Deformed Wing Virus is the most widespread virus in honey bee colonies and one of the leading causes of winter losses. It is spoken of together with Varroa, because the relationship between the two has fundamentally changed the virus's impact.

Before and after Varroa

The virus existed before Varroa. It was present in colonies at low levels, mostly without symptoms; it spread by mouth and rarely did harm.

Varroa changed two things:

• Direct transmission: As the mite feeds it injects the virus into the bee; the natural barriers of the digestive tract are bypassed
• Increased load: The viral titre jumps from a symptomless level to a disease level

DWV today is therefore not an independent problem but largely a consequence of Varroa.

Signs

• Crumpled, wrinkled or undeveloped wings
• Shortened, stunted abdomen
• Loss of colour
• Young bees crawling in front of the hive, unable to fly
• Greatly shortened lifespan

The real danger is the invisible form

Seeing a bee with deformed wings is already an advanced stage. The real damage occurs in symptomless infection.

A bee that looks normal may carry a high viral load. That bee lives a shorter life. For a winter bee this is fatal: a bee that should live for months lives for weeks, and the colony dwindles away in midwinter.

For this reason the judgement "I see no deformed wings, so there is no problem" is misleading.

Why the colony collapses in winter

The chain runs as follows: the mite load is high in late summer → winter bees are born infected → the fat body is weakened by both mite and virus → the bees die earlier than expected → the colony dwindles in January or February.

Plenty of honey and few bees in the hive is the typical picture.

Control

There is no treatment that acts directly on the virus. The only realistic course is to control the vector:

• Reduce Varroa before late summer. The mite load must be down before the winter bees are reared
• Measure regularly. The viral load follows the mite load
• Act urgently if you see deformed bees. It indicates the mite level has been high for some time
• Work with lines showing strong hygienic behaviour. Colonies that clean out infected brood have the advantage
• Requeen. A young, healthy queen speeds the colony's recovery

Summary

DWV is the clearest illustration of why Varroa control is a matter of timing rather than of the calendar. The mite level in August determines the number of colonies in February.