Thymol as a ‘Winter’ Varroa Control

Richard Ball

Thymol is often used as a late summer varroa control with an efficacy of between 80% and 90% often being quoted as typical in the UK climate. One of the known factors of efficacy is temperature, the warmer it is giving better results but in extremes of heat or with overdosing it is known to kill bees and brood. ‘Apiguard’ is a commercial thymol-based product, which has been developed with a sophisticated base gel that helps to regulate the dose so that as it gets hotter it reduces the dose, and as it gets cooler it increases. Beekeepers often report that they consider efficacy as being negligible if the late summer is cool. During normal beekeeping management if thymol is used at this time and varroa mites remain at a high level after treatment it is common practice to use Oxalic Acid in November or December.

If a colony that has been treated or controlled by another method and varroa mites need to be controlled in winter what would happen if thymol was used. Many experts suggest that an efficacy of 60% can be achieved. Is this right?

As I monitor and record mite drop on a daily basis in my home apiary it seemed a good idea to put this to the test. Colony 1 was showing a concerning mite drop whilst the others were not so became control colonies. This assessment was made using information available in the NBU ‘Managing Varroa’ leaflet. ‘Apiguard’ was the chosen product as it was to hand in my medicine store and also because of the gel formulation.

During the six weeks prior to treatment application the average daily mite drop was:

 Colony 1  2.44

 Control Average  0.745

One tray of ‘Apiguard’ was applied on 23rd November 2010 – end of week 5 on the graph below. This coincided with a period of unseasonably cold weather where for four weeks night time temperatures averaged at sub-zero and day time less than 5°C. However on some days night-time temperatures were higher than the day time so the temperature graph shows high and low. No increase in mite drop was noted until after this time when temperatures were warmer though still unseasonably cold for the South-West of England. Mite drop continued to be monitored daily. It was clear that mite drop increased in Colony 1 when the lowest daily temperatures were above 0°C.

During the month of April daily mite drop averaged:

 Colony 1  3.81

 Control Average 1.225

This represents an increase prior to treatment of:

 Colony 1  x 1.5614

 Control Average x 1.6443


The control average in April 2011 shows no need for concern. However with a daily drop of 3.81 mites in Colony 1 varroa management controls such as drone brood culling, artificial swarm techniques or queen trapping should be used to prevent the colony becoming at risk.

Colony sizes were not significantly different at the last examination of 2010 and the first in 2011. However the presence of brood during the winter was not checked. If colonies are brood-less or little brood is present mite reproduction is not possible. Whether colonies become brood-less in SW England winters could make an interesting project.

The number of colonies used is insufficient to draw any meaningful conclusion but it would seem that there was a small benefit in the treatment. If the winter had been more normally milder it could have had a greater impact

Graph showing:

Daily mite drop averaged over weekly periods

Graph showing:

Maximum and Minimum temperatures over same period

Chris Slade’s Beekeeping Blog Link

Glyn Davies’ Surrey and WBKA Presentation

in pdf  format.

‘Honeybee Mating Process;

 Evolution & Consequences’