August 22, 2019

Varroa Destructor, the Pest

The Varroa Destructor seems to have come to Maine in 1987, the year after the Tracheal Mite. Both created devastation in the beekeeping world and caught beekeepers off-guard. At that time it was referred to as the Fall Dwindle. It took a closer look to see that the die-outs were coming from a pesky little parasitic mite. The Varroa is now known to suck the blood-like body fluids, hymolymph, from the pupa and adult bees. They are also responsible for the deformed wing virus and some 22 other reported viruses. The life cycle of the varroa is interesting in that as the mite enters the hive on a bee, it will drop off when it collects the scent, juvenile pheromone, of the brood in royal jelly. Here it scurries into the cell and hides under the prepupa in the royal jelly. As soon as the cell is sealed, the fertile female will lay an egg. This first egg is that of a male. About 30 hours later, the mother mite will lay an additional egg which will be a female. The male will hatch, develop, and mature a bit faster than the female. The mother will continue laying until she has laid 2 to 6 eggs. The male mite will be mature and ready to mate with his sisters when they mature, and then he will die in the cell. The mated sisters will continue to feed on the pupa until it emerges from the cell. They will then scurry about to find another cell with brood to continue the cycle of mite rearing. The female mites are believed to live as long as 30 days to continue rearing little mites.

You can see how exponentially these little pests can multiply. Eight mites could turn into 32, and then 192 in the following twelve days. So in other words, in about three weeks, the population has increased by nearly 2400%. So treating for mites isn’t something we can afford to put off if we see we have a potential problem.

Varroa Destructor - Adapted from illustration by B. Alexander

Varroa Destructor - Adapted from illustration by B. Alexander


It’s that time of year again when we need to be aware of what the mite load is in our hives. In the spring and fall, it’s especially important to know your mite population. In the spring, if left unchecked, a mite bloom will occur when your about to go into a peak nectar flow, only to have a hive dwindle out. Then your concern becomes whether you can save the bees rather than what to do with all that honey. In the fall, if left unchecked, the population will dwindle until the hive finally expires from lack of bees, whether it be in the fall or through the winter months. If the timing for medicating for mites isn’t right, the hive can get PMS, parasitic mite syndrome. This is a scenario where you killed off a large population of mites, but the mites were so prevalent in the hive that they left behind a large amount of disease. Then the bees dwindle from a collection of viruses, fungi, and bacterium—all things that can’t be treated for. So this is why we need to stay on top of the mite situation and keep our bees healthy and productive.

There are several methods that we can use to check for mites and how extensive they are in the hive. Now we understand how they can contribute to our bees’ poor health. We know they are carriers of other viruses and when unchecked, will eventually kill a hive. So I have listed the common methods for testing your mite population and tried to simplify the results. As far as treatments, I’ll leave that up to you to decide.


Some methods of mite detection are much more reliable than others. Some kill the bees, whereas others will promote hygienic behavior. Each test involves a different number of bees; the more bees involved, the more reliable the test. Each method will look for mites. A certain mite count will trigger a need for treatment.

Drop Board: A screened bottom board [SBB] generally is equipped with a drop board [a.k.a. SBB insert]. Clean the insert of any debris and apply a thin layer of vegetable oil, shortening, Pam cooking spray or Vaseline to the insert for the purpose of adhering the dropped mites to it, ultimately killing them. Put the insert into place and leave it there for 2 to 4 days. When you remove it, count the mites and divid them by the number of days the insert was in place. This method is perhaps the least reliable of the mite checks.

Drone Check: Break the cell cap and pull the pupa out with a toothpick. In checking drone brood for mites, you’ll try to find brood that is at the purple-eye stage. Here you will find the hatched mites for an accurate account. Check 10 purple-eye drone pupa.

Sugar Roll: Checking for mites using the sugar-roll technique is much more reliable than the previous two methods, and doesn’t kill the bees. You’ll need approximately 300 bees, (¼ to 1/3 of a cup). Lightly rub your jar down on a brood frame to catch your bees. Do this several times to get the number of needed bees. Place a screened cover (#8 hardware cloth) on the jar and add one heaping tablespoon of powdered sugar. Gently roll the container, mixing the sugar into the clump of bees. Add another spoonful if needed, to thoroughly coat the bees. Let the bees sit for a minute or two. Gently shake out the excess sugar, along with the mites that have slid off the bees, onto something white so that the mites can be easily seen—a small plastic wash pan, a white bucket lid, or similar container works well. Also, adding a little water to the shaked-out sugar will dissolve the sugar and allow the mites to stand out more easily for counting.

Alcohol or Ether Roll: Follow the same directions as in the sugar roll, with the exception of using alcohol or ether instead of sugar, and using a solid lid on your jar. This method kills the bees and the mites. Either of these are by far the most accurate tests, as the mites die and so are fully dislodged for an accurate count. You can recycle the alcohol through a clean rag or quality paper towel for future use.

Varroa Destructor Mite Treatment Threshold Chart for Honey Bees

Varroa Destructor Mite Treatment Threshold Chart for Honey Bees

* The drone pull is not a reliable mite-testing method. If you find 4 or more mites in the fall, you either need to do a more accurate check with a sugar, alcohol, or ether roll, or just treat them, knowing that there are mites in the hive.

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