Parasitic Mites

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.

Varroa Management Options

For the most current information on pest and disease treatment recommendations for bee colonies, consult Insecticide Recommendations for Arkansas - MP144.

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 combs.

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 repeatedly.

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 offspring.

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 dose.

Read and follow all product labels, and protect yourself with appropriate personal safety equipment!

Organic Acids

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 wear an 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

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 food grade 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.

Essential Oils

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 instructions.

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 Apiguard®, 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 affected.

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 varroa populations by vigorously grooming mites from themselves and nest mates. Some bees may exhibit varroa sensitive hygienic (VSH) 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 population

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 bees.

example:

9 mites ÷ 300 bees × 100% = 3% infestation or 3 mites per 100 bees

Alcohol Wash

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 Véto-Pharma (vetopharma.com)

Photo courtesy Kelley Beekeeping (kelleybees.com )

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).

Sugar Shake

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.

Photos courtesy Brushy Mountain Bee Farm (brushymountainbeefarm.com)

CO2 Sampling

Carbon dioxide is commonly used to anesthetize queen bees for instrumental insemination. It can also be used to quickly knock out a sample of bees and phoretic mites, which can be shaken through a screen to be counted.

This treatment should not harm bees, but they often expel their stomach contents, making the bees and container sticky, so some mites may remain and not be counted. Therefore, when making colony assessments beekeepers should consider that this method may dislodge only about 60-70% of the mites

Photo courtesy Daniel Ruck(www.Bienen-Ruck.de)

Sticky Boards

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 treatment.

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.

Sticky board

Mite Square

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.