Deformed Wing Virus (DWV)

Deformed wing virus (DWV) is the most widespread virus in honey bee colonies and, together with varroa, one of the main causes of winter losses. Seeing bees with crumpled wings means the mite load has been high for a long time. There is no medicine against the virus itself; the treatment is to bring varroa down in good time.

What is it, and what did varroa change?

DWV was present in colonies before varroa; it spread orally through food, stayed at low levels and mostly caused no symptoms. Varroa upset this balance:

• Direct transmission: While feeding on the pupa in the capped cell, the mite injects the virus straight into the haemolymph, bypassing the natural barriers of the digestive tract.
• Rising load: As the virus is carried inside the mite and passed from bee to bee, its load climbs from a symptomless level to a disease level.
• Selection of variants: With the spread of varroa, virus variants that are carried well by the mite (DWV-A and DWV-B) have become dominant.

For this reason DWV today is not an independent disease but largely a consequence of the varroa problem.

How to recognise it

Symptoms appear in bees that picked up a high virus load during the pupal stage:

• Crumpled, curled, small or undeveloped wings
• Short, stubby and sometimes swollen abdomen
• Pale colour and weak appearance; sometimes paralysed legs
• Young bees unable to fly, crawling on the frames or in front of the hive
• These bees dying soon in the hive or being thrown out by the workers

The same colony usually shows the other signs of varroa as well: perforated cell cappings, a patchy brood pattern, dead pupae, and mites on the bees or on the floor.

The real danger is the invisible form

Seeing bees with deformed wings already means an advanced stage. The real loss comes from symptomless infection: bees whose wings look normal can also carry a high virus load and live shorter than expected.

The chain works like this: the mite load is high at the end of summer → winter bees are weakened by mites and virus during the pupal stage → bees that should live for months die early → the colony dwindles in mid or late winter. A hive with plenty of honey but very few bees is the typical picture of this situation.

How it differs from similar conditions

How to confirm it in the hive

• The wing deformity is visible to the eye; a definitive virus diagnosis is not needed.
• Measure the varroa level straight away: the mite rate on adult bees with the icing sugar roll or alcohol wash, or the daily mite drop with a natural-drop board.
• Open capped drone brood with an uncapping fork and look for mites on the pupae.
• A laboratory can measure the virus load by PCR, but the beekeeper's decision is based on the varroa count.

What to do

1. Do not delay varroa treatment. If you see bees with deformed wings, the mite level has been high for a long time; act immediately if there is no honey on the hive or the product is approved for use during the honey flow.
2. Bring the mites down before the winter bees are raised. In most regions this falls at the end of summer. A late treatment will not save winter bees that are already damaged.
3. Count again after treatment. If the mite level has not dropped, there may be resistance or reinfestation (robbing, drifting).
4. Help a weak colony recover. Feed it; if necessary, unite it with another colony whose varroa has been brought down.
5. Requeen. A young queen and lines with strong hygienic behaviour make recovery easier.

Treatment and medicines

There is no effective medicine against DWV itself; the treatment is aimed at varroa. Authorised options fall into the following active substance groups:

• Oxalic acid: Most effective in the broodless period (winter cluster, artificially created broodless period), applied by trickling or vaporisation.
• Formic acid: Can also reach mites in capped brood; sensitive to temperature.
• Thymol-based products: Temperature-dependent effect; used in mild weather.
• Amitraz strips: Effective, but resistance can develop with continuous use. Resistance to the pyrethroid group (tau-fluvalinate, flumethrin) is widespread in many regions.

Take the product and dose from the package leaflet or from your veterinarian. Use it with honey supers on only if the label allows it, and do not repeat the same active substance year after year. Biotechnical methods such as drone brood removal and creating a broodless period by caging the queen increase the effect of the medicine.

Prevention

• Measure varroa regularly from spring to autumn; act on the measurement, not the calendar.
• Treating at the same time as neighbouring apiaries reduces reinfestation.
• Do not leave a dying or very weak colony open to robbing; mites are carried to strong colonies by robber bees.