Honey Bee Disorders: Honey Bee Parasites
- View from across the pond, opinion piece on Varroa by Dr. Delaplane.
- Guest editorial by Dr. Delaplane on the significance of mite non-reproduction.
- Invitational article by Dr. Delaplane on the theory and practice of Varroa IPM.
- Resource posters, courtesy of Dr. Marla Spivak, University of Minnesota
- Sampling for Varroa mites: poster / paper
- Powdered sugar roll for Varroa mites on honey bees
- HopGuard® Varroa Miticide Receives Section 18 Approval for Georgia
Varroa mites are external, obligate parasites of worker and drone honey bees. Varroa mites are visible to the naked eye and look somewhat like a tick (Fig. 1 & 2). 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 (Fig. 3) is a natural parasite of Apis cerana, the Asian honey bee on which it causes only minor damage. These mites were accidentally introduced into the United States in the mid 1980s.
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) cause death to the pupa. The end result of unchecked mite populations is an eroding adult bee population and eventual colony death.
Most infested colonies die within 1 to 2 years if the beekeeper does not take 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 four treatments available for Varroa mite control in the US: coumaphos (trade name Check Mite), fluvalinate (Apistan; Fig. 4), two formulations of thymol (Api-Life VAR and Api-Guard), and fenpyroximate (Hivastan). Always follow manufacturer's instructions when using these products. 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 synthetic miticides 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
overnight sticky sheets: 59-187
Overnight sticky sheets can be executed with little effort. Various beekeeping equipment vendors sell sticky sheets (Fig. 5). Place these into your colony and the next day (18-24 hours) remove them and count the number of mites (Fig. 6). If the mite number exceeds those above it is time 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.
Screened bottom boards are another cultural control against Varroa mite (Fig. 7 & 8). It is commonly thought that the mites fall through the screen onto the ground; however, this has not been experimentally verified. What has been verified is the fact that in colonies with screen bottoms the mite reproduction rate is lower. It should be stressed that cultural controls alone may not rid your colonies of Varroa mites and should be thought of 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. Bees expressing Varroa-sensitive hygienic behavior or auto-grooming are especially promising.
Tracheal mites (Fig. 9) were first detected in the United States in 1984 and have since caused the loss of tens of thousands of colonies and millions of dollars. Trachealmites infest the tracheal system of the adult honey bee (Fig. 10). Levels are highest during the winter and spring. Mites prefer adult bees less than four days old. Once they are on the bee, mites are attracted to carbon dioxide emissions and enter the spiracles located on the thorax which lead to the tracheal system. They puncture the wall of the trachea and suck the hemolymph of the bee. Tracheal mites live, breed and lay eggs in the tracheal system. The adults and eggs plug the tubes of the trachea which impairs oxygen exchange. They also spread secondary diseases and pathogens since they puncture the trachea in order to feed. Individual bees die due to the disruption to respiration, damage to the tracheae, microorganisms entering the hemolymph, and from the loss of hemolymph. Honey production may be reduced when over 30 percent of the population is infested with tracheal mites. Also, the likelihood of winter survival decreases with increasing infestation of the mite. Mites are transmitted from bee to bee within a colony and to other colonies by robbing or drifting bees.
Infested bees will be seen leaving the colony and crawling on the grass just outside the hive. They will crawl up the blades of grass or the hive, fall back down and try again (Fig. 11). The wings may be disjointed and the bees unable to fly. The abdomens may be swollen. In late stages of infestation, bees will abscond from the hive. If you are unsure if you have tracheal mites, send a sample of bees in alcohol to your local county extension agent for verification.
One method for controlling tracheal mites is the use of menthol, available from most bee supply companies. The temperature must be above 60° F in order for the menthol to work. The bees breath in the vapor which, it is believed, desiccates the mites. Menthol must be removed during a nectar flow in order to not contaminate honey.
Another less caustic treatment for tracheal mites is an oil extender patty. It consists of two parts sugar to one part vegetable shortening. Make a small patty about four inches in diameter and sandwich it between wax paper (Fig. 12). Cut the wax paper around the edges so the bees have access to the patty. Center the oil patty on top of the frames within the hive body. The bees will be attracted to the sugar and obtain oil on their bodies. The oil acts as a chemical cloak and the tracheal mites are unable to identify suitable bee hosts. The oil patties are acceptable for prolonged treatment since the oil will not contaminate honey supplies.