Asian honey bees are natural hosts to several species of parasitic mites. Movement of European honey bees
around the world has placed them into contact with exotic parasites and pathogens for which they have little
or no natural resistance or tolerance, and has helped to spread new pests around the globe. Mites attack and
feed on honey bees, physically weakening them and impairing their immune systems, while vectoring viruses
and other pathogens. The result is a mite-virus complex that can be more dangerous to bee health than either
mites or viruses alone. Because there are currently no practical treatments for honey bee viruses, managing
for low mite populations is the best strategy to limit the transmission of of these viruses.
The varroa mite (Varroa destructor) is considered the greatest threat to honey
bees and beekeeping in most
the world, and are widespread across the U.S. While only about 1/16” wide, these parasites are very large in
proportion to the body size of their host, and can have a severe impact on honey bee health
Photo by Gilles San Martin (flickr.com/photos/sanmartin).
Varroa mites spend much of their lives hidden in cells, and even if they are not apparent, beekeepers should
assume mites are present. These external parasites feed on fluids and protein reserves in adult honey bees
and pupae, and reproduce exclusively within sealed brood cells. Mites severely weaken the developing pupae,
and their feeding introduces viruses.
Varroa mite on honey bee pupa.
Photo by Gilles San Martin (flickr.com/photos/sanmartin).
Varroa mite on adult honey bee.
Photo by Stephen Ausmus, USDA-ARS.
Life Cycle of Varroa
The life cycle of the Varroa mite has two phases. During the phoretic phase, mated female mites
attach themselves to adult honey bees (1), and are carried through the hive. They are often
found on nurse bees that remain in the brood nest, tending and feeding larvae (2).Phoretic mites
must enter a honey bee brood cell to begin their reproductive phase. A
mature female mite, called
a foundress, enters a brood cell just prior its being capped (3). Once sealed inside, The mite
opens a feeding wound on the bee pupa, often infecting it with viruses she carries (4). The
foundress will soon begin laying eggs (5) of which the first is always male, followed by several
female eggs over several days. Offspring hatch and mature, feeding on the same pupa, removing
nutrients from the developing bee and picking up viruses that they can later vector to other
bees (6). Mature sibling mites mate within the brood cell (7). When the adult bee emerges (8),
the foundress is released along with her mature female offspring. Male mites and immature
females remain in the cell to die (9) and will be removed by housecleaning bees. The male mite’s
entire life is spent in the capped cell. Mites that emerge will quickly seek a host bee (10),
and may change hosts multiple times. Phoretic mites prefer middle-age nurse bees that tend
late-stage brood, increasing opportunities to invade larval cells just prior to capping.
Varroa mites prefer drone brood to worker brood. Worker bees develop in 12 days, while drone cells
remain capped for 15 days. This extra time allows mites higher reproductive potential when infesting
drone cells. Varroa infesting worker brood have an average of 1.5 mature female offspring, while mites
on drone brood average 2.5 viable daughters. Therefore, limiting excess drone comb within a hive can
limit the population growth of varroa mites.
When brood is available, varroa typically spend 4-5 days in the phoretic stage before seeking a suitable
cell for reproduction. Mites spend the majority of their lives in the reproductive phase, sealed
protectively inside brood cells. However, most treatment options affect only the phoretic mites.
Understanding the life cycle of the varroa mites is key for beekeepers to effectively treat hives and
manage varroa infestations.
So-called hard chemicals were among the first successful treatments that U.S.
beekeepers found when
varroa first appeared in the late 1980s. Synthetic miticides such as pyrethroids and organophosphates
act on the central nervous system of the mites. They were formulated for hive use by impregnating
plastic strips with pesticides. Strips were placed between brood combs, and phoretic mites were killed
as bees contacted the material.
These strips were convenient to apply and reasonably economical for beekeepers to use. However, over-use
quickly led to resistant mites, which required more frequent or stronger treatments. Many miticides are
also easily absorbed by beeswax with each repeated use. Not readily water-soluble, miticides will not
easily leach into honey, but they do accumulate in the wax over time. Because bees and mites develop in
beeswax cells, they are both potentially exposed to increasing levels of chemical contamination in older
Bee health can be affected by this exposure, potentially impairing immune system response, shortening the
lifespans of workers, and reducing fertility in queens and drones. Due to over-dependence on a narrow
range of chemistry, populations of varroa are largely immune to some chemical treatments. Particularly,
tau-fluvalinate (Apistan®) and coumaphos (Checkmite®) may no longer be effective treatments in some
places. Products containing amitraz (Apivar®) remain effective at this time, but over-reliance on a
single product will potentially render it ineffective as well. To prevent or delay resistance,
beekeepers should rotate treatments each time they are required. By alternating products with different
modes of action, pests are less able to rapidly develop tolerance than when a single product is used
When using any pesticide product, remember that the label is the law.
Read and follow all product
directions, including the instructions for removing the product from the hive after a prescribed period
of time. This period is often 42 days, or two brood cycles, which insures that mites sealed in brood
cells will be exposed to a treatment product at least once during their brief phoretic phase. Following
recommended timing insures the product is removed from the hive before it dissipates, exposing the mites
to a weaker dose, which they are more likely to survive and pass on resistance traits to their
Due to problems associated with miticide use, many beekeepers are now choosing to treat colonies with
soft chemicals, which include organic acids and essential plant oils. When
used correctly these
naturally-derived compounds can be effective against mites, with limited impact on bees. Many commercial
products are available in convenient prepackaged doses. Be advised that many “natural” compounds can
still be dangerous, or even deadly, to bees or to beekeepers when used improperly or at the wrong
Read and follow all product labels, and protect yourself with appropriate personal safety equipment!
Some concentrated organic acids can effectively kill varroa, without significantly impacting bees or
affecting the quality of honey. However, if not used properly, these can cause serious effects on bee
health, including queen or brood mortality, or complete colony death
Formic acid is highly effective at killing varroa mites. Available as a
prepackaged gel formulation (Formic Pro®,Mite
Away Quick Strip®), it is placed directly on top of the brood frames, and must volatilize in the hive.
It is temperature dependent, and should be applied when the outside daytime temperature will remain
between 50-92°F (10-33 C) for at least 5 days. If too cool, the product will not evaporate effectively,
and if used when too warm, it will evaporate too rapidly and cause significant brood or queen mortality.
Hives should not be opened for at least 72 hours after application. This product must be handled with
acid-resistant gloves (not leather bee gloves) and applicator should
appropriate respirator. See
product package for specific details. Formic acid is a natural component of honey, and is approved for
use in certified organic production. The vapors are also capable of penetrating cell cappings, and is
the only treatment known to kill varroa in the sealed brood.
Oxalic acid is a naturally occurring compound in many plants that can be used
to effectively and
inexpensively treat for varroa mites. This compound affects only phoretic mites, and should not be
applied when honey supers are on hives. Oxalic acid treatments can be applied to hives in two ways
Trickle method: Dissolve 35 grams of oxalic acid crystals in 1 liter
of warm 1:1 sugar syrup to make
a 3.5% solution. Measure accurately, as a weak solution may not be effective; and a solution that is
too strong can damage bees. Using a syringe, trickle or drench 5 ml (1 tsp) directly onto adult bees
in each occupied space between brood combs. Do not use in honey supers. This method works best when
bees are clustered in cool weather, and no brood is present. Avoid application to the same bees more
than once per year. This method may not be appropriate for use in warm climates, where broodless
periods are short. Oxalic acid becomes unstable in sugar solution, so unused material should be
Vaporization method: When heated, oxalic acid sublimates, going
directly from solid to vapor.
Numerous applicator devices are available to quickly treat bee hives. Prepare hives by removing any
honey supers and sealing screen floors and other cracks in the hives. All burr comb should be
removed from solid bottom boards to prevent fires when using an in-hive heater on the floor. Place 2
grams (1/2 teaspoon) oxalic acid crystals in the vaporizer device, insert the applicator into the
flight entrance of the hive, cover the entrance with a towel and turn on the device. Follow the
directions for your specific applicator. Honey supers can be replaced on the hive 20 minutes after
application. Treatment is most effective when broodless, but application can be repeated after 2
weeks if brood is present.
Hops Beta Acids are derived from the hops plant (HopGuard 3®). Treated
cardboard strips are placed over
frames, and phoretic mites are killed as bees contact the material. Once strips have dried, bees will
chew up and begin to remove them. Treatment should be repeated after two weeks. Product is made from
material, and can be used when honey supers are present, but works best when no brood is present for an
extended period. Follow manufacturer’s label directions.
A number of plant essential oils have acaricidal properties. Concentrated thymol , isolated from the thyme
plant, is one of the most effective. It may be formulated with menthol, camphor, eucalyptus, wintergreen
oil, or other ingredients in commercial products. These products must volatilize in the hive, and their
effectiveness is temperature dependent. Each product formulation has its own specific recommended
temperature range, which is typically between 65-85°F (18-30 C). Consult product label for specific
In general, these products should be placed into hives for an initial treatment period, and then replaced
approximately two weeks later with a second dose, to ensure that mites from multiple brood cycles are
exposed. Products may be available in pre-measured doses or in bulk quantities. Some commercially
available essential oil products include Apivar®, Thymovar®, and ApilifeVar®.
Read and follow the specific manufacturer’s directions or each product package for best results. Note
that some essential oils can be toxic to honey bees, and experimentation with non-commercial mixtures is
done at the beekeeper’s own risk to the hive. Volatile essential oils should never be applied to a
colony while honey supers are in place because the quality and value of the honey can be severely
Genetic Mite Resistance
European honey bees have not had a sufficiently long association with varroa mites to develop a stable
host-parasite relationship to better withstand this novel pest on their own. Some genetic lines of bees
have begun to show resistance to varroa, and queen producers have had some success with breeding these
traits into commercially available bee stock. Russian honey bees reduce
populations by vigorously
grooming mites from themselves and nest mates. Some bees may exhibit varroa sensitive hygienic
behavior by detecting reproducing mites in capped brood cells, and then removing both pupae and mites
and disrupting mite reproduction.
Other lines of bees may also demonstrate various mechanisms for mite resistance, but honey bee queens and
drones mate in the air at random, far from their hives. This behavior can make maintaining specific
genetic traits difficult in areas crowded with unselected bee stock. Precise control of genetics is only
possible through instrumental insemination, which is too costly and labor intensive to be practical for
most hobbyists. When a colony swarms or supersedes, the genetic composition of the hive changes, and
desirable genetic traits can be potentially lost or minimized. However, if bee swarms establish as feral
colonies, the progeny of strong survivor stock may eventually repopulate wild areas with bees that carry
beneficial combinations of genetic traits, and begin to reverse the severe losses caused by varroa
mites. These survivors can also serve as a healthy source of drones for managed queens.
While these genetic stocks can reduce the frequency of mite treatments, there are not yet any lines of
bees available that demonstrate 100% resistance to varroa mites. Beekeepers should monitor their
colonies during the season and remain aware of the mite population levels.
Drone Brood Trapping
Varroa mites appear to prefer reproducing on drone brood over worker brood. Beekeepers can use this
behavior to trap and remove mites from the hive without chemical treatments by placing drone-sized
foundation into a hive. Drone foundation is available from commercial suppliers. The cell pattern on
this foundation is slightly larger than standard worker-sized foundation, and the colony will draw comb
with larger diameter cells into which the queen will place only unfertilized drone eggs. Beekeepers can
also purchase a single-piece plastic frame and foundation combination, which is often colored green to
make it easily identifiable.
When a majority of these drone cells are capped, but before adult drones begin to emerge, the entire comb
should be removed from the hive and frozen for 3-5 days This kills both the drone pupae and the mites
hiding in their cells.
After the frame has thawed, it can be placed back in the hive. Worker bees will uncap and remove the dead
pupae and mites, and prepare the cells for the queen to deposit eggs again. Some beekeepers will use two
such frames, which can be swapped out as needed.
In the photo below, there are 860 sealed cells on this side of the comb. If we assume that the other side
contains about the same amount of sealed brood, and a foundress mite can produce an average of 2.5
viable daughters per cell, then this frame could potentially hold over 6,000 mites that can be removed
without chemicals. Even if only half of these cells contain reproducing varroa, there is still potential
to easily eliminate a large number of mites. However, if beekeepers fail to remove these combs as
intended, the drones will emerge and release their mites, increasing the mite population far more than
if drone combs had not been employed. A good rule of thumb is to swap and freeze drone frames every 21
days during the brood rearing season
Trapping and killing varroa mites in a drone comb frame such as this one can help to reduce the
mite population without the use of chemical treatments in the hive. However, if drones are not
removed on a regular schedule, the presence of extra drone brood will likely increase the varroa
Sampling for Varroa Mites
To maintain colony health, varroa populations should be kept below a 3% infestation rate ,
or fewer than 3
mites per 100 bees. Sampling for mites is not difficult, and numerous methods have been developed for
beekeepers to use. For an accurate estimate, count the number of mites in a sample of at least 300 bees
(about 1/2 cup). Adult bees should be collected from combs containing open brood, as these nurse bees
are most likely to carry phoretic mites. When sampling bees, be sure to avoid the queen!
Bees can be brushed or shaken into a tub, and then a measuring cup can be used to scoop out the
appropriate number of bees for a sample. A jar or other container can also be marked to show 1/2 cup,
and bees can be sampled directly into it. Hold a comb covered in bees vertically above the hive, and
gently move the jar down, barely touching the backs of bees. Many will flip over into the container as
it brushes past them. Tap the bottom of the jar to knock the bees down and estimate if additional bees
will be needed.
Once mites are counted, divide the number of mites by the number of bees in the sample, then multiply by
100 to determine the infestation level. Varroa estimates are often expressed n terms of mites per 100
9 mites ÷ 300 bees × 100% = 3% infestation or 3 mites per 100 bees
Honey bees can be sampled directly into a jar to determine the mite infestation level. About
300 bees are needed for an accurate sample. Video by Sheri Burns (honeybeesonline.com)
Washing bees with alcohol is the most accurate method to determine varroa infestation level. Shake bees
in alcohol for a minute or more to dislodge mites, then pour the liquid through a mesh screen to
separate mites from bees. Liquid is then poured through a fine sieve or white cloth, or into a white tub
to make mites visible. For a precise count, bees can be washed again with water until no additional
mites are dislodged, but this may be unnecessary if the threshold is clearly exceeded. Commercially
available tools make this technique quick and easy to conduct. Effective tools can also be made from
simple materials on hand
Because it is among the most reliable ways to determine varroa infestation level, washing mites from a
sample of bees with alcohol is a standard method in scientific research. However, the method is fatal
for the honey bees sampled, and many hobbyists don’t enjoy the idea of killing hundreds of their honey
bees. For this reason, several other sampling methods, which do not harm the bees, are available for
beekeepers to use.
In reality, individual honey bees are-short lived, and healthy colonies are quite populous. If a colony
is so weak as to be significantly endangered by the loss of 300 workers, the future of that colony may
be at risk anyway.
Photo courtesy Randy Oliver (scientificbeekeeping.com)
Tools for sampling varroa mites by alcohol wash include commercially
available devices, such as the Varroa EasyCheck® (left), Varroa Sampling Gizmo® (middle) and a homemade device (right).
A wide-mouth canning jar can be modified by replacing the lid with a circle of 1/8” mesh. Mark the jar at
the 1/2 cup level with a permanent marker. Once sufficient bees have been added to the jar, screw on the
lid, and add ~2 tablespoons of powdered sugar through the screen lid. Gently roll the jar for about 1
minute to thoroughly coat every bee with sugar, then invert the jar and thoroughly shake out all sugar
into another container (usually 1-2 minutes). Varroa are unable to hold onto bees when coated with dust,
and the sugar will induce vigorous grooming from the bees, further dislodging mites. Dark colored mites
are clearly visible in the white sugar. While not quite as reliable as alcohol wash, sugar generally
recovers 80-90% of mites. Extremely humid weather can cause the sugar to clump, making the test less
accurate. Beekeepers should be aware of these considerations when making their assessments. Bees survive
this treatment (although they will not be pleased) and can be returned to the hive.
Carbon dioxide is commonly used to anesthetize queen bees for instrumental insemination. It can
used to quickly knock out a sample of bees and phoretic mites, which can be shaken through a
This treatment should not harm bees, but they often expel their stomach contents, making the bees
container sticky, so some mites may remain and not be counted. Therefore, when making colony
beekeepers should consider that this method may dislodge only about 60-70% of the mites
Photo courtesy Daniel Ruck(www.Bienen-Ruck.de)
Some phoretic varroa mites fall from bees on a daily basis. A cardboard or plastic sheet, coated with a
sticky or greasy substance, can be placed beneath a hive’s screen floor to capture and count the number
of mites falling, and indicate their relative population
Sticky boards cannot estimate an accurate level of mite infestation, since the number of bees cannot be
accurately counted with this method. But it can be used to track changes in mite population growth in
the same hive over time, so that a beekeeper can be aware if the mite level is increasing from month to
month. Sticky boards can also be used to evaluate the immediate knock-down effect of a particular mite
Sheets of light colored corrugated plastic board work well, and a printed grid makes counting mites
easier. Leave the board in place for 3 days for an accurate sample, remove and count all visible mites,
then divide by the number of days sampled to determine the average mite-fall per day. In the spring, the
threshold for varroa should be much lower than in the fall, but the specific number is highly variable
with the honey bee population. In early spring, fewer than 3 mites per day may be acceptable. In the
late summer, finding more than 30 mites per day will likely prompt treatment.
Screen Bottom Boards
Using 1/8” screen for the floor of a bee hive can help to passively eliminate some mites continually
throughout the season. As phoretic mites are dislodged, they fall through the screen and are unable to
climb back into the brood nest. This may be particularly effective with genetic lines of bees that
aggressively groom mites from themselves and their nest mates. While this modification will not
eliminate all varroa, it can reduce mite reproduction and may delay their build-up as part of an overall
IPM strategy. However, in areas with high populations of small hive beetles, a screen floor may allow
adult beetles to enter unchecked, and should be used only with an oil tray in place.
Screen bottom board.
The tracheal mite (Acarapis woodi) is an internal parasite of honey bees. They infest and breed in the
tracheal tubes (breathing passages) within the bees’ bodies. These mites feed directly through the
tracheal walls on a host’s hemolymph (blood), causing damage, potentially vectoring diseases, and
impairing the host’s breathing. Young mites must disperse to find new hosts younger than 3 days old to
infest. The short-lived bees of spring and summer can only host a single generation of mites, but
long-lived overwintering bees can host multiple generations within each bee. For this reason, tracheal
mites are generally associated with winter losses, especially when colonies are under additional stress.
Common symptoms of tracheal mite infestation are nonspecific, but include K-wing, and generally weakened
or dwindling hives. A microscopic examination of the tracheal tubes is needed for precise diagnosis.
Some lines of bees have been bred with good genetic resistance this pest. Formic acid treatment, menthol
(Mite-A-Thol®) or any of the essential oil products used against varroa should also help to control
tracheal mites, since bees breathe in these vapors for an extended period. Oxalic acid, however, does
not remain in the vapor state long enough to affect tracheal mites
Photo by Ron Ochoa & Gary Bauchan, USDA-ARS.
Two species of Tropilaelaps mites found on Asian honey bees can also infest European honey bees. These
mites will feed and reproduce on both worker and drone pupae, can vector pathogens, and cause damage to
colonies similar to that of varroa mites. While their life cycle is similar to varroa,Tropilaelaps have
a much faster reproductive rate, and can quickly overwhelm colonies during the brood-rearing season.
They may be observed running rapidly across the comb, rather than remaining phoretic on adult bees.
These tropical mites are unable to feed on adult bees, and appear able to survive for only three days
with no brood in the laboratory. Therefore they may be unable to establish in much of the United States
where winter conditions create an extended broodless period. However, if these mites are able to adapt
and feed on alternate hosts, the extent of their potential range cannot be known.
Comparison of size between a varroa mite (left) and a Tropilaelaps mite (right).
Photo courtesy UK Food and Environment Research Agency (FERA), Crown Copyright.
These mites are not currently found in North America, but with increasingly rapid global trade,
Tropilaelaps be accidentally introduced here, as have many other harmful species. Beekeepers should
be aware of this potential pest, and immediately report its suspected presence to their state
entomologists or apiary inspectors.