The Cloudy Wing Virus (CWV) is closely associated with the deadly varroa mite parasite; in fact, severe infestations by this mite generally coincide with the presence of CWV. This has led to the suggestion that varroa mites are a primary vehicle for spreading the virus, although definitive proof is still lacking.
Though one of the least researched honey bee viruses, CWV is frequently found in colonies undergoing collapse. Migratory beekeepers in the US have publicized the widespread losses caused by collapsing hives, which demonstrates the serious challenges honey bees face and the urgent need for intervention. Multiple factors contribute to bee mortality, including parasites, diseases, environmental stresses, and exposure to harmful chemicals like pesticides and insecticides.
Beekeeping plays a key role in global agriculture, with pollination by honey bees and other insects generating billions in economic value each season in the US and worldwide. As of October 2019, pollinators contributed an estimated $235 to $577 billion to global food production. Recently, significant hive losses reported by beekeepers worldwide threaten beekeeping and food security. The decline in bee populations disrupts numerous industries that depend directly or indirectly on bee products and pollination services.
How Viruses and Infections Enter
Viruses are ubiquitous and pose threats to many species, including humans and honey bees. Most bee viruses are detected through the identification of specific symptoms or diseases caused by infections.
The strong link between viral infections and honey bee deaths has prompted intensified research. At least 18 viruses affecting bees have been identified, including Chronic Bee Paralysis Virus (CBPV), Sacbrood Virus (SBV), Deformed Wing Virus (DWV), Black Queen Cell Virus (BQCV), among others.
Many of these viruses exhibit visible clinical signs at various stages, making laboratory testing essential for accurate diagnosis. Advances in genetic sequencing have provided nucleotide data for most bee viruses, enabling the development of nucleic acid-based diagnostic assays.
How Viruses Spread
Viruses like the Cloudy Wing Virus transmit within bee populations through different routes, categorized as either vertical or horizontal transmission.
Vertical Transmission
Vertical transmission occurs when an infection passes from one generation to the next—for example, a queen bee may transfer a virus to her eggs or developing brood, either coating the egg surface or infecting it internally. Additionally, viruses present in drone sperm can infect the queen during mating.
Horizontal Transmission
Horizontal transmission involves the spread of infection among bees of the same generation. This can happen via direct contact, ingestion, contaminated feces, sexually transmitted infection during mating flights, contaminated food sources, or through vectors. Vectors can be mechanical, carrying the virus without becoming infected themselves, or biological, where the virus replicates within the vector before transmission.
Detecting the Cloudy Wing Virus
To diagnose CWV, samples are taken randomly from bee colonies and transported on ice at -25 ˚C until processing. Each bee is ground using a pestle and mortar with 1 ml of Trizol reagent, then the mixture is placed into microcentrifuge tubes for RNA extraction.
Chloroform separates proteins from other components, and RNA is precipitated from the upper aqueous phase using 1 ml of isopropanol followed by centrifugation at 14,000 rpm. The resulting nucleic acid pellet is washed twice in 70% ethanol before being resuspended in nuclease-free water.
Subsequently, RT-PCR amplification is performed on the RNA samples to identify viral sequences, with further analyses conducted to confirm the virus’s presence.
Preventing and Managing the Cloudy Wing Virus
There are no specific treatments available for most viral diseases affecting honey bees, including CWV. However, several measures can reduce the virus’s spread and severity.
- When symptoms are severe, it is best to destroy all affected hives by burning the bees and combs. With milder symptoms, replace the queen and remove all infected combs for destruction. Beehives should be thoroughly cleaned and disinfected before reuse, using bleach or heat treatment such as passing a blue flame over the hive components. Infected combs must never be recycled.
- Acquiring or breeding bee stocks that demonstrate resistance through hygienic behavior or naturally stronger immune responses can help combat infections.
- Implementing a quarantine period for new queens to monitor brood health before integration is important to prevent virus transmission through egg infection, as viruses can be passed from queens to eggs and developing bees.
- Maintaining strict hygiene and proper beekeeping methods is essential to prevent diseases and stress factors in bee colonies. Neglecting cleanliness can foster an environment where viruses and infections thrive. To ensure hive health, regularly clean and ventilate the beehives, replace brood combs annually, and avoid swapping combs between different hives. Additionally, enhance the bees’ nutrition by providing high-quality, uncontaminated supplements to support their overall well-being.
- Enhancing the bees’ immune defenses can be achieved by introducing beneficial nonpathogenic bacteria. These bacteria stimulate the production of antibacterial peptides within the bees, strengthening their resistance to infections. This method can be applied throughout all honey bee developmental stages, promoting an innate ability to combat diseases.
- Utilizing propolis can be beneficial since it contains phenolic compounds akin to essential oils, which possess antimicrobial properties that inhibit bacteria and fungi. Beekeepers can encourage bees to deposit propolis inside the hive by placing cotton cloth as an inner board or roughening the wooden inner surfaces of the hive to facilitate accumulation on these areas.
- Radiation treatment has been employed to sterilize wax and honey without observed harmful effects, though it may alter honey’s physical and chemical attributes, such as reducing enzymatic activity and changing color. This technique is also effective in disinfecting contaminated equipment, helping to prevent the spread of viruses and other pathogens.
- Managing varroa mite populations is crucial, as these parasites serve as carriers for the Cloudy Wing Virus as well as other viruses affecting honey bees.
Preventing the Cloudy Wing Virus by Managing Varroa Mites
Among all parasites targeting honey bees, varroa mites are the most widespread. First introduced to the United States in 1987, they have since become a major threat to bee colonies worldwide. Female varroa mites lay eggs inside both drone and worker bee cells; however, they primarily reproduce in drone cells due to their larger size and longer developmental time, which allows for a faster mite population increase per reproductive cycle.
The following methods are commonly used to control varroa mite infestations:
1. Cultural Methods
Cultural methods focus on reducing varroa mite reproduction and can be highly effective. One approach is breeding or acquiring resistant bee strains, such as Russian honey bees, which possess natural tolerance to mites. Some bee populations have also developed behaviors like grooming and mandible biting to help manage parasite infestations.
Another cultural tactic involves using small cell comb foundations that have smaller hexagonal cells. These reduce the duration of the post-capping stage, making it harder for mites to reproduce successfully within the brood.
Lastly, implementing brood breaks by temporarily removing or caging the queen for at least three weeks interrupts mite reproduction. During this period, varroa mites are forced onto adult bees, where they can be more easily targeted and controlled.
2. Mechanical Methods
Mechanical controls complement other treatments and consist of hive and colony management techniques designed to reduce mite numbers.
One mechanical method is mite trapping, which exploits the mites’ preference for drone brood with longer post-capping times. Beekeepers introduce a drone comb as a trap and remove it before drone emergence, effectively eliminating many developing mites.
The use of screened bottom boards also aids in mite control. These allow naturally fallen varroa mites to drop out of the hive, preventing them from climbing back and thereby diminishing the mite population and reproductive potential.
Applying powdered sugar to bees stimulates grooming behavior, causing mites to detach and fall off. Although not sufficient on its own, this practice can support other control measures.
3. Chemical Treatments
While some beekeepers avoid harsh chemicals, mild, naturally derived treatments are preferred and should be applied judiciously. Formic acid, a component naturally found in bee venom and honey, effectively kills mites and penetrates wax cappings but must not be used at high temperatures above 85 °F (29.4 °C).
Oxalic acid, extracted from plants like spinach and rhubarb, is another commonly used treatment, applied as vapor or dribble. It’s most effective during broodless periods such as early spring or winter because it does not penetrate capped brood cells. Care must be taken to avoid overuse, as it can be harmful in excess.
Thymol, derived from plants, is a widely favored natural treatment available in various formulations. Like oxalic acid, it does not penetrate capped cells and has limited effectiveness, so it is best used alongside other mite control methods.
Hops beta acids from the hops plant contain potassium salts safe for bees, and can be applied at any time of year, preferably when brood levels are low. These acids are resilient to temperature extremes and do not penetrate capped cells.
4. Synthetic Chemicals
Synthetic acaricides and miticides have been developed to manage varroa mites, demonstrating approximately 95% effectiveness. However, they are seldom recommended due to their residual presence in bee colonies and potential risks.
Older synthetic treatments like coumaphos and fluvalinate have largely been discontinued because mites developed resistance to them, and their residues accumulate in wax, posing health hazards for bees and beekeepers alike.
Amitraz is currently considered one of the best synthetic remedies and is marketed under brands like Apivar. It does not leave significant residues or contaminate honey or wax, yet overuse can harm bees and may lead mites to develop resistance.
Distribution of the Cloudy Wing Virus Worldwide
The Cloudy Wing Virus is prevalent across Europe and has been identified on other continents as well. In Britain, serological surveys found the virus in about 15% of sampled bee colonies, with infection rates varying throughout the year without a clear seasonal pattern. Research also indicates a higher incidence of the virus in colonies infested with varroa mites compared to those without, highlighting the mite’s role in virus spread across Britain.
Throughout numerous studies, serological testing revealed that the Cloudy Wing Virus (CWV) was present in a large proportion of deceased adult honey bees during the season, as well as in honey bee pupae during the summer. Initially, it was suspected that varroa mite feeding played a significant role in spreading CWV, since the virus was detected both in infested pupae and in individual varroa mites themselves. Later findings, however, suggested that the virus’s presence did not directly depend on the extent of varroa mite infestation.
Research conducted in Nordic countries—including Denmark, Sweden, Finland, and Norway—showed somewhat different outcomes. There, CWV was found to be widespread across apiaries regardless of the degree of varroa mite presence. Moreover, the widespread prevalence of the virus in these regions did not appear to correlate with occurrences of colony collapse.
Conclusion
Currently, there is limited data explaining how the Cloudy Wing Virus persists and spreads in natural or field conditions, leaving its true impact on honey bee populations unclear. Despite this, the virus is not frequently found in adult bees or brood, and its relatively common occurrence in these groups implies it may not be highly pathogenic. Still, CWV can be lethal, often leading to colony death. Bees infected with the virus typically become lethargic and die within a few days.
Diagnosing Cloudy Wing Virus based on symptoms alone is unreliable, and its presumed association with varroa mites remains unconfirmed. Past laboratory investigations have produced inconsistent evidence regarding the link between varroa mite infestation and CWV presence. Nonetheless, controlling mite populations remains a valuable strategy to mitigate viral infections in honey bee colonies. Additional management approaches—including cultural practices, mechanical treatments, and chemical controls—also contribute to reducing viral outbreaks among bees.