FIG. 1 Varroa jacobsoni is a parasitic species of mite that subsists by infesting honey bee (Apis mellifera) colonies. Varroa mites have become an increasing problem for commercial and hobbyist beekeepers alike. Damage caused by Varroa can be extensive. Obviously, continued loss of hemolymph (insect blood) has detrimental effects on the host. Decreases in longevity, weight, wax production and disease resistance are all results of Varroa infestation of the workers. Drones and even queens may suffer a decreased ability to mate successfully. Once it is infested with these minute arachnids, a colony may die within a few months if left untreated.
A mature female Varroa mite is approximately 1.2mm x 1.6mm and are a reddish brown in color making it fairly easy to spot when it walks around on its host. However, the shape of these mites, flattened and oval, enables them to fit into the narrowest of spaces making them difficult for the bees to remove and also making them less conspicuous to the eye of a beekeeper. Other more accurate methods of Varroa detection are necessary. [Erickson's excreta method]
FIG. 2 As a member of the class, Arachnida (other mites, ticks, spiders, scorpions), these organisms have four pairs of walking legs (only three pair pictured here), and do not have antennae or compound eyes. As other arthropods, Varroa mites lay eggs. A female Varroa mite will enter the larval bee cell 1 or 2 days before the cell is capped by workers. The mite feeds of the hemolymph of the larva which gives her the energy to produce one unfertilized and one or more fertilized eggs, hatching into male and female mites, respectively. The male matures first and mates with the females. In this way the females can lay up to 7 eggs in a cell. Not all of these progeny reach maturity, however. When the bee reaches maturity and chews its way out of the cell, all immature mites perish. Because of this the mother mite may only reproduce herself once per cell (one live female offspring). In drone cells the reproduction rate is higher and indeed Varroa seem to favor drone cells to worker cells. Once the bee emerges the mites (including the mother) are free to roam about the hive and repeat the cycle. The average life expectancy for Varroa mites is about 50 days.
FIG. 3 The adult mites are able to pierce both the soft skin of the honey bee larva and tougher integument between the thoracic segments of adult bee with their specialized mouth parts. Not only do the mites remove hemolymph from their host, but they inject their own saliva which, like other arachnids, contains proteases (various enzymes that predigest the bee tissues). With digestion occurring externally, the mite simply slurps up the liquid meal.
Indeed, a "good" parasite will not kill its host or else it too will die. Why then, are bee colonies dying because of Varroa infestation? What are safe and efficient ways of treating colonies with Varroa? These and other questions are being investigated now in order to help alleviate the problem of Varroa jacobsoni in the United States.
Text and SEM Micrographs by Kevin Kehl (kehlk@setmms.tusd.k12.az.us.
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