SLU news

The timing of varroa treatment is important for the health status of overwintering bees

Published: 18 September 2017

A honeybee colony that is infested by varroa mites will succumb within a few years without control treatment. But it's actually not the mites that kill the colony, rather the viruses that the mite vectors and spreads. Deformed Wing Virus (DWV), the most common mite-associated virus causes bees to have deformed wings that cannot fly. SLU researchers have looked at what happens to the virus infection levels when the mites have been removed from the honeybee colony after a mite control treatment. One result is that the timing of the treatment is important.

The study shows that:

  • High virus levels in honeybee colonies can be maintained by oral transmission between bees even after the mites are removed
  • It is important to reduce mite infestation as early as possible in the late summer so that a turnover of the adult bee population can replace virus-infected bees with healthy bees beforing rearing the sensitive long-lived overwinter bees.

The varroa mites feed on bee haemolymph of developing pupae and adult bees and thereby contribute to the rapid spread of the virus. But the bees can also spread the virus to each other, especially when they feed their larvae. This study demonstrates that these oral transmission pathways between bees are important for maintaining the virus epidemic even when the mites have been removed.

 Treated with Apistan

"When it comes to honeybee health, we usually discuss the varroa mites and how to treat them, but what about the virus? It is actually the virus that causes the colony to collapse and what happens with the virus infection dynamics after a mite control treatment is not well understood and that was what we wanted to study, " says Barbara Locke, researcher at the Department of Ecology.

Barbara and her colleagues used six bee colonies that were infested by Varroa mites. They divided every colony in two, and treated one of the splits against mites with Apistan, while the other split was not treated.

 The virus begin to increase again

As expected the mites almost completely disappeared from the treated colonies and rapidly increased in numbers in the untreated colonies. To see what happened to the viruses the researchers used molecular techniques to quantify the virus levels in adult bees and pupae every other week after treatment throughout the summer for 14 weeks. In the treated colonies the amount of viruses initially decreased sharply but began to slowly increase again in the autumn to subclinical levels: below the threat of mortality but high enough to cause suboptimal health conditions for the colony. Why the virus levels increased again was not due to mite reinvasions and remains unclear, but it is probably linked to the major physiological and functional differences between 'summer bees' that live about six weeks and is constantly replaced with new bees during the season and 'winter bees' that lives for six months from autumn to spring.

"What we saw was that the turnover of adult bees played a greater role than previously thought for the timining considerantions of mite control treatment," said Barbara Locke.

 Timing of treatment

If no or a few mites are present in the summer months, a treatment in the fall is enough to minimize the amount of mites present in the colony in the following spring when the bee colony starts building up again after the winter. However, if the colony is highly infested with mites the treatment should be done earlier in the season so that the colony has enough time to replace weak virus-infested bees with healthy bees before feeding the sensitive long-lived overwintering bees.

“That means that a treatment for highly infested colonies should preferably be done at least six weeks before the winter bees are produced but of course after harvesting the honey”.

This study has a very practical application for treatment highlighting the importance of monitoring mite infestation levels to assess how and when to treat, while from a scientific aspect we have new insight into the virus dynamics in relation to the fascinating physiology of the honeybee colony.  

Bees with varroa mites on their back. Photo Barbara Locke Grandér