Varroa mites are external, obligate parasites of worker and drone honey bees. Varroa mites are visible with the naked eye and look somewhat like a tick. They feed on the hemolymph of adult bees and the developing brood. The reproduction cycle of the mite takes place inside the cells. Female mites (foundresses) enter the brood cells of last stage worker or drone larvae just prior to the cells being capped. There she will deposit five to six eggs over a period of time while feeding on the brood. The first egg laid will be unfertilized and develop into a male. The subsequent eggs will be fertilized and develop into females. The eggs hatch and the young mites begin to feed on the developing pupa. It is normal for mating to occur between siblings. The adult female mites along with the original female mite(s) leave the cell when the bee emerges. The female mites will enter another cell or attach themselves to an adult bee to feed. Varroa mites are transported from colony to colony by drifting or robbing bees.

Varroa destructor is a common mite found on Apis cerana, the Asian honey bee on which it  does not cause serious damage like it does on Apis mellifera. These mites were accidentally introduced into the United States in the mid 1980s. Before this time, honey bees were found coast to coast across the United States. Now only an estimated 2% of the feral honey bee population remains, and even this derives annually from honey bee swarms from beekeeping operations. Practically speaking, the wild honey bees have become extinct in the United States due to infestation of the Varroa mite.

Visible symptoms of Varroa mite damage can be evident on newly-emerged bees which is due to the mite feeding on the immatures within the cell. The newly-emerged bees may be smaller than normal, have crumpled or disjointed wings, and shortened abdomens. The lifespan of the newly emerged bee is also reduced. Severe infestations of Varroa mites within the cell (5 or more foundresses) can cause death to the pupa. Other symptoms of mite infestation are rapid colony decline, reduced adult bee population, evacuation of the hive by crawling bees, queen supersedure, spotty brood, and abnormal brood with symptoms resembling European foulbrood and sacbrood disease.

Infested colonies will die within 1 to 2 years if the beekeeper does not take necessary actions against Varroa mites. If upon initial examination of your colony you do not see visible mites, use a capping scratcher on drone brood to see if Varroa are inside cells. Varroa mites prefer drone brood over worker or queen. If mites are detected you may need to treat in order to save your colony. At this time there are only two chemical treatments available for Varroa mite control in the US: Check Mite+ strips (active ingredient coumaphos) and Apistan (active ingredient fluvalinate). Always follow manufacturer's instructions when using Apistan or Check Mite+ strips. Also, never treat during a nectar flow because the chemicals can contaminate the honey, and never leave strips in hives after the recommended time because this encourages resistance. In recent years mite resistance to Apistan strips has become a problem throughout the world. Therefore, rotating chemicals, delaying treatment and using cultural controls are recommended to manage mites in a more sustainable manner.

Delaying treatment can be accomplished if you monitor the level of Varroa infestation in your colonies. Treatment is justified only when the economic threshold is achieved. Economic thresholds are defined as the pest level that justifies treatment in order to prevent the pest from reaching damaging levels. For the southeast Piedmont region, the economic threshold has been determined to be:

mite populations: 3172-4261
ether roll levels: 15-38
overnight sticky sheets: 59-187

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Ether rolls are easy to do and require little work. Take a quart jar and fill with about 300 bees (1.5 inches of bees) (Fig. 1). Then spray some ether engine starter fluid into the quart jar and replace the lid (Fig. 2). Shake the jar for 30 seconds. The mites will dislodge from the bees and stick to the sides of the jar. Count the number of mites and compare them to the numbers above. If the numbers are at or above the economic threshold, you should treat your colony. If the numbers are below then you can wait. Overnight sticky sheets can also be executed with little effort. Various beekeeping equipment vendors sell sticky sheets (Fig. 3). Place these into your colony and the next day (18-24 hours) remove them and count the number of mites (Fig. 4). If the mite number exceeds those above it is ime to treat.

Cultural methods for Varroa mite control include using drone comb or bottom screens to trap Varroa mites. Varroa mites prefer drone brood. Using a few frames of drone comb per colony draws mites into the cells which are then capped by the worker bees, trapping the mites within. The frames are removed and put into the freezer 24-48 hours to kill the mites.

Bottom screens are also an effective control method. They are basically a wood-bound screen that is placed underneath the brood chamber. The mites fall through the screen onto the bottom board or ground. The mites are thus separated from the bees and eventually die. It should be stressed that these two treatments alone will not rid your colonies of all Varroa mites and should be used as a means to delay the economic threshold and the need for a chemical application. Hopefully in the future, genetic bee stocks resistant to Varroa mites will become more available to beekeepers.

Honey Bee Disorders / UGA Honey Bee Program / UGA Entomology Department