Finnish beekeepers routinely treat their bees with oxalic acid as a part of their varroa mite management protocol. Most commonly they use the dribble method of inserting oxalic acid sugar syrup between the frames. That’s what I also did a year ago with my Farrar frame hives. Even though it seems a little strange to open the hives in the middle of the winter and pour syrup right on them they didn’t seem to be too bothered about it. The bees will lick the syrup off each other. The reason the treatment is done in the winter is that it is not effective to varroa mites that are in the brood – therefore the treatment should be done in the broodless period. The Italian bees that are most common in Finland go on laying eggs late into the autumn and early winter so oxalic acid treatment should be postponed to December. In a warm winter like this (today 20.12.2015 it was 9-10C – almost the same as last midsummer!) there might even be brood in December but it’s not a good idea to open the hives for checking at this time. The black Nordic bees would stop laying eggs much earlier (they are better adapted to the climate up north) but unfortunately hardly anyone has them anymore in Finland.
With my horizontal top bar hives I am basically trying to follow the same varroa management program as with the frame hives – so far I am not confident that the bees would survive the mites any better in the top bar hives. But using the dribble method of applying oxalic acid is problematic. The top bars form the roof of the hive – there are no spaces between the top bars similar to the spaces between frames in a frame hive. Therefore using the dribble method would require moving the top bars and thereby breaking the propolis sealing the bees have made between the top bars and the attachments between the combs and hive walls. In the winter the bees would not be able to fix the sealing anymore and therefore the conditions in the hive in terms of insulation, ventilation and draft would have changed without the bees being able to do anything about it. With the Nordic bees the treatment could be done earlier and this would not be such a problem.
When discussing this with Patrick Sellman – the Swedish horizontal top bar hive missionary – he suggested using a vaporiser instead. I started searching for vaporisers but could not really find a satisfactory solution before coming by the name Varrox vaporiser in Gunther Hauk’s book ”Toward saving the honey bee”. Looking at the website of the Swiss manufacturer ”Andermatt Biovet AG” I was convinced enough to order the device for a little over 100 €. Today was the day to test it in practise.
The vaporiser comes with clear instructions and its use is pretty straight-forward. You measure 1-2 g crystal oxalic acid (a measuring cup comes with the device) depending on hive size into the vaporiser spoon, push it in the hive, close the hive with wet towels and put the current on using a car battery. After 2,5 minutes you take the current off, wait 2 minutes before taking the vaporiser out of the hive and keep the hive closed for an additional 10 minutes. Dip the vaporiser in cold water before filling it again with the oxalic acid. Of course proper precautions need to be taken to avoid any contact with the vapor – bees might tolerate it well but the varroa mites and you don’t. You need to wear a protective breathing mask, glasses and gloves. Read the safety manual!
I first used the vaporiser with my 4 Farrar frame hives. I first cleaned the hive floors from dead bees and varroa mites. Without counting exactly I would say there was about 50-100 mites on the floors – and quite a few dead bees. After the treatment I checked the floors again finding some 20-30 dead bees and roughly the same number of mites. So some bees were definitely not happy with the treatment but maybe those were the ones in bad shape already anyway. The number of mites did not seem dramatic either taken that the manufacturer promises a >96% result. However my understanding is that they would not all die at once so I better check again after a week or so.
Treating the horizontal top bar hives relieved some problems that could be corrected with hive design. First of all the entrance in my hTBH’s are wine bottle size round holes (22 mm) so there is no way of putting the vaporiser in from there. The solution was to push it under a follower board. I am using the ”eco floor” à la Chandler, so I could push some bark chips aside and push the vaporiser to the other side putting a piece of wood under it to avoid igniting the bark. The problem here was that it was not possible to get all the way under the winter ball and therefore it is not certain if the vapor actually affected the whole bee colony. Following the vaporising from the observation window showed that it would not necessarily spread all over the hive – at least that is how it visually looked (see photo).
I think the solution is to make a vaporiser entrance in the hive body. It should be placed where the winter ball will be, i.e. in the center of the hive body in a Chandler design (entrance in the middle of a long side) or in the other end of the hive if the bee entrance is at the end of the hive (I call it type Mangum). The vaporiser entrance needs to be some 2 x 10 cm.
So what happens with the oxalic acid in the hive? According to manufacturer ”as a result of the heating action of the pan, the oxalic acid liquefies and vaporises. Oxalic acid vapour fills the hive and all the bees and surfaces are then covered with a very thin layer of oxalic acid crystals. These fine crystals are tolerated well by the bees, but have a deadly effect on varroa mites.” ”The oxalic acid will break down to water and carbon dioxide and will be cleaned by the bees in spring.” However if you need to get in the hive before bees have cleaned it be careful – the oxalic acid crystals could still be there (f.ex. emptying a dead hive).
Oxalic acid is definitely not healthy for you but it will totally break down into water and carbon dioxide without leaving any traces. Use of oxalic acid for Varroa mite management is allowed in organic bee keeping according to the EU Organic Regulation. Of course one could still consider not using it as a more natural way of bee keeping. But that is another discussion.