Identifying Varroa Resistant Bees

Key traits

The key observations in a varroa resistant colony are uncapping of worker brood, chewing out of infected pupae resulting in pupal exoskeletons on varroa insert boards, leading to low mite counts as the breeding cycles of the mites are interrupted. With low levels of varroa, there are fewer vectors of deformed wing virus enabling the colony to survive.

Uncapping, chewing out and exoskeletons

The key trait that varroa resistant colonies show is the chewing out of mite infected worker pupae in the brood nest. Some worker brood will be uncapped, typically at the pink/purple-eyed stage of pupal development, and infected pupae will be chewed out of the cells, with the parts falling to the floor or varroa insert board.

Workers have learnt to detect the odour coming from a worker cell infested with varroa. But in a busy hive chemical cues are wafted around, making it difficult for detector bees to pinpoint the exact cell. As a result, a group of cells are uncapped in the search process and the beekeeper observes patches of uncapping during inspections. Infected pupae will be removed by the workers, with other un-infested cells recapped and the pupae left to develop into adult bees. You can learn more about the science of this process at Varroaresistant.uk.

 

During an inspection this uncapping and chewing out is observed at the pink/purple eye stage of pupal development, where the interruption stops mites from reproducing and contributes towards infertility in the mother mites as they can only reproduce two or three time in their lifetimes.

Uncapping will be observed regularly during inspections, but the behaviour may stop during a major flow as the bees focus on nectar collection and processing. In some colonies, a very high proportion (c.60%) of worker brood is uncapped by stages.

 

When infected pupae are removed from the cells, the workers chew them into pieces which then fall to the varroa insert board:

There will be certain times of year when that aren't any exoskeleton parts on the board, such as when uncapping stops during a flow, or during a brood break when the queen stops laying for a period in late autumn/early winter.

Mite counts

It is very important to track and record mite numbers in colonies to ensure that the bees are continuing to manage mite numbers as they disrupt mite breeding cycles by uncapping and chewing out.

There are a number of ways that beekeepers have used to monitor mite numbers in a colony, including sampling (e.g. sugar shaking or alcohol washes), Harbo assays and drone brood uncapping, The simplest and easiest method is to count the mite drop on the varroa insert board.

Whatever method is chosen, it's important that results are recorded regularly and consistently so that the seasonality of mite drops can be tracked, which is described in Steve Riley's book The Honey Bee Solution to Varroa.

A good way of doing mite counts, particularly across several colonies, is to take a picture of each varroa insert board which can then be blown up for a more accurate count away from the pressures of an inspection or manipulation. Each board can be marked with an identifier so pictures aren't mixed up, and some beekeepers mark grid lines on the board to help.

As a guide, an average mite count of five or fewer per day across a whole year for a British National brood box would be varroa resistant.

The mite population calculator on varroaresistant.uk takes account of the monitoring method you may decide to use.

In summary:

• • The key observations in a varroa resistant colony are: uncapped worker pupae, with infected pupae being chewed out resulting in exoskeleton parts on the varroa insert boards. This should result in low seasonal mite counts.

 

• • Keep varroa insert boards in all year round for regular mite counts and pupal exoskeleton observations.

 

• • Look for uncapped worker cells in the brood chamber at each inspection, typically at the pink/purple eye stage, and record this.

 

• • Do a mite count from the varroa insert board at each inspection or hive visit. An easy way is to take a photo and do the count at home. During winter, do these counts at least once a month - it is far harder when there is a build-up of debris on the boards.