Varroa

The Varroa mite, Varroa destructor, is the most damaging pest to honey bees in Slovenia and most of the world. Since its discovery in our country in 1980, the pest has spread rapidly throughout the country aided by the movement of infested, commercial honey bee colonies. Varroa is so widespread and its affect on bee colonies is so serious that beekeepers must routinely treat or their colonies will likely perish. Resistance to traditional chemical miticides has developed in recent years. Fortunately, the number of management tools and strategies for Varroa has recently increased, providing beekeepers with a wider array of options for more sustainable mite management.

Varroa destructor was until recently thought to be a closely related mite species called Varroa jacobsoni. Both species parasitize the Asian honey bee, Apis cerana. The mite species originally described as V. jacobsoni by Oudemans in 1904 is part of the same species complex, but not the same species that made the jump to Apis mellifera. That jump probably first took place in the Philippines in the early 1960’s. Only after Apis mellifera were imported to the Philippines, it came into close contact with Apis cerana. Varroa as a parasite of Apis cerana, also became a parasite of Apis mellifera. Up until 2000, scientists did not positively identify Varroa destructor as a separate species. In 2005, we know that the only varroa mites that can reproduce in colonies of Apis mellifera (Western honeybee) are the Korea and Japan/Thailand genotypes of Varroa destructor. Varroa jacobsoni is a fairly benign parasite of Apis cerana. This late identification in 2000 by Anderson and Trueman led to some confusion and mislabeling in the scientific literature.

Biology

Varroa is an external parasite, feeding on the hemolymph, or blood, of immature (brood) and adult bees. They live either on adults or within brood cells. A mated, adult female mite (called a foundress), enters a worker brood cell 15 to 20 hours prior to capping (40 to 50 hours pre-capping for drone brood) and feeds on the larva after the cell is capped. She lays her first egg about 60 hours later and may lay as many as six eggs at 30-hour intervals. Her young feed on the bee prepupa and pupa, taking 7 to 8 days (females) or 5 to 6 days (males) to mature. Mating occurs in the capped cell. Although several eggs are laid, the average number of mature, viable female mites produced per foundress in a cell is less than two. When the bee emerges from the cell, the new female(s) may stay attached to the bee or may attach to another bee. They feed on these bees and may stay on them for a few days to a few months depending on the time of year. Eventually they will enter a brood cell to begin the reproductive cycle over again. The original foundress may survive to infest and reproduce in another cell.


Damage


Symptoms of Varroa infestation may not be obvious until mite numbers have reached damaging levels. Adult bees parasitized by mites as brood may exhibit deformed legs and twisted wings
which is thought to be caused by a virus transmitted by mites. Infested colonies will appear weak, the brood pattern may be spotty, bees may be overly defensive and may be seen discarding larvae and pupa. A colony may abscond (leave the hive) if heavily infested. Colonies entering winter with a Varroa infestation may not survive. A colony dying from Varroa during the winter may be found with a small amount of dead bees and perhaps a moderate amount of honey. There
may be no bees remaining.

Varroa mites can be treated with commercially-available miticides. Miticides must be applied strictly according to the label in order to minimize the contamination of honey that might be consumed by humans. Proper use of miticides will also help to slow the development of resistance among the mites.