The Varroa Mite

   Background   

The Varroa mite, Varroa Jacobsoni, is an external parasite of honeybees. It feeds on the hemolymph of both brood and adult bees. The entire life cycle of these mites is spent with the bees. The Varroa Mite originated in Southeast Asia where it is a parasite of the Eastern honeybee, Apis cerana. It was first discovered on the Western honeybee, Apis mellifera, in 1960. The crossover resulted from beekeepers intermingling the two species, and further spread has been encouraged by beekeepers transporting colonies. This mite is now found on every continent except Australia. No one is sure how it came into the United States but it is most likely that they arrived with queen bees, which were brought in illegally. By 1992 Varroa mites have been found in at least 40 of the United States and continue to spread. They were first found in Wisconsin in late 1987 and shortly thereafter were reported in Florida. They are assumed to have been in Florida first and moved to Wisconsin with migratory bees. A further assumption is that they were in the U.S. for at least two years before discovery. Varroa mites were first found in Massachusetts in 1988, and by 1992 had spread through the eastern half of the state and were slowly moving west, although the heaviest infestation continues to be in the cranberry growing areas. We can reasonably expect these mites to spread to all parts of the state within two to three years.


Figure 1- Development Cycle    Life History   

The numbers of mites found in a colony of honeybees varies with the season. The fewest mites are found in the spring, increasing over the summer to a high point in the fall, and then falling off over winter and into the spring. During the spring and summer the mites are found mostly on brood. In fall and winter they are mostly on adult bees. Within this cycle, only adult female mites are found outside the brood cells in the hive. The mite's life cycle begins when an adult female leaves a host bee and enters a brood cell as that host is feeding a young larva. Though the mite is found on all three castes of the honeybee, its preference is for drone brood; the mites enter drone cells in greater numbers than they enter worker cells. Once the mite is in the cell she hides in the brood food. As many as 21 mites have been found hiding in a single cell, though smaller numbers are more common. The mite is liberated from the brood food as the food is eaten by the bee larva. Soon after the cell is capped the mite moves to the pupa and begins feeding. Sixty hours after the cell is sealed the mite lays her first egg, with succeeding eggs at approximately 30-hour intervals. The first egg results in a female, the second a male, and the third and beyond, females. New young mites reach maturity within the cell and mating takes place. The male mite then dies, along with any immature females, and the newly mated females leaves the cell with the emerging bee, moving to a new host bee to continue the cycle.

For feeding, the mites move to secluded locations on the bees' body where they are difficult to detect visually. During the feeding stage mites may transfer to other adult bees as the bees brush against each other in the hive. The feeding stage may last for several days. The total development time for an individual mite from newly laid egg through maturity and mating is 10.5 days. Most of the races of Apis mellifera are good hosts for the Varroa mite because of the length of the bees' pupa or post capping development time. This is typically about 12 days. In this time at least one mite can come to term in each cell. However, some races of A. mellifera (e.g., the African bee, A.m. scutellata and the Cape bee, A. M. capensis) have a shorter development time and fewer mites can come to maturity.

   Colony Damage And Mortality   

Individual bees infested with Varroa are harmed in two ways - first, by loss of hemolymph, which in it self is serious, and second, by the puncture wound, which allows entry of infections and disease. Even in low infestations, bees suffer weight loss and shortened life. If the per bee infestation is less than 6 mites, the bee usually reaches maturity. Developing mites, therefore, also reach maturity. However, the adult bees are weakened and their lives are shortened as much as 50%. Bees infested with a single mite during the brood stage can lose 6% to 7% of adult weight. Bees infested with 6-8 mites during the brood stage can lose 25% of adult weight. However, many of these pupae never mature to emerge. Other damage includes asymmetrical wings, misshapen legs, and shortened abdomens. Drones have a reduced number of spermatozoa, reduced weight, and lesser frequency of flight activity. The sum effect on a colony infested with Varroa is that it is seriously harmed, and if no control measures are taken the colony will die. Without beekeeper intervention, the probability of mortality is 10-15% the first year, 20% to 30% the second year, and perhaps 100% in the third year. At most, an untreated colony is unlikely to live more than five years after infestation.

   Detecting Mites   

Finding mites by inspecting individual live bees is usually not successful. The mites are very difficult to see on the bees, even with several mites present. They blend in with the bee, and when not actively feeding they hide; some method of removing the mites from the bees is necessary. Even then, mites may be difficult to find in a light infestation.

    Inspecting Brood   

Mites are easier to detect on brood, especially when dealing with drone brood. Uncapping and removing some of the drone pupae best do inspection. Very young pupae are difficult to remove; they break up easily. Older pupae may be removed from their cells identified by their eyes, which have color. The younger pupae have little or no eye pigment. Once the cells of the older pupae have been located, slide the points of the capping scratcher horizontally under the cells capping so as to impale a number of brood at one time. Lift the capping and the brood from the cells to inspect. If mites are present they should be obvious on the white or light colored bodies of the pupae.

A capping scratcher, or uncapping fork, can be used to uncap cells and remove immature honeybees. Older pupae are best to examine since they do not break apart as easily as younger immature.

Examining a few cells is not enough. A casual inspection should include at least 200 cells (about 13 sq. in.), but at the height of colony development, about 450 cells (about 28 sq. in.) need to be examined to identify a mite infestation of 1% at an accuracy of 99%.

A method to help detection is to place a frame of drone foundation in the brood area. This will serve to concentrate a large number of drone brood in one place. Drone foundation is available from some bee supply sources. Otherwise, you can place some damaged comb in the brood area, which the bees will probably rebuild as drone comb.


Figure 2 - Uncapping Drone Cells    Inspecting Adult Bees   

Following are two methods for testing adult bees for mites. These methods are most effective in the late season when mites are more concentrated on the adult bees, although they do not give good results with light infestations. *The Ether Roll is relatively simple and uses a mini- mum of equipment. Required are a glass jar of about 12 to 16 once with a tightly fitting lid, and a pressure can of ether (such as automobile starter fluid). Place at least 500 live bees in the jar. Crack the lid and give them a one second shot of ether. Seal, tilt the jar on its side, and roll the contents. Moisture will form on the inside of the jar and after twenty to thirty seconds, if mites are present, they will appear adhering to the film of moisture as the jar is rolled. Do this test quickly, and then dump the bees on a clean, exposed surface. They may recover. *The shaking method is similar to the ether roll. Place a quantity of bees in a jar of alcohol, detergent, diesel fuel, gasoline or even hot water. Shake vigorously for about one minute. If mites are present on the bees, they will fall off. Sieve out the bees and then strain the liquid and look for mites.

    Using Inserts   

Bottom board inserts are commonly used in detecting mite infestations. These can be purchased, though many beekeepers make their own board. The inserts comprise two parts - a sheet of sticky paper or cardboard which covers the entire bottom board, and a screen that covers the sticky paper and serves to keep bees and larger hive debris off. The screen is raised slightly above the paper and is of a size that prevents the bees from passing through it. Mites, which are killed or stunned by appropriate hive treatment, Oxalic Acid aerosol, for instance, will fall to the bottom of the hive, pass through the screen, and be caught on the paper. The screen should be of fine enough mesh to pass mites while retaining the bulk of other debris, but at the same time the debris should not be allowed to accumulate and prevent the mites from falling through.