Varroa destructor


Varroa destructor

We have a problem !

In fact we have hundreds of them – varroa destructor mites. Despite applying an autumn treatment we have discovered, when applying the later winter treatment, that the autumn treatment appears to have been rather unsuccessful. So we have seen a massive dead mite count over the last few weeks, much higher than expected. On the upside, I suppose, at least the winter treatment seems to have worked …

Image: courtesy of National Bee Unit, © Crown copyright

What is varroa ?

The varroa destructor mite is a dangerous threat to honey bee colonies. It can, as I mentioned in a previous blog, lead to the collapse and death of the hive. And it is prevalent throughout the UK, with very few exceptions (remote parts of the western highlands, for example). It is present on every (inhabited) continent with the exception of Australia and was first identified in the UK only in 1992.

These are tiny mites who reproduce by dropping into a cell with the developing honey bee larvae just before the cell is capped. The mite is then free to feed off the pupae and lay little mite eggs undisturbed over the next number of days. The worker bee emerges after 21 days (24 days for drones – which is not good news for them, see the paragraph on the Impact of Drones below!). Any surviving mites will cadge a lift on the newly emerged bee and look for a new open cell with larvae to drop into and start the cycle again. It is not even the potential weakening of individual pupae that is the issue, but the fact that the mites can transmit viruses such as deformed wing virus.

What can be done ?

Many beekeepers will disagree on appropriate treatments and the timing of them. But we followed the generally accepted principle of applying an approved miticidal treatment in August, after the honey supers (the boxes with the honey frames) had been collected and removed from the hives. The idea of this timing is to decimate the mite population before the winter bees start to be developed. Most worker bees survive only 6 weeks; however winter bees are actually physiologically different from the summer bees and will survive several months to see the colony through the cold winter. It is vital, therefore, to help the hive obtain the highest possible number of healthy winter bees. It is thought that varroa have developed a resistance to the miticide we chose, unwittingly because it seemed the least threatening to the actual bees themselves.

There is a period in winter where the hive will be broodless, i.e. there will be no eggs, larvae or pupae, therefore no sealed cells for the little blighters to hide in. This is the perfect time to apply another treatment – oxalic acid. This sounds scary but is actually an organic compound found in many plants. It does a great job of killing off the mites but it cannot penetrate through any capped cells, which is why it needs to be used during this broodless period.

Impact of drones

Before I go into the gruesome detail of how drones can aid in varroa control, I’ll just remind everyone that drones are the male bees. They don’t make honey, they don’t clean the hive or raise the baby bees. They don’t even have a sting to protect the colony. They have one purpose and one purpose only – to mate with a queen on her mating flights. If they succeed at this, they die straight afterwards, albeit possibly with a wee bee smile on their face. If they fail to mate the queen, then they’ll probably hang around the hive for a few weeks, making the nurse worker bees feed them honey, roaming aimlessly until the colony gets fed up of them and throws them out in autumn.

However, a key difference is that the drone takes those 3 extra days to emerge from the capped cell. This is just enough extra time to allow the varroa mite a full two life cycles, rather than the one and almost a second that they get in a worker bee cell. The mites actually know this and will by choice jump into drone cells rather than worker cells! The beekeeper can use this to their advantage in spring by introducing a frame of what is called drone foundation into the hive. This is a frame that has imprinted cells exactly the right size for drones which encourages the queen to lay a whole double-sized frame of drone eggs. The mites all go rushing into these drone cells and congregate in there, on that one frame. Some time before the 24 days is up, therefore, an astute beekeeper can remove this still capped frame, full of drone pupae and hopefully the majority of any mite population, and, errrr, humanely despatch the whole lot in the freezer.

Quite what we are then meant to do I have no idea and haven’t had the stomach to ask yet. All I know is that somehow this frame gets emptied and can be reused. If anyone insists I’ll go and find out.Until then, it’s fingers crossed that our colony managed to get strong enough to survive the pests and will manage through a cold Scottish winter.

Image: courtesy of National Bee Unit, © Crown copyright

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