Diseases of the honey bee or abnormal hive conditions include:
Pests and parasites
Varroa mites
Varroa destructor and V. jacobsoni are parasitic mites that feed on the fat bodies of adult, pupal and larval bees. When the hive is very heavily infested, Varroa mites can be seen with the naked eye as a small red or brown spot on the bee's thorax. Varroa mites are carriers for many viruses that are damaging to bees. For example, bees infected during their development will often have visibly deformed wings.
Varroa mites have led to the virtual elimination of feral bee colonies in many areas, and are a major problem for kept bees in apiaries. Some feral populations are now recovering—it appears they have been naturally selected for Varroa resistance.
Varroa mites were first discovered in Southeast Asia in about 1904, but are now present on all continents except Australia. They were discovered in the United States in 1987, in New Zealand in 2000, and in Devon, United Kingdom in 1992.
To the untrained eye, these mites are generally not a very noticeable problem for a strongly growing hive- as the bees may appear strong in number, and may even be very effective at foraging. However, the mite reproduction cycle occurs inside the capped pupae, and the mite population can surge as a result of colony growth. Careful observation of a colony can help identify signs of disease often spread by mites. When the hive population growth is reduced in preparation for winter or due to poor late summer forage, the mite population growth can overtake that of the bees and can then destroy the hive. It has been observed diseased colonies may slowly die off and be unable to survive through winter even when adequate food stores are present. Often a colony will simply abscond (leave as in a swarm, but leaving no population behind) under such conditions.
Varroa in combination with viral vectors and bacteria have been theoretically implicated in colony collapse disorder.
It is known that thymol, a compound produced by thyme, naturally occurring in thyme honey, is a treatment for Varroa, though it may cause bee mortality at high concentrations. Provisioning active colonies with crops of thyme may provide the colony with a non-interventional chemical defense against Varroa.
Treatment
A variety of chemical and mechanical treatments are used to attempt to control Varroa mites.
"Hard" chemicals
"Hard" chemical treatments include amitraz (marketed as "Apivar"), fluvalinate (marketed as "Apistan"), coumaphos (marketed as CheckMite), flumethrin (marketed as "Bayvarol" and "Polyvar Yellow").
"Soft" chemicals
"Soft" chemical treatments include thymol (marketed as "ApiLife-VAR" and "Apiguard"), sucrose octanoate esters (marketed as "Sucrocide"), oxalic acid (marked as "Api-bioxal") and formic acid (sold in liquid form or in gel strips as Mite Away Quick Strips and Formic Pro, but also used in other formulations).
According to the U.S. Environmental Protection Agency, when used in beehives as directed, chemical treatments kill a large proportion of the mites while not substantially disrupting bee behavior or life span. Use of chemical controls is generally regulated and varies from country to country. With few exceptions, they are not intended for use during production of marketable honey.
"Mechanical" treatments
Common mechanical controls generally rely on disruption of some aspect of the mites' lifecycle. These controls are generally intended not to eliminate all mites, but merely to maintain the infestation at a level which the colony can tolerate. Examples of mechanical controls include drone brood sacrifice (Varroa mites are preferentially attracted to the drone brood), powdered sugar dusting (which encourages cleaning behavior and dislodges some mites), screened bottom boards (so any dislodged mites fall through the bottom and away from the colony), brood interruption and, perhaps, downsizing of the brood cell size.