Pest description and damage Varroa mites are external parasites of honey bees, feeding on the fat bodies and hemolymph 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). Varroa parasitism results in reduced longevity or mortality of individual bees and heavy parasitism can lead to death of the colony. Varroa mites also vector viruses, and are implicated in the transmission of several lethal honey bee viruses.
Varroa mite monitoring is crucial in the development of an integrated pest management (IPM) plan. Varroa mite levels can be determined by placing roughly 300 bees (a half cup) in a jar fitted with a screened lid and adding 2 tablespoons of powdered sugar. The powdered sugar dislodges the mites from the bees. The sugar and mites are shaken from the screened jar 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 1 cup of 70% isopropyl alcohol. Shake the jar vigorously for one minute, 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, however only the powdered sugar shake or alcohol wash can provide an accurate value (percentage) of the mite infestation.
For more information: Sampling for Varroa Mites https://extension.oregonstate.edu/video/sampling-varroa-mites
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. Later, this drone comb can be re-inserted into the colony for the bees to remove and recycle the nutrient-rich dead brood. This technique 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 (Apistan) and coumaphos (CheckMite+)) 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.
- formic acid (Mite Away Quick Strips, Formic Pro)-Optimal when daytime temperature highs are between 50°F and 85°F. Brood loss or queen mortality may occur if daily temperature highs exceed 92°F during first 3 days of treatment. This product targets mites that are on the bees and the reproductive mites inside the capped brood cells. May be used during nectar flow.
- hop beta-acids (HopGuard 3)-Derived from the hop plant and labeled for use even during the nectar flow. Most effective during times with little or no brood present.
- thymol, menthol, eucalyptus (ApiLife Var, Apiguard)-Essential oil-based treatments. Do not use during honey flows, or when surplus honey supers are installed. Do not harvest honey from brood chambers or colony feed supers. Do not use at temperatures above 105°F (Apiguard) or 95°F (ApiLife Var). Two treatments per year are permitted.
- oxalic acid dihydrate (Api-Bioxal)-Available to use as a solution or vapor. Oxalic acid application is most effective when there is little or no brood present in the hives. May be used during the nectar flow.
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