varroa

Varroa destructor is a parasitic mite roughly 1.1 mm long that feeds on the hemolymph, or blood, of adult honeybees and their larvae. The mite originated in Asia on the eastern honeybee Apis cerana, where local bee populations evolved resistance. When it jumped to the western honeybee Apis mellifera in the 1950s, it became catastrophic. A single infested colony can collapse within two to three years without treatment.

The mite's life cycle is tightly synchronized with the brood cycle of the host colony. A female mite enters a brood cell just before it is capped, reproduces inside the sealed cell, and the mated daughters emerge with the bee. This hidden breeding ground makes the mite exceptionally difficult to control. Winter is the critical period: mites concentrate on the tightened cluster of bees, feeding heavily and transmitting viruses that weaken the colony before spring foraging begins.

Heavy infestations produce visible signs: deformed wings, stunted abdomen, and shortened lifespan in adult bees. Larvae in infested cells may fail to pupate or emerge as crippled adults. More insidiously, varroa transmits at least five viruses to bees, notably deformed wing virus and acute bee paralysis virus. These viruses do more damage than the mite's direct feeding. Beekeepers monitor mite levels by counting mites on a sticky board placed under the colony floor, or by alcohol-washing a sample of bees.

Management relies on both chemical and non-chemical methods. Synthetic miticides such as tau-fluvalinate and coumaphos are applied as strips or sprays, but mite resistance to these compounds has developed in many regions. Organic acids including formic acid and oxalic acid are also used; oxalic acid is applied as a trickling treatment in late autumn or winter when the colony is in cluster and brood is minimal. Mechanical methods such as drone removal, screen floors, and split hive management reduce populations without chemicals but require consistent labor.

The term "varroa" in beekeeping now refers both to the mite species itself and to the infestation condition. A beekeeper might say a colony has "high varroa levels" or needs "varroa treatment." The economic impact extends beyond hobby beekeepers to commercial pollination operations and migratory beekeeping; untreated varroa is one of the leading causes of colony loss in temperate regions. Research into selective breeding for mite-resistant bee lines continues, but resistance remains incomplete and requires ongoing management attention.