Varroa mite drops using thymol treatment

By John Mumford

Making sense of mite drops counts using any Varroa treatment can be most confusing! How many mites are actually in the bee population is a complete unknown, and if you see deformed bees it could be that you have a lot of mites, OR that you have a bad virus problem, OR both. Equally a colony showing little signs of Varroa problems may have a very large Varroa population.

When any varroa treatment is first applied there are 20% or so of the mite population on the bees, and the Apiguard/ Grease Pattie is fresh and the treatment gives off more Thymol, hence there is a large initial mite drop. When the treatment has been in the hive a few days things begin to settle down and the mite drops more nearly reflect the numbers of mites emerging with the bees. The efficacy of Thymol can be very dependent on ambient temperatures and the results can sometimes get a bit distorted.

It should be remembered that If left untreated a Varroa mite population would naturally increase at some 40% over a sealed brood cycle (13 days), so we need to do mite counts on set days during the cycle to be able to make some meaningful comparisons.

Natural Mite Mortality is said by the experts to give some indication of a mite population within a colony of bees depending on the time of year. And so I have adopted the same reasoning that, “All things being equal”, the proportion of mites killed by a treatment is dependant on the number of mites present and the efficacy of the treatment.

So, if we make an average daily mite count over days 6, 7, and 8 after the treatment is first applied, and we make another average mite daily count over days 18, 19, and 20 after the treatment was first applied and the treatment is refreshed on day 13, the second average divided by the first average will give the SURVIVAL RATIO.

A Survival  Ratio greater than 60% is just keeping the mite population stable. A Survival Ratios of 50% will reduce the mite population to acceptable numbers, but the treatment will need to continue over a longer period, (about 4 sealed brood cycles), 7 or 8 weeks. A typical Survival Ratio with Thymol treatment when applied correctly is between 25% and 30%, and the treatment will have done it’s job in just 3 sealed brood cycles, 5 to 6 weeks.

If Apiguard is to reduce mite numbers to safe proportions over 2 sealed brood cycles, 4 weeks, then a Survival Ratio of  less than 20% is essential. If there is any doubt about how well your Apiguard treatment has worked then continue with another tray – “Dead bees don’t gather honey.”  I have heard that some Queens are reluctant to lay anywhere near Apiguard but have no personal experience of it.

I like to continue the Thymol treatment until the mite drop is below 2 or 3 mites, per week.

Where bees are kept in large hives with lots of ventilation that can’t easily be sealed off, then it is worth considering transferring them into a normal hive for their treatment and then putting them back once the mites have been reduced to safe proportions. Happy counting!

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