larvae. It rejoices in the latin name varroa destructor.
It first appeared in the UK in 1992, and has since spread across the
entire country and is found in every beehive. Despite its latin name, the
varroa mite does not immediately destroy a colony. To do so would
result in killing itself as well. Instead the mites reproduce gradually, build
their numbers over time. Beekeepers have learnt to monitor and manage
varroa mite numbers so they do not get too high.
Varroa mites prefer to feed on larvae when they are sealed in their
honeycomb cells developing into adult bees. This results in weak and
disabled bees emerging. These are unable to contribute to the work of
the colony and are instead a burden. If the number of varroa mites gets
too high the final result is colony collapse, where the healthy bees can
no longer sustain the colony.
Varroa mites on the back of a bee.
How do we monitor varroa mite numbers?
Modern beehives incorporate a tray at the bottom of the hive that allows a sheet of board to be inserted to catch anything that falls from within the hive. Much of what falls is pollen, but occasionally a varroa mite will fall too. The “varroa board” is generally left under the hive for a week and then removed and the number of varroa mites counted. The number of mites on the board can be used to extrapolate the varroa population in the hive.Varroa mites are tiny (less than 1mm across), but they have a distinctive shape and shine to their outer shell. With practice they can be counted fairly easily. This is my daughter counting mites from our bees.
How do we reduce the number of varroa mites?
It is not possible to eliminate varroa mites all together. But there are a number of techniques that can be used to reduce their numbers. Once the numbers are reduced to tens rather than thousands in the hive it takes some months for their numbers to increase again.Mite numbers can be reduced by using various pesticides that have been developed specially for beekeeping. In addition, there are non-chemical techniques that can help to reduce mite numbers, such as having an open mesh floor rather than a solid floor to the hive. Most beekeepers use a mixture of these techniques to keep the mite numbers as low as practical.
One very effective pesticide treatment is oxalic acid, which is mixed with sugar syrup and applied directly to the bees. This is best done when there is no brood – that is, no larvae sealed in honeycomb cells. This tends to happen in winter when the bees are dormant. With all the mites out of honeycomb cells they are much more vulnerable to the effect of the pesticide.
So in mid January we applied oxalic acid treatment to our bees. It’s a quick job: once the chemical is mixed with the sugar syrup it simply needs to be gently squirted over the bees in the hive. Because it is cold outside the hive should not be open for long. We made sure the hives were only open for a couple of minutes. As you can see from the photos, we still got covered in sleepy bees!
A week after the treatment we checked the varroa boards again. If you look closely at the photo below you can see hundreds of mites have died and dropped out of the hive. Result.
