History of Varroa in North America
The first discovery of Varroa jacobsoni (later renamed Varroa destructor) in the United States was made on Sept. 25, 1987 in Florida on colonies owned by a Wisconsin beekeeper. This beekeeper, Gary Oreskovic, was a supplier of package bees that were combined with packages from other beekeepers and sold to companies and individuals in Wisconsin. The initial infested hives were depopulated (killed). Yet by October 20 of that same year 19 of Florida’s 67 counties had positive finds for the mites, and within two years the mites were found in 19 states within the United States. They are now found in every part of the Americas where bees are kept.
Cornell Prof. Roger A. Morse reported that the source of the queens that introduced the mites into North America was somewhere in South America, imported illegally by a commercial beekeeper. Between beekeeper movement of hives and packages by beekeepers and the natural interchange of bees from one colony to another, mites were efficiently distributed. It has been shown, for example, that bees foraging on flowers will join a swarm as it moves through the air. The foragers are apparently attracted to the pheromone odor of the swarm and the swarm’s overall behavior. This is one method mites could be spread from colony to colony in nature. Drones, migrating from colony to colony during their reproductive flights, also provide a critical vectoring of the parasite. Also, when a colony eventually dies from the mites, remaining workers carry mites to other colonies, a notorious behavior often called the ‘varroa bomb’.
Because European beekeepers had experienced the wave of varroa across that continent, there were pre-existing chemical control methods in production that were easily adapted within the United States. A compound called Apistan (fluvalinate) was impregnated into wooden strips in initial treatments in Florida and elsewhere. These were replaced by the availability of plastic Apistan strips that were widely sold to control mite numbers. There was little discussion about ‘should we treat’, but rather the driving insistence that we develop treatment methods that were cost effective and relatively inexpensive. This did not stop the wide-scale use of home-made delivery methods to administer fluvalinate (tau-fluvalinate is sold as Mavrik and Klartan for insecticidal and acaracidal action on aphids, trips, leafhoppers, whitefly, beetles and spider mites). Fluvalinate is a pyrethroid that acts on the insect nervous system. Because it was available for purchase by agricultural producers, many beekeepers developed their own control methods using the agricultural preparation of the compound. In doing so they both over- and under-dosed the colonies where they were attempting to achieve mite control. Over-dosing provided evidence of toxic effects to queens, drones, workers and lead to widespread comb contamination. Where lower than recommended levels were used, there were a larger number of mites that survived the treatment, ultimately leading to mite resistance to the compound. Because of the wide-spread resistance, the compound is now used only when in rotation with other mite control molecules.
Finding Tolerance Against Varroa Mites
Untreated colonies suffered horribly as the mites swept across the country. Beekeepers reported the deformity of worker bees at the time they should emerge from their cells, and the appearance of damaged wings as a result of the mites feeding on the bees. The eventual outcome was the death of the colony. Within a few years there were reports of colonies that were still alive after the mites had destroyed the rest of the colonies in the apiary. Feral bee colonies died from the mites as well, and gardeners and naturalists quickly noticed the lack of honey bees on vegetable and flower gardens, as well as a decline in the natural food bees pollinate for birds and wildlife. With the absence of completion from honey bee foragers for the same food supply, many naturalists and scientists noticed an increase in the number of bumble bees and other native pollinators. It was an interesting exercise to observe the change in the ecosystem as honey bees were somewhat suddenly removed.
The survival colonies were of interest to many beekeepers, and many small-scale producers used these few remaining colonies as the basis of their slow rebuild of bee colony numbers. The progress was slow. Researchers noticed too, and Dr. Roger Hoopingarner (Michigan State University) and Dr. John Harbo (USDA Honey-Bee Breeding and Physiology Lab, Baton Rouge) sent out a call for beekeepers to ship them their queens and they started a stock of survivor queens. With Hoopingarner’s retirement Harbo maintained the program, developing the Suppressed Mite Reproduction or SMR strain of honey bees.
Harbo was joined by Jeff Harris at the Baton Rouge Bee laboratory (who continued the program until his departure for Mississippi State University) and in 2003 they reported on the project and its results to date. They called this the Suppressed Mite Reproduction (SMR) trait (not a line or stock, but set of genes anyone could breed for). They established that they had identified a heritable trait of the honey bee showing that by selective breeding they could bring the mite reproduction rate to zero in worker brood. They established that the trait is additive, so that a SMR queen mated to non-SMR drones resulted in an intermediate level of mite reduction.
They showed that once a colony was given an instrumentally inseminated SMR x SMR queen, the colony had normally reproductive mites for a period of about two months. Then, the level of mite reproduction was reduced. They also reported “Sometimes brood production is poor in colonies with artificially inseminated SMRxSMR queens, even though a queen may produce a very solid brood pattern in her first brood cycle. This does not always happen and we don’t know why it happens. Consequently, a colony with an SMR breeder queen may not grow rapidly enough to become a productive field colony. Free-mated daughters of these SMR breeder queens have not had this problem, for tests have shown that colonies with free-mated SMR queens produced as much brood and bees as colonies”.1
Later it was shown that the SMR trait is a form of hygienic behavior, and the SMR trait was renamed the VSH trait. The mechanism explaining the SMR trait has not been described, but Ibrahim and Spivak (2003, ABJ 144: 406) found that bees with the SMR trait were very hygienic and were able to remove varroa-infested pupae from capped brood cells. They also suggested that SMR bees may selectively remove pupae having reproductive mites. It was shown that the brood cells with reproductive mites were opened and removed by the bees with the hygienic trait, while the single foundress mite that is nonproductive) was not removed, and was assumed to emerge from the worker cell when the worker bee emerged. Because she had not produced offspring, she was not removed from the cell by hygienic bees. There is a group of VSH bee breeders that may be reached through VSHBreeders.org. However, beekeepers may want to place orders for VSH breeder queens through Adam Finkelstein of VP Queens (in Maryland) at www.vpqueenbees.com.