Good timing is one of the keys to controlling Varroa mites in honey bee colonies. Knowing the life cycle of the parasite, and its population growth through the season, will help beekeepers to treat in a timely and effective manner, and keep the Varroa mite populations down to levels that the bees are able to tolerate.
There are three common methods of monitoring the population of Varroa mites in honey bee colonies:
- Drone brood sampling
- Natural drop on sticky board
- Powdered sugar roll
Here is a brief explanation of each of those methods.
1. Drone brood sampling
Capped drone brood is impaled on the tines of a wire capping scratcher. The beekeeper looks for Varroa mites, which are fairly easy to see against the white pupae. Note that we don’t count the total number of individual mites with this method, but the percentage of brood that’s infected.
2. Natural drop on sticky board
Natural mite drop onto a sticky board: this is probably the most common method used to monitor Varroa mites among the beekeepers of Atlantic Canada at present.
A sticky or Vaseline-coated board is placed on the floor of the hive, usually with a wire mesh screen on top to keep the bees up off it, and the board is left in place for a set period of time. At intervals — usually after 24 hours and again at the end of 3 days — the board is removed and the beekeeper counts the number of mites that are on the sticky board.
The 24-hour mite drop is adequate for a quick idea of how badly infected a hive might be, but a more accurate count is obtained by waiting 3 days and then working out the average number of mites dropped per day.
3. Sugar shake
Powdered sugar sampling is the third common method of monitoring varroa mite populations. In this method, a sample of approximately 300 live nurse bees (1/2 cup of bees) is scooped up in a jar and shaken gently with powdered sugar (also known as confectioner’s or icing sugar) for about one minute.
The sugar will cause the mites to fall off the bees, and the mites are dumped out into a light-coloured dish to be counted. The number that the beekeeper will record, in this instance, is usually the number of mites per bee — or mites per sample may be used if the beekeeper takes care that all samples taken through the season are of the same size.
Shaking in powdered sugar does not harm the bees — they will just clean off the powdered sugar and go back to their normal life when they’re released back to the hive.
For more accurate mite counts, however, the sampled bees may be killed with a wash of alcohol or soapy water and the sample poured through a double strainer. A coarse mesh catches the bees but allows the mites to pass through, while a second finer screen will catch the mites and allow the liquid to flow away.
Why do we count Varroa mites, anyway?
The idea behind periodic sampling is to monitor the population levels of the Varroa mites within a given hive, to best judge when to treat for mites for maximum impact — and to check how well a treatment has been working.
At a minimum, beekeepers are generally advised to do a “head count” on mites at least twice a year. A mite count late in the spring, once the queen has started to lay well, gives the beekeeper a chance to treat as needed before the main honey flow begins. After the main honey flow, about mid- to late-August, a second mite count will allow the beekeeper to treat for Varroa mites as needed in order to ensure winter survival of the honey bees.