What Are Varroa Mites?

— Written By and last updated by Patricia Burch

The varroa mite is an external parasite that is a very serious pest of honeybee colonies. They affect honeybees by attacking the adult bees and the developing bee larvae. These mites have a flat oval shape and are reddish-brown in color. Comparatively, they are about the size of the head of a straight pin.

Mites affect colonies of bees by the adult entering the cell of a developing bee larvae and laying eggs. As the mites develop, they feed on the larvae and can cause deformity or even death depending on the number of mites. Male mites die within the cells, while the female mites repeat the cycle by entering other cells containing developing larvae. If preventive measures are not taken, then varroa mites will eventually kill the colony.

There are several detection methods to determine if you have a varroa mite infestation among your honeybee colonies. These methods are the Sugar Shake, Sticky Board, Alcohol Wash, and Drone Brood Inspection. They are all described below.

Sugar Shake Method
This method estimates the mite prevalence within the colony (the percentage of adult bees with mites).

1. Obtain a clear one-pint jar or other container with a lid made from one-eighth inch hardware cloth or similar mesh material. If you can’t find a jar with a mesh lid, make a mesh lid for your container.

2. Brush or shake approximately two hundred adult bees from a frame with an emerging brood into the jar.

3. Close the mesh lid on the jar, and add two to three tablespoons of powdered sugar through the lid.

4. Set the jar aside for several minutes to allow the bees (and mites) to be covered in sugar.

5. Shake the sugar (and dislodged mites) out of the jar onto a clean, flat surface (preferably white). The bees, although covered in sugar, are not killed and can be returned to the colony.

6. If ten or more mites are found per two hundred bees, take appropriate measures to control the mite population (a magnifying glass may be necessary to see the mites).

Sticky Board Method
This method estimates the total mite load of the colony (the total number of mites in the hive).

1. Purchase a commercial sticky board from a beekeeping supply company. A sticky board has a pre-applied adhesive and sampling grid drawn on the surface. Alternatively, a sticky board can be constructed with a stiff sheet of white paper.

2. Spray the upper surface of the paper (facing the bees) with an aerosol cooking spray, or apply a thin layer of petroleum jelly to the upper surface of the paper to create a homemade sticky board.

3. Place the board or paper between two eight-mesh wire covers (with one cover on the top and one on the bottom) so that the bees do not adhere to the sticky surface.

4. Place the sticky board on the bottom floor of the hive. A portion of the mites will fall off the bees, fall through the mesh screen, and stick to the white board.

5. Remove the board twenty-four hours later, and count the total number of mites on it. If the number of mites is between sixty and one hundred ninety (depending on the size of the colony), then appropriate control measures should be taken.

Alcohol Wash Method
Similar to the sugar shake, this method requires that the beekeeper brush or shake adult bees from a frame into a clear container to measure the prevalence of varroa mites.

1. Pour one to two inches of rubbing alcohol (isopropy1 alcohol) into a clear one pint jar or container with a solid lid.

2. Brush or shake approximately two hundred adult bees from a frame with emerging brood into the container.

3. Vigorously shake the container for at least thirty seconds, and then examine it for dead mites sinking to the bottom. If you see ten or more mites per two hundred bees, then you should treat the colony.

 Drone Brood Inspection
Because of the variation in sampling, this method is not always a reliable indicator of mite levels in a colony. However, it can be used to verify the relative degree of varroa infestation.

1. Find any capped drone brood within the hive, which is typically located on the periphery of the brood nest.

2. Uncap the cells and gently remove the pupae.

3. Closely inspect the drone pupae for adult varroa mites. If ten percent or more of the drones are infested, then you should take appropriate measures to reduce the mite population.

 You should monitor each colony for varroa mite infestation many times during the season to determine if and when it is necessary to apply treatment. There are several control options. Mechanical control can be conducted by using screened bottom boards, drone brood trapping, or purchase mite tolerant bees. Biopesticides are naturally occurring and there are several that can be used to control varroa mites, Thymol, Sucrose octanoate, and Formic acid. Synthetic pesticide treatments include Fluvalinate and Coumaphos. Keep in mind to use several products on a rotational basis to prevent resistance buildup by varroa mites.

 Source: NCSU Publication AG-662

Reminder:  If you would like to learn more about Horticultural related topics, then join the “Sampson County Friends of Horticulture”. This program offers monthly “How To” Horticultural Seminars. Please call (910) 592-7161 for more information. Please call the Sampson County Cooperative Extension Center at (910) 592-7161 with your horticultural questions and to register for any upcoming events. Be sure to check out the Ask An Expert Widget at sampson.ces.ncsu.edu for any questions you may have.