The external parasite Varroa destructor is the most significant pest threatening the health of the western honeybee, Apis mellifera. These mites feed on adult and developing bees, weakening them by targeting their fat bodies. Mites also act as vectors for debilitating RNA viruses, such as Deformed Wing Virus (DWV). This combination accelerates colony decline, leading to colony collapse if left untreated. Effective management is necessary to maintain mite populations below damaging levels.
Assessing Mite Levels Before Treatment
Management decisions should rely on accurate data regarding the infestation level, not guesswork. Beekeepers determine the “mite load,” or the percentage of adult bees carrying mites, and compare this to an established economic threshold. Treatment is warranted when the infestation rate reaches 2 to 3 mites per 100 bees, though this threshold varies by season and region.
The Alcohol Wash is the most precise method for determining mite load, recovering 90-100% of mites present. This test involves collecting approximately 300 bees and shaking them in alcohol or soapy water to dislodge the mites for counting. The main drawback is that the collected bees are sacrificed.
The Sugar Shake is a non-lethal alternative, though less accurate. This method uses powdered sugar to coat the bees, causing the mites to fall off, generally recovering about 70-90% of the mites. The final mite count calculates the infestation percentage used to decide if treatment is necessary. Sticky boards are useful for general monitoring but are not accurate enough for threshold assessment.
Synthetic Chemical Treatment Methods
Synthetic acaricides are compounds formulated to kill mites. These products are applied via plastic strips impregnated with the chemical and placed directly into the brood nest area. As bees walk across these strips, they pick up the chemical and distribute it throughout the colony.
Two common classes include pyrethroids (e.g., tau-fluvalinate) and organophosphates (e.g., coumaphos). These chemicals interfere with the mite’s nervous system, causing paralysis and death. While synthetics are highly effective initially, repeated use carries a high risk of resistance development; tau-fluvalinate efficacy may drop significantly after only a few years.
A concern is chemical residue accumulation in the beeswax. To prevent honey contamination, beekeepers must remove all honey supers before application. Efficacy depends on ambient temperature, and strips must be removed after the prescribed window to slow resistance development.
Organic Acid and Essential Oil Treatments
Naturally derived substances like organic acids and essential oils are often called “soft” chemicals. They are less likely to lead to mite resistance than synthetics but require precise application and strict attention to temperature and safety.
Oxalic Acid
Oxalic acid is applied using two main techniques: vaporization and dribble. Vaporization involves heating solid oxalic acid crystals, which sublimate into a vapor that fills the hive. This vapor re-crystallizes into fine particles on hive surfaces, killing mites on contact. Vaporization is highly effective against mites on adult bees, but the beekeeper must wear a respirator, goggles, and gloves due to vapor toxicity.
The dribble method mixes oxalic acid dihydrate with a warm sugar-water solution and trickles it onto the bees clustered between the frames. Both methods are most effective during a broodless period (late fall or winter) because the acid does not penetrate capped brood cells. The dribble method is less equipment-intensive but is temperature-dependent, typically requiring 40-50 degrees Fahrenheit or higher.
Formic Acid
Formic acid is widely used, often applied via pads or saturated strips. Formic acid is unique because its vapor can penetrate the wax cappings of brood cells, killing reproducing mites inside. This fumigant is extremely temperature-sensitive; the optimal range is generally between 50 and 85 degrees Fahrenheit, as higher temperatures risk bee and queen loss. Proper ventilation is required during application.
Essential oil treatments, primarily utilizing thymol, are available in gel or strip form. Thymol works as a fumigant, but its efficacy is highly reliant on ambient temperatures. Like oxalic acid, thymol does not penetrate the capped brood cells, and its overall efficacy is generally lower than that of acid treatments.
Non-Chemical Management Strategies
Non-chemical management strategies focus on manipulating the colony’s environment or biology to slow mite population growth without acaricides. These methods are important components of a comprehensive mite control plan.
Drone Brood Removal
This technique capitalizes on the Varroa mite’s preference for reproducing in drone brood cells. Mites are drawn to drone cells because they have a longer post-capping period. Beekeepers insert a specialized frame to encourage drone comb building. Once the drone cells are capped, the frame is removed and frozen to kill the mites inside, delaying the need for chemical treatment.
Brood Cycle Interruption
This strategy involves temporarily caging the queen or splitting the colony to create a period when no new brood is laid. This deliberate broodless period forces all mites onto the adult bees, making them vulnerable to chemical treatments. A brood break also allows more workers to engage in foraging.
Screened Bottom Boards
Screened Bottom Boards replace the solid floor of the hive with a fine mesh screen. As mites are dislodged from the bees, they fall through the mesh and out of the hive, preventing them from crawling back onto the bees.
Selective Breeding
Selective breeding for hygienic behavior, particularly the Varroa Sensitive Hygiene (VSH) trait, is a long-term genetic strategy. These bees detect and uncap cells containing mite-infested pupae, effectively removing reproducing mites from the colony.
Developing a Seasonal Treatment Strategy
An effective seasonal plan is built on Integrated Pest Management (IPM), combining accurate monitoring, non-chemical methods, and targeted chemical treatments. This approach discourages reliance on a single tactic and uses a variety of tools throughout the year.
The timing of treatment is important, with late summer or fall being the most important period. Treating after the main honey harvest and before winter bees are produced ensures the colony goes into winter with a low mite load, necessary for successful overwintering. If mite levels are high in the spring, an early treatment may be necessary, or a winter treatment with oxalic acid during a natural broodless period is highly effective.
IPM requires the rotation of chemical classes to prevent mite resistance. Beekeepers should avoid using the same active ingredient in consecutive years, switching instead between chemical types like organic acids and synthetics. This rotation strategy prevents mites from being repeatedly exposed to the same mode of action.
Post-treatment verification is the final step. After any treatment, beekeepers must re-check mite levels (typically ten to fourteen days later) to confirm the treatment was effective and that mite numbers have dropped below the established threshold. Maintaining accurate records of treatment dates, products used, and resulting mite counts is invaluable for future management decisions.