Background
The Varroa mite, Varroa jacobsonii, is an external parasite
of honey bees. 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 honey bee,
Apis cerana. It was first discovered on the Western honey bee, 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.
![[varroa development cycle]](../images/varroa1.gif)
Life History
The numbers of mites found in a colony of honey bees varies with the season.
The fewest mites are found in the spring, increasing over the summer to a high
point in the fall, 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 honey bee, 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 prepupa 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' pupal or postcapping 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 itself 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. Inspection is best done by uncapping and removing some of the drone
pupae. 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 cell cappings so as to impale a number of brood at one
time. Lift the cappings 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 honey bees. Older pupae are best to examine since they do not break
apart as easily as younger immatures.
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.
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 capacity 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, 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. The inserts comprise
two parts - a sheet of sticky paper or cardboard which covers the entire bottom
board, and a screen which 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, ApistanTM strips, for instance, will
fall to the bottom of the hive, pass through the screen, and be caught on the
paper. As of late 1992, ApistanTM (fluvalinate) is our only approved medication
for killing Varroa mites. A second material, MiticurTM (amitraz) may become
available in 1993. Apistan comes in the form of impregnated plastic strips that
are placed in the hive for a prescribed period. It is a contact miticide and
does not harm the bees when used properly. Directions for use are carried on the
label. Use only in accordance with these directions. Bottom board inserts can
also be used without medicated Strips. When so used they will serve to retain
any mites which die naturally in the hive and fall from the bees. They are most
effective towards the end of winter, before the start of active season. 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.
Tobacco Smoke
Tobacco smoke may be used in conjunction with inserts and screens. About 2-3
grams of tobacco should be ignited in the smoker (without other fuel present)
and blown into the hive in the evening after the bees have stopped flying. The
entrances should then be blocked with newspaper or other material to contain the
bees and the smoke. The hive may be opened in the morning and the insert
examined. Treatment Although the label directions allow, it is not advisable to
use ApistanTM as a routine prevention treatment against Varroa. Reserve the use
of Varroa for treatment of an actual Varroa infestation. This reduces the possibility of mites establishing a tolerance for ApistanTM. After an infestation is
detected, use ApistanTM strictly in accordance with the label directions.
Dangers Other treatment materials have been discussed in the beekeeping
literature, as have other methods of treatment. lt is important to remember
that unauthorized, untested, or otherwise unproven treatment can be detrimental
to bees and often to humans. Further misuse of approved or experimental
materials can lead to the development of resistance to treatment in the mites.
With only one or two chemicals available for treatment, we can not afford to
lose any because of beekeeper carelessness or ineptitude. Always use only
approved materials and in accordance with the label instructions.
References
Following are listed several sources for further information on diagnosis
and treatment:
1. Cobey, Susan and Lawrence, Timothy, 1988, "Varroa Mite: Potential Methods of
Control," American Bee Journal, 128(2); 112-117.
2. Dietz, Alfred and Hermann, Henry R., 1988, Biology, Detection and Control ~
Varroa jacobsonii: A Parasitic Mite of Honey Bees, Lei-Act Publishers, 82 p.
3. Needham, Glen R., 1988, "Status Report on Varroa jacobsonii," American Bee
Journal, 128(2): 10~110.
4. Shimanuki, Hachiro and Knox, David A., 1991, Diagnosis of Honey Bee D*eases,
U.S. Department of Agriculture, Agriculture Handbook No. AH-690, 53p.
5. Witherell, Peter C. and Bruce, William A., 1990, "Varroa Mite Detection in
Beehives: Evaluation of Methods....," American Bee Journal, 130(2); 127-129.
Richard E. Bonney Department of Entomology, Fernald Hall University of
Massachusetts Amherst, MA 01003 (413) 545-2283
Issued in furtherance of Cooperative Extension work, Acts of May 8 and June
30, 1914, in cooperation with the United States Department of Agriculture.
Robert G. Helgesen, Dean and Director, University of Massachusetts Cooperative
Extension System. The Cooperative Extension System offers equal opportunity in
programs and employment. AG951:12/92-2M
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