Deformed Wing Virus (DWV) poses a major threat to honey bee populations across the globe. This viral pathogen is a major concern for beekeepers due to its association with declining bee numbers and colony losses. Understanding DWV and its impact is crucial for protecting these vital pollinators.
Understanding the Virus
Deformed Wing Virus is a single-stranded RNA virus that belongs to the Iflaviridae family. It is one of the most prevalent viruses affecting honey bees worldwide, found in a large percentage of apiaries. DWV exists in two forms: overt, with visible symptoms, and covert, where bees are infected but show no outward signs.
While DWV is primarily known for infecting the European honey bee (Apis mellifera), it has also been detected in other pollinator species, including various bumble bee species like Bombus terrestris and Bombus pascuorum, as well as wasps and ants, highlighting its broader host range. The virus can infect all developmental stages of honey bees, from eggs to adults, though it tends to replicate more significantly in pupae. There are several genetic variants of DWV, with DWV-A and DWV-B being the most commonly studied, and their prevalence can shift over time.
Symptoms and Effects on Bees
The most striking symptom of an overt DWV infection is deformed, crumpled, or underdeveloped wings, rendering affected bees unable to fly. These wing deformities are typically seen when bees are infected during their larval or pupal stages, particularly when transmitted by mites. Affected bees have a severely shortened lifespan, often surviving only 4 to 6 days after emergence, and are frequently expelled from the hive by other bees.
Beyond wing deformities, bees with DWV may exhibit other signs, including shortened or rounded abdomens, discolored bodies, and paralysis. Even covert infections, without obvious physical symptoms, can harm individual bees and the colony. Covert infections can lead to increased mortality rates, cause worker bees to begin foraging at a premature age, and reduce their total foraging activity span, ultimately weakening the colony over time. These individual bee health issues collectively contribute to a compromised colony, impacting its ability to forage effectively, maintain immunity, and, when combined with other stressors, can lead to overall colony collapse.
How Deformed Wing Virus Spreads
The primary mechanism for DWV transmission is the ectoparasitic mite Varroa destructor. These mites feed on the hemolymph of honey bees, directly injecting the virus into the bee’s bloodstream. This direct injection by the mite bypasses the bee’s natural defenses and leads to much higher viral loads and more pronounced symptoms. Varroa mites significantly increase DWV infection rates within a colony.
While the Varroa mite is the main vector, DWV can also spread through other routes. Oral transmission occurs within the colony, for example, through contaminated food sources like royal jelly and pollen, or during trophallaxis (the exchange of food between bees). Vertical transmission is another pathway, where the virus can be passed from the queen to her offspring through infected eggs or from drones via their semen. Additionally, horizontal transmission can occur between bees within the same colony, or between different colonies through activities like robbing (bees from one colony stealing honey from another) or drifting (bees accidentally entering a different hive).
Strategies for Management
Managing Deformed Wing Virus focuses on controlling its primary vector, the Varroa destructor mite. Reducing Varroa mite populations directly lowers DWV transmission rates and the severity of infections within bee colonies. Beekeepers often employ Integrated Pest Management (IPM) techniques to control Varroa mites effectively.
IPM strategies for Varroa involve a combination of approaches. Chemical treatments, such as oxalic acid or formic acid, are frequently used to reduce mite loads in hives. Cultural practices also play a role, including techniques like drone brood removal, as Varroa mites prefer to reproduce in drone cells. Mechanical methods, such as using screened bottom boards to allow mites to fall out of the hive, can also contribute to mite reduction.
Beyond mite control, maintaining strong and healthy colonies through good nutrition and minimizing other stressors helps bees better resist the virus. Research is also exploring genetic selection for honey bee strains that exhibit resistance to Varroa and, consequently, to DWV.