Jeremy Quinlan

The beekeeper’s most important November activity is the winter treatment of Varroa. Professor David Evans, a virologist at St Andrew’s University who blogs as The Apiarist, has covered the subject in detail; much of what follows is taken from his posts, with permission. He has my grateful thanks.

                                                                                                                                                                          Photo by Jeremy Quinlan

If colonies are to survive, Varroa numbers must be controlled. If the mites are not killed, the open wounds they inflict (arrowed in the photo above) and the viruses they transmit threaten colony survival. Some do not treat in the hope their bees may become tolerant; miracles are rare! At high levels, deformed wing virus reduces the lifespan of worker bees. A reduced lifespan does not matter in May when there are many workers and they only live for some six weeks. Reduced longevity matters a very great deal in the winter when overwintering bees must survive for months to maintain the colony and raise new brood in the spring. If these bees die early, in weeks not months, the colony will dwindle and die.

When to treat
The timing of the late summer treatment is probably the most important decision about Varroa control a beekeeper makes. Treatment in mid-August makes a follow-up winter treatment essential, because surviving mites continue to reproduce. It is doubly essential for those who used thymol as it is only ~80% effective. Oxalic acid is ~96% effective. It is dangerous to people, so the minimum PPE should be nitrile gloves and eye protection. It can also kill open brood when trickled, but seems well tolerated when sublimed/vaporised. The simplest, cheapest and quickest method of application is trickling or dribbling at 5ml per ‘seam’ of bees. It should only be applied when colonies are broodless, but judging when they are is difficult. In a warm winter they may never be broodless, but in midwinter, sealed brood within which mites can shelter, is at a minimum. As soon as there is a proper cold spell, the queen will stop laying, and 21 days later there will be no brood; treat then. David Evans’s records show this minimum brood period has varied between the third week in November and the third week in January. Treating between Christmas and the New Year is often too late.

Estimate Varroa numbers
An imperfect estimate of the state of the brood may be had from daily inspections of clean Varroa trays. New biscuit-coloured, wax capping crumbs will show that brood is still emerging. Getting it a little wrong means the loss of a few eggs and larvae, a worthwhile sacrifice; but mites in capped cells will survive. David says: ‘The easiest way to determine whether the colony has sealed brood is, on a slightly better day, to open the box and look. Done quickly and calmly I suspect this is more distressing for the beekeeper than it is for the colony. You think the bees will be aggressive or distressed. In reality they are usually pretty lethargic and often very few fly. You only need to look at the frame in the centre of the cluster. If there is brood present it will be where the bees are most concentrated.’

How to treat with oxalic acid
David Evans describes the method he uses. ‘Remove the roof and insulation, lift one corner of the crown board and give a gentle puff of smoke under it. Wait thirty seconds and gently remove it. There will be bees on its underside; stand it carefully to the side, out of the breeze. Some bees will probably crawl to the upper edge when you put it back, so remember first to shake them off into the hive. The colony is likely to be clustered if the weather is 8°C or cooler. To give space to work, first remove the outer frame furthest from the cluster.

                                                                                                                                                                         Photo courtesy of the

In the photo above the bees occupy five seams with a few stragglers between frames 6 and 7. I used my hive tool between frames 3 and 4 to split the colony, just levering them a centimetre or so apart, so I could then separate frame 3 from 2 and lift it out. The queen was on the far side of frame 3. The top of the frame was filled with sealed stores, the lower part of the frame was almost full of uncapped stores. There was no sealed brood and no eggs or larvae that I could see.’ Light levels are low so a head-torch would be useful. Remember, colonies vary; can you assume any one is ‘typical’?

Be prepared. You need: oxalic acid in the form of Api-Bioxal, Oxuvar or Oxybee, a syringe or, better still, a Trickle2 bottle, a vacuum flask dedicated for this use only, granulated sugar, nitrile disposable gloves and eye protection. Get these soon; do not leave it until midwinter.

These treatments are enough for ten or more colonies, so join with your beekeeping neighbours. They should be delivered at an air temperature of 3°C or more. Usually only one trickle a year, but vaporisations may be repeated. While any left-over treatment is best disposed of in line with the manufacturer’s instructions, it may be stored in the fridge at 4°C, where it needs to be clearly labelled and kept out of reach of children. David says: ‘In my experience the daily mite drop is highest 24–48 hours after treatment. I usually try and monitor it over the next five to seven days.’

Editor’s note:
All products used should be licenced and manufacturer’s usage instructions should be followed. Usage should be recorded on a Veterinary Medicines Directorate (VMD) record sheet, which must be kept for five years.