So what is a mite anyway? Generally, a mite is an invertebrate animal in the class Arachnida—a name that comes from the Greek word for spider. Like most other arachnids, mites have eight jointed legs.
A simple leg count is probably the easiest way to tell an arachnid from an insect. Insects—including bees—have six legs. In addition, arachnids have no wings or antennae. However, since both arachnids and insects belong to the phylum Arthropoda, they have many structures in common—one of these being a protective exoskeleton.
As a general rule, mites tend to be smaller than insects; some are even microscopic. Many types of mites—at least fifty or more—can be found inside a beehive, and most of these were carried there by the bees themselves. For the most part these are harmless, non-parasitic mites that were feeding on flowers, pollen, nectar, detritus, or other mites when they were picked up inadvertently and flown to the hive. These will usually die and become part of the frass that routinely collects on the bottom board.
Since mites have no wings, they often attach to insects and hitch a ride to a new location. Mites that move this way are called phoretic. Whether the mites in a colony of bees arrive by design or by accident, an overwhelming majority do no harm and may even be beneficial for consuming detritus.
However, some mites are parasitic, and two of these are famous for wreaking havoc on honey bee colonies. The first of these, Acarapis woodi, is known as the tracheal mite. As the name implies, this microscopic creature lives and reproduces inside the tracheae (or breathing tubes) of the honey bee. They bite into the wall of the trachea and suck the hemolymph or “bee blood.” This not only weakens the bee, but the wound allows the entry of secondary infections.
The most famous parasitic mite, Varroa destructor, is found on the external surface of both pupal and adult bees where it also feeds on the hemolymph. It is closely related to several other species of mite that have long been known to affect Apis cerana, the Asian honey bee. By mite standards, Varroa destructor is very large, and it is huge compared to the size of the host honey bee. Besides weakening the bees by consuming their body fluids, it is thought that Varroa mites carry a number of bee viruses that transfer to the bee through its bite.
Mites are spread easily and quickly from hive to hive. Beekeepers spread them during routine hive management and migratory beekeepers spread them from one apiary to another and one region to another. Bees also spread them when drifting, swarming, or robbing. Mites can even be spread when the bees are foraging. Several other species—including bumble bees, scarab beetles, and flower flies–have been found to carry Varroa mites from place to place. Although Varroa are harmless to these species due to their vastly different life cycles, the mites are glad to hitch a ride whenever the opportunity presents.