Washing adult bees in alcohol was one of the first methods devised for measuring
mite populations in beehives, and it is still one of the most accurate. Alcohol
(25%) is often not available, and is also costly, so a technique has also
been devised using soapy water instead. Methylated spirits (25%) can also
be used, but care is needed to avoid breathing in the fumes.
Method – Mix a level tablespoon of liquid or dry laundry detergent in one litre of water. Select a detergent that doesn’t foam very much, since foaming can make the procedure more difficult. Collect 200 or more bees from the brood nest (about 1/4 of a 500 ml jar), making sure to take bees from at least three brood frames. Put the lid on the jar and shake the bees and the soapy water for one minute. Expect to remove 80-90% of the mites in that time period, but it takes a full 30 minutes to free almost all of the mites (99%). Pour the contents of the jar over a piece of cotton cloth (e.g., a piece torn from an
old bed sheet) placed above a bucket to catch the soapy water.
Count and carefully remove the bees, then count the number of mites. Finally work out the number of mites per 100 bees. To save time, you can use a piece of 8 mesh for the lid, as in the ether roll, and pour the bees and liquid through it. The mesh will collect the bees, but let the varroa come out with the liquid onto the cloth. Another way to improve accuracy is to thoroughly wash the bees with a sprayer into a bowl. Use a strong spray and count the number of mites that float on the top of the water in the bowl.
Advantages – The soapy water/alcohol method is both
lowcost and accurate, and involves only one trip to the apiary.
Disadvantages – The method takes more time than the ether roll because the contents of the jar have to be filtered.
Thresholds – The same British researchers who worked out thresholds for varroa populations based on examining drone brood have also developed thresholds for using the adult bee washing technique. The threshold depends on the size of the colony and the time of year. In a colony of 20,000 bees in the winter, if more than one mite is found in 300 bees, the mite population could build up to a level (2500 mites) needing treatment by the following autumn. For a colony with 60,000 bees at the end of the honey flow, the threshold number of mites is between two and ten in 300 bees. This threshold is variable since it depends on when the honey flow ends and the amount of worker brood and drone brood present in the colony in the preceding weeks.
Washing adult bees in alcohol is one of the first methods devised for measuring mite populations in beehives.