I first found varroa in the autumn of 1997.

I decided that I did not wish to introduce pesticides into my hives and so, since then, I have treated only with thymol crystals and oxalic acid. The strategy appears to be successful so far and the thymol appears to have also reduced chalkbrood disease to an insignificant level [the chalkbrood levels have risen again since writing this, possibly due to the appalling autumn, winter and spring - PE 24/6/2001]. The cost is extremely low and both treatments are very quick to apply.

The thymol treatment is based on research by Mariano Higes and Jesús Llorente of the Regional Apiculture Centre, Agrarian Investigation Service Joint Communities of Castilla - La Mancha, which was reported in Bee Biz, July 1997. Effectiveness is quoted at up to 97.6%.

The oxalic acid treatment is based on research by the Swiss Bee Research Institute.

Methods used:

• Thymol crystals are placed in an old honey jar lid placed on the queen excluder directly over the brood; supers will - just - fit over the lid to allow the colony space. The dose used is 8g (approximately 2 heaped teaspoonfuls) per week for four weeks and is applied in March - April, before the flow starts and during August immediately after the main crop is removed. Cost is approximately 32p per four week treatment.
  Thymol is a relatively safe substance to handle with gloves (datasheet), but is very pungent and therefore best kept out of the house.
• Oxalic acid is used in November when the colonies are practically broodless to ensure that the colony goes in to winter clean - this is the time to remove mites from any re-invasion during the Autumn.
  A solution is made with 1 litre water, 300g sugar and 45g oxalic acid dihydrate (but see update below); this is then dripped from a pipette (actually a cheap kitchen baster) over the seams of bees at a dose of 5ml per seam. Cost is approximately 5p per treatment.
  Oxalic acid in its crystalline form is poisonous and should be handled carefully. It should not be allowed to contact the skin and it is essential that the dust is not inhaled (datasheet). I weigh and mix outdoors, preferably in a light breeze to take any dust away from me. In the solution given above it is safe to handle with rubber gloves.
  Update 2 December 2002: Recently published research by the Swiss Bee Research Institute suggests that the solution should now be made with 35g oxalic acid dihydrate in 1 litre of 1:1 (sugar:water by weight) sugar syrup. This tolerated better by the bees and has a similar efficiency (98% mite kill in broodless colonies). Further research suggest that it may be possible to reduce the dose to 2.5ml per seam of bees.

I used the supplementary treatment with oxalic acid for the first two years after finding varroa, but then discontinued it as numbers seemed to be well controlled with thymol alone - and after six years with varroa I am very pleased with the results. I am still applying thymol in March/April for four weeks before the spring flow and also for the four weeks immediately after the main flow, putting on the thymol as I remove the crop.

I monitor in the spring by uncapping drone brood when inspecting colonies for swarm control and was hard pressed to find any varroa mites this year. I also have perhaps 10% of colonies on varroa floors and again have found it difficult to find any mites, the highest count being after the application of thymol in August when one colony yielded 10 mites after two weeks.

One potential problem is the treatment of colonies going to the heather; it would be good to treat them before they go, but a full four-week treatment is obviously not possible. There is then a danger, perhaps now lessened by global warming, that the weather will turn too cold after their return in September for the thymol to work effectively. However, should this happen then oxalic acid could be used and, with varroa numbers so well controlled during the earlier part of the season, it seems unlikely that damage thresholds will be approached.

Although bees do move away from the thymol, I have not experienced any exodus of bees from the hive, as some have reported, despite applying it this year when temperatures exceeded 30°C, nor have I seen any adverse effects on brood, adult bees or queens. Since the arrival of varroa my average crop (based on the number of colonies the previous autumn) has actually improved and there has been no noticeable taint of the honey.

I put around thirty colonies on open-mesh floors in the autumn of 2002 and have found that this appears to have no adverse effect on the efficiency of the thymol treatment.

Peter Edwards
7 September 2003