Deformed Wing Virus (DWV) is a widespread viral pathogen that infects honey bees, representing one of the greatest threats to apiculture. The virus is a single-stranded RNA virus from the Iflaviridae family, and its prevalence has increased since the global spread of the parasitic Varroa destructor mite. DWV exists in several variants, including DWV-A and the highly virulent DWV-B, and is now considered a pandemic among honey bee populations. This viral infection, often combined with its primary vector, is a major driver of colony losses.
Recognizable Symptoms and Colony Impact
The virus is named for the primary sign of acute infection: the malformed or shriveled wings of newly emerged adult bees. These bees, infected with a high viral load during their pupal stage, possess wings that are too short, crumpled, or twisted, rendering them unable to fly. Additional overt symptoms include a reduced body size, a shortened and rounded abdomen, and discoloration. Symptomatic bees have a drastically reduced lifespan, often surviving for less than 48 hours, and are typically expelled from the hive by their nest mates.
While these visible signs are alarming, DWV often persists as a covert, or asymptomatic, infection in adult bees. These outwardly normal bees still carry high viral loads, which weakens their immune system and reduces their lifespan, even without physical deformities. DWV infection impairs cognitive functions, such as learning and memory, and can negatively affect the bee’s ability to forage and navigate. The accumulation of these chronically infected, short-lived bees causes the colony population to dwindle rapidly, especially during the autumn. When the long-lived winter bees die prematurely due to the virus, the colony cannot maintain the necessary cluster temperature and often collapses before spring.
The Varroa Mite Connection: A Deadly Partnership
DWV’s destructive nature lies in its relationship with the parasitic mite, Varroa destructor. DWV naturally exists at low, asymptomatic levels in honey bee colonies, where it is transmitted horizontally through bee-to-bee contact, such as trophallaxis, or vertically from the queen to her eggs. However, the introduction of the Varroa mite fundamentally changed the virus’s transmission route and its overall virulence. The mite acts as an efficient biological vector, transforming DWV from a mild pathogen into a high-virulence killer.
The mechanism of transmission involves the mite feeding on a developing bee, which facilitates the direct injection of the virus into the bee’s hemolymph, bypassing natural defenses. The mites primarily consume the bee’s fat body tissue, which is the insect equivalent of the liver and fat combined, rather than just the hemolymph. This feeding on the fat body causes immunosuppression and directly facilitates the high-titer DWV infection.
This direct inoculation dramatically increases the viral load in the individual bee, selecting for and propagating more virulent strains of DWV. Studies consistently show a direct correlation: high Varroa populations lead to dramatically increased viral concentrations within the colony, which then results in the emergence of symptomatic bees and subsequent colony death. The mite essentially selects for a more aggressive form of the virus, turning DWV into the single greatest factor in the decimation of honey bee colonies worldwide. The spread of Varroa is now considered the main driver of DWV’s pandemic status, leading to colony failure in areas where mite control is not practiced.
Mitigating DWV: Focus on Mite Control
Since DWV is a virus, there is no direct antiviral medication to treat an infected bee or colony. The only effective strategy for managing DWV is the control of its primary vector, the Varroa destructor mite. Keeping mite populations below established thresholds is the way to keep the viral load in the colony at a low, asymptomatic level. This preventative approach is central to maintaining colony health and preventing acute infections.
Beekeepers must regularly monitor mite levels using methods such as alcohol washes, which provide an accurate count of the mite population per bee. When monitoring indicates that mite levels exceed thresholds, timely intervention is necessary. Integrated Pest Management (IPM) techniques involve a combination of monitoring, physical controls, and the application of approved miticides, particularly during late summer and autumn to protect the winter bee population. Promoting hygienic behavior in bees, where workers detect and remove infected brood, acts as a secondary biological defense against both the mite and the virus.