What are some ways to reduce the population of Varroa mites in honey bee colonies, without the use of pesticides?

Mite-resistant Bees. In response to development of resistance to chemical miticides, and in order to provide more sustainable mite management, honey bees have been selectively bred for resistance to, or tolerance of, Varroa. There are two known mechanisms of resistance: hygienic behavior and suppression of mite reproduction (SMR). Hygiene is the removal of diseased (including mite-parasitized) brood by workers; SMR is the reduction in reproduction of female mites within brood cells. Types of resistant queens include; Minnesota Hygienic, the Russian and the SMR. The Minnesota Hygienic, as the name implies, has been selectively bred to be hygienic against diseases such as American Foulbrood and against mite parasitism. Russian bees, originated from far-eastern Russia, where developed by the USDA, and are resistant to Varroa. SMR bees, also developed by the USDA, reduce Varroa numbers by interfering with reproduction, although host factors affecting mite reproduction are not well understood. Open Bottom Boards. The use of open bottom boards takes advantage of the natural fall of Varroa from the colony to reduce mite numbers by exclusion. Mites continually fall from bees and when exiting capped cells. Many fall to the bottom board where they are likely to re-attach to bees. But if the floor of the bottom board is screened rather than solid, the bees will fall to the ground below where they perish. Open bottom boards have been shown to reduce Varroa numbers by about 15%. And they can enhance the performance of treatments by removing mites that fall from bees during a treatment, but are not killed directly by the treatment. Removal of Drone Brood. The preference of Varroa for drone brood can be used to help delay buildup of mite populations. Wax drone brood foundation, which encourages bees to build larger cells and the queen to lay drone eggs, can be purchased, or empty frames with a starter strip can be given to colonies during drone rearing season. After capping, the entire frame can be discarded, or the brood can be destroyed (with a capping scratcher or by freezing) and the frame can be used again. Drone brood foundation should be inserted in early spring within or directly next to the brood cluster and it must be removed before drones begin emerging. Removing naturally occurring drone brood may not be practical because it is usually scattered throughout the cluster and is not numerous enough to affect Varroa numbers if removed. Apiary Isolation. Even if you are diligent about managing your colonies, they can be re-infested if Varroa-infested colonies are located nearby. Workers with mites can “drift” to other colonies; and workers from stronger colonies can rob weak, mite-infested colonies, and bring Varroa back with them. The greater the distance between apiaries, the less likely re-infestation will occur. This tactic is not always feasible because worker bees may fly several miles from their colony when foraging, and, of course, you probably will have no influence on the management of your neighbor’s colonies. Integrated Management. Reliance on traditional chemical mite treatments may be reduced by using a combination of management tactics. For example, combining resistant bees and open bottom boards may help to maintain Varroa below damaging levels and thereby reduce the number of treatments required. Perhaps the most important component of an integrated management program for Varroa is monitoring. Before development of resistance to Apistan™, few beekeepers considered monitoring mite populations because they knew this product would provide control. Now control is not certain, and monitoring has become a necessity. At the very least, monitoring should be conducted after treating to determine treatment effectiveness. When using control tactics which require more time to affect Varroa numbers, such as open bottom boards or resistant bees, monitoring should be conducted about once a month over the course of a season. Regardless of your management program or mite monitoring schedule, colonies should be sampled for Varroa in late August so that if a treatment is necessary, it can be applied and affect mite numbers before cold weather sets in. -John Skinner, University of Tennessee