varroa mite became established on honey bees first in Africa and then in Europe. Quickly, it spread around the world. It was first detected in the U.S. in 1987; Mexico and Canada quickly closed their borders to U.S. bees. Varroa has now been in the U.S. for over two decades and a robust history exists published in two parts: 1 and 2.
Only one continent, Australia, remains free of the mite, however, it is expected to be introduced in the near future and the continent continues to have scares about various reports of the mite. It is now known that at least five species (18 haplotypes) of Varroa mites can be found in the tropics and Dr. Denis Anderson, an Australian researcher, has renamed the specific mite (Korean in origin) that is so damaging worldwide as Varroa destructor. This concept of renaming organisms might become more common in the future as DNA technology improves.
Varroa continues to be considered the most devastating parasite of honey bee colonies in existence. The mite is absolutely dependent on the honey bee and cannot complete its life cycle without being in contact with the honey bees. Here’s an animated take on the relationship between honey bees and Varroa: 2 minutes long.
One reason the mite is so damaging is that it is relatively new to the Western honey bee (Apis mellifera). Most parasites have evolved mechanisms so that they do not kill their hosts, in the long range disadvantageous. Thus, the original host, Apis cerana, is somewhat resistant to predation by mites. However in temperate areas, almost every Apis mellifera colony infested by Varroa will be killed unless there is intervention to reduce the mite population. There is mounting evidence, however, that certain European honey bees and other populations might be somewhat resistant/tolerant and incipient breeding programs exist to cultivate and enhance this “Varroa survivor stock.” At the moment the Russian Honey Bee Breeding Program and those based on removal of mites through colony hygiene are the most promising. Read contributor Randy Oliver’s ideas on the possibility of breeding honey bees more resistant to Varroa now and in the future.
Detection of Varroa can be accomplished by several methods. Most regulatory agencies use the “ether roll.” A sample of bees are put in a glass jar and a squirt of ether mixture (commercially available engine starting fluid) added. The jar is agitated and the mites stick to the sides. There is a technology called the sugar shake, which doesn’t kill honey bees; it appears to be ok for detection, but not treatment. Beekeepers can also visually examine the brood (capped brood is best) and/or hive debris that has fallen on the bottom board covered with sticky white paper (“sticky board“). See a rather complete hive examination by contributor Oliver here.
Perhaps the best source of information on detection of Varroa mites is found in what is known as the Coloss Bee Book. There is information that in the summer when colonies have sealed brood, it is easier to detect mites in hive debris than sampling brood itself.
Very little information exists on determining the beginnings of a Varroa infestation and subsequent thresholds for treatment. Varroa mite infestation is both a honey bee and beekeeping community issue and treatments should be tailored to this fact. Many, beekeepers prefer to use no chemical treatment. Treatments continue to have evolved over the years as mites have become resistant to specific materials. In addition, the use of so-called “soft chemicals,” such as essential oils and/or organic acids are now considered feasible approaches. However, only those currently approved by the Environmental Protection Agency (EPA) are legal. It may come as a surprise that some sixteen chemicals are on the list of approved materials. What is missing is what specific conditions are necessary for adequate control for each. Nevertheless, researchers and beekeepers continue to look for a balance in controlling Varroa via Integrated Pest Management.
Most recently, scientists and beekeepers have realized that Varroa infestation is more complex than originally thought. It turns out that viruses vectored by the mite may be a huge factor in honey bee colony losses. For a comprehensive study on Varroa, see Biology and Control of Varroa destructor by Peter Rosenkranz a,*, Pia Aumeier b, Bettina Ziegelmann, The Journal of Invertebrate Pathology.
A new technology on the horizon known as RNAi may have some utility in the future as a “silver bullet.” However, this is only a dream at the moment. Treatment still relies on a delicate balancing act. as one wag put it, “it’s not easy killing a bug on a bug without killing both.”
The Africanized honey bee is a special case with reference to Varroa and appears to be much more tolerant in Brazil as well as its parent stock in South Africa. Other countries may have tolerant stock due to absence of treatments, including war torn areas like Iraq. At least one dissertation has dealt with this situation in Mexico.