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Varroa Mite (Varroa jacobsoni)
Known as the varroa mite, Varroa jacobsoni is an ectoparasitic member of the class Arachnida found on honeybees (genus Apis). Originally described from Java in 1904, the mite was located in Hong Kong and the Philippines by 1963 and imported to the United States on infected queen bees by 1979. Now the parasitic mite has spread to most of North America except for isolated locations in Canada.
In Florida, where thousands of migratory bee colonies are moved annually, this parasitic mite is found on flower-feeding insects such as the American bumblebee (Bombus pennsylvanicus), syrphid flies (Palpada vinetorum), and scarab beetles (Phanaeus vindex). Although the varroa mite cannot reproduce on other insects, these hosts serve as a means of spreading the acarian species.
Exhibiting a crab-like shape, adult female mites are brown in color and have dimensions of about 1.5 millimeters long and 1.75 millimeters wide. Well adapted for their hosts, the curved bodies of the ectoparasite fit into the abdominal folds of the adult bees and are held there by specialized ventral setae. This mechanism protects the hitchhikers from the normal grooming habits of the bees. The male mites are yellowish with light tan legs and a spherical body that is approximately 0.85 millimeter in diameter. Males feature modified chelicerae that transfer sperm to females.
The life cycle of the varroa mite is so synchronized with its honeybee host that some researchers speculate that hormones or pheromones of the host are necessary for the mite to complete its development. Female mites lay eggs in the bee brood cells. Developing mites feed on developing honeybee larvae, and then male and female mites copulate in the cell. The male then dies, but pregnant females emerge from the cell along with a bee host, or seek another cell to repeat the cycle. The longer the postcapping period of the honeybees, the more time there is for additional female mites to develop, leading to a heavier episodic infestation. Once the cells are capped in the hive, mites stop invading. Only seven days are required for the female mite, and six days for the male, to reach sexual maturity. If the worker bee host does not emerge from its egg phase after 21 days in the hive, the mite dies.
As the most significant parasite of honey bees, Varroa jacobsoni is of great economic importance, not only to honey producers, but also to farmers and horticulturists that depend on bee pollination for crops such as alfalfa, ornamental flowers, and oranges. If the parasite is left unchecked, weakened bee colonies may die for lack of workers to manage broods and gather nectar. Under certain weather conditions, as occur in Florida, bee colonies may fail in only a few months. This may appear to be a case of a "bad parasite" wiping out its host, but within the mites' native range, the original hosts, Apis cerana and African honey bees (A. mellifera scutellata, support populations of mites without collapsing.
Cynthia D. Kelly, Thomas J. Fellers and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
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