It was a beautifully sunny but chilly day yesterday for a demonstration by Tom of oxalic acid sublimation. Amazingly some of our bees were still flying even at temperatures of under 10°C (50°F). The snowdrop shoots haven’t come on much further.

Inside our nuc the bees were still active but clustered more tightly than before. I smeared some extra fondant on top of a few of the frames. They have fondant left in the side feeder still, but I thought it would be nice for them to have some nearer the cluster.

In our bigger hive the ladies remain up in their fondant and pollen bags, but they have plenty of the sweet stuff left still. Nibble nibble.

A good number of us had gathered to see Tom demonstrate how oxalic acid sublimation (vaporisation) works. His subliminator cost around £35 (see Thorne’s Vapmite one) and the car battery charge around £35 too, so initial equipment costs are about £70. He did say this was a cheap subliminator and it’s starting to fall apart, there is a more expensive version available for around £100 which would probably last longer. You will also need oxalic acid in the approved form of Api-Bioxal, which conveniently contains extra sugar that identifies it as Api-Bioxal and makes the subliminator tray extra-sticky.

As the apiary bees were already treated by the drizzle method before Christmas, this was only a demonstration on an empty hive. The hive had a glass crown board so we could see the effects of the gas in a confined space. We were all wearing masks, this is very important as the oxalic fumes are dangerous to humans.

The equipment

Tom began by putting foam in at the entrance, this helps keep the fumes inside the hive. He left the subliminator in for 3 mins 20 seconds attached to the battery to heat up the oxalic acid, then another 3 mins without the battery to cool down. You can then remove the subliminator and leave the hive sealed up with foam for 5-10 mins after that, before removing the foam (please read the official instructions before doing it yourself rather than relying on these timings, in case I am mis-remembering anything!).

As the oxalic acid vaporises, the vapour fills the hive, coating the bees and hive surfaces with a very thin layer of oxalic acid crystals. The bees cope well with these crystals, but they have a deadly effect on varroa mites.

The advantages of sublimation are:

• You don’t need to open up the hive, which breaks propolis seals and can potentially disturb the bees.
• You can carry out repeated treatments, whereas the drizzle should only be carried out once annually.
• Research carried out by the University of Sussex Laboratory of Apiculture and Social Insects suggests that, compared to spraying or dribbling, sublimation has a higher varroa kill rate (see Integrated control of Varroa mites on the LASI website).

The disadvantages of sublimation are:

• The extra costs of the subliminator and battery equipment.
• Oxalic acid is toxic to humans, so you have to be very careful when handling it; including wearing gloves and a mask to avoid breathing in the fumes.

The instructions on Thorne’s website have some interesting details – they say the air temperature should not be below +4°C and the last cleansing flight should not date back more than four weeks. This is probably not something we need to worry about in London, but further north daytime temperatures might drop lower than 4°C. The LASI guidelines recommend applying oxalic at outside temperatures of 4-16°C.

Useful links:

• How to Apply Oxalic Acid Via Sublimation to Control Varroa – LASI guidelines
• How to Apply Oxalic Acid to Honey Bee Colonies via Sublimation – LASI video
• The Best Way To Kill Varroa With Oxalic Acid – Bee Culture article by Francis L.W. Ratnieks, Luciano Scandian, Hasan Al Toufailia, February 2016