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, for the first two years, oxalic acid.  I no longer use oxalic acid as I am concerned about the damage to bees (especially queens) and because I find that the control achieved with thymol is satisfactory.

The strategy appears to be successful so far and the thymol also appears to have reduced chalkbrood levels - although some of this is now probably due to selection.  The cost is extremely low and treatment is very quick to apply.

The method of application 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%.

Method used:

• Thymol crystals are placed in an old honey jar lid placed on the queen excluder directly over the brood; an eke is used to provide the necessary air circulation (the ekes are also used for feeding fondant). 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 35p 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.

I monitor in the spring by uncapping drone brood when inspecting colonies for swarm control; mite levels are variable, but appear to remain below the economic damage threshold.  I consider that some exposure to mites is highly desirable as this will lead eventually to resistant bees.  Over the past five years I have changed all floors to permanently and fully open mesh; this does not appear to have affected the efficiency of the thymol.

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 lessened now by global warming, that the weather will turn too cold after their return in September for the thymol to work effectively.  In practice this does not seem to happen, probably because colonies return from the heather with large amounts of brood and the temperature immediately above the queen excluder will therefore be maintained.  Heather colonies are nearly always our best colonies during the following spring.

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.

Peter Edwards
Updated 5 August 2009

In 2009 I decided to reduce the number of treatments from 4 to 3 following tests which showed no adverse effects.
In 2010 I have reduced the number of treatments again to 2 for some colonies.

With lower temperatures I find that thymol works well in colonies with plenty of brood, but is much slower to evaporate (or deliquesce) in colonies with little brood.  This is very noticeable with our bees (A.m.m.) as poor weather in July can cause queens to go off lay temporarily. 
So in my view it is the warmth from the brood that makes all the difference.

Peter Edwards
Updated 28 November2010

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