Transmission of V. destructor
Robbing by bees is a major source of transmission. As an infected colony become progressively weaker, its defensive capabilities decline, and it becomes susceptible to invasion by workers from nearby colonies (the robbers) seeking its valuable cache of honey. In the process of removing the honey, robbers become infected with mites and transport them back to their own colonies. Swarms from infected colonies also contribute to the local reservoir of mites. These colonies are particularly susceptible to being robbed because they do not receive any treatment for mite control. They weaken and die within a year or two and may be robbed by workers from nearby colonies. Drifting bees, especially in apiaries where colonies are kept close together, also contribute to the spread of mites among colonies.
Beekeepers also play a major role in the transmission of mites. Moving brood among colonies for the purpose of strengthening or equalizing colonies is a common practice that transmits mites. In addition, beekeepers often purchase colonies of bees in the spring to replace winter losses or to increase colony numbers. Some beekeepers purchase small nucleus colonies, usually called ”nucs,” from local or regional suppliers. These colonies consist of one to five combs of bees and brood and usually come with a queen. Others purchase package bees (2, 3 or 5 pounds of bees, usually with a queen) from a southern location. An estimated 1 million packages are shipped throughout the country each year. Each of these practices spread mites, including various types of pesticide resistant mites.
Migratory beekeeping also plays a role in transmitting mites. Over a million colonies are moved throughout the country each year as migratory beekeepers fulfill pollination contracts. After the bloom is over, colonies are widely dispersed to other locations for honey production. During the season, some of these colonies may issue mite-infested swarms into local environments, while others may succumb to mites and be robbed by local colonies. Each fall, surviving colonies are returned to a few states in the south where colony numbers are restored. This brings colonies from many different regions of the country into close proximity to one another and provides many opportunities for the transfer of mites among colonies, including various types of pesticide resistant mites. In the spring, these colonies resume their migratory routes throughout the country, and the process is repeated.