Pest description and damage These mites are external parasites of honey bees, feeding on the lipid reserves of adult bees, pupae and larvae. Mites are brown to reddish brown: females are the size of a pinhead, males are smaller (but are never seen alive outside of the brood cell). Parasitism results in reduced longevity or mortality of individual bees and heavy parasitism can lead to death of the colony. Varroa mites are also implicated as significant vectors of honey bee viruses.
Varroa mite levels can be determined by placing a half cup of bees in a jar and adding 2 tablespoons of powdered sugar. Shaking the jar dislodges the mites which can be emptied onto a surface and counted. Similarly, an "alcohol wash" can be performed on a similar volume of bees by placing them into a jar with a lid and 2-3 cups of alcohol. Shake the jar vigorously for one minute and then pour the liquid through a mesh that allows the mites to pass while retaining the bees. Count the mites in the discarded liquid. Typically, mite infestation rates are reported as "mites per 100 bees", so by counting both the number of bees and mites, this value can be calculated. Vaseline coated trays placed under screens on the hive bottom board will catch falling mites for monitoring purposes (see reference).
Varroa mites are preferentially attracted to drone brood during their reproductive phase, thus removal of mite-infested drone combs from colonies (drone trapping) can be used to reduce mite populations. Drone comb can be inserted into colonies, removed in the capped brood stage and then frozen to kill the mites. Once re-inserted into the colony, the bees will remove and recycle the nutrient rich dead brood and the combs can be reused. This is most effective in the spring and early summer.
Resistant honey bee stocks
Genetic differences in resistance or tolerance to Varroa mites are known to occur among honey bees subspecies and some commercial strains. A number of behavioral or physiological mechanisms appear to be involved in the resistance, including "Varroa sensitive hygiene," in which mite-infested cells are opened and cleaned out by the worker bees.
Resistance to some mite-control chemicals has been reported (including fluvalinate and coumaphos) and beekeepers should evaluate mite levels before and after treatment to ascertain that the products used are providing effective control (see monitoring above).
- amitraz (Apivar)-Currently available in a strip formulation and approved for use in all states.
- coumaphos (CheckMite Strips)-Remove all surplus honey before treatment.
- fluvalinate (Apistan)-Treat before honey flow or during the summer dearth. Destroy any honey left in hive during treatment. Do not re-use strips. If pesticide resistance is suspected, use alternative control measures.
- formic acid (Mite Away Quick Strips, Formic Pro)-Colony must be reduced to 1 to 2 supers deep. Allowable daytime temperature highs are between 50°F and 92°F. Remove pads from hive if daily temperature highs exceed 92°F during first 7 days of treatment.
- hop beta-acids (HopGuard 3)-Derived from the hop plant and labeled for use even during the honey flow. Currently available with full Section 3 registration. Most effective during times with little or no brood present.
- menthol, eucalyptus, thymol (Api Life Var, Apiguard)-An essential oil based treatment. Do not use during honey flows. Do not use when surplus honey supers are installed. Do not harvest honey from brood chambers or colony feed supers. Do not use at temperatures above 90°F. Two treatments per year are permitted.
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