The parasitic mite Varroa destructor is the greatest threat to the health of the western honeybee, Apis mellifera. Infested colonies face a significant risk of collapse, often succumbing to mite-vectored viruses like Deformed Wing Virus (DWV). Without intervention, most honeybee colonies in temperate zones will fail within two to three years. Therefore, prevention means continuous, proactive integrated management to keep mite populations suppressed below a level that causes colony damage.
Establishing Baseline Mite Levels Through Monitoring
Effective mite management begins with accurate measurement, as treating a colony without knowing its infestation level is wasteful and potentially harmful. Beekeepers must establish a “treatment threshold,” which signals the need for intervention. This threshold is often set at 2% to 3% infestation (two to three mites per 100 adult bees) during the active season, though some experts recommend keeping levels below 1% year-round.
The gold standard for a precise count is the alcohol wash, which involves collecting a standardized sample of 300 adult bees and submerging them in rubbing alcohol. The alcohol instantly dislodges the mites, providing a highly accurate count that often exceeds 90% detection. While this method sacrifices the sampled bees, the loss is negligible compared to the thousands of bees produced daily in a healthy colony.
Monitoring should occur frequently during the active season, ideally monthly, and always immediately before and after any treatment to confirm efficacy. Other monitoring methods include:
- The powdered sugar roll, which is less invasive but significantly less reliable due to inconsistent mite dislodgement.
- Using a sticky board placed on the hive bottom to measure the “natural mite drop” over several days.
The sticky board method is useful for tracking long-term trends but should not be relied upon for determining an immediate treatment threshold.
Non-Chemical and Mechanical Control Techniques
Beekeepers can employ several non-chemical and mechanical techniques to disrupt the mite’s reproductive cycle before resorting to chemical applications. These methods are used proactively to slow population growth and delay the need for chemical treatment.
Drone Brood Removal
Drone brood removal is a highly effective cultural practice that exploits the mites’ preference for drone cells, which offer a longer development period conducive to higher mite reproduction. Beekeepers insert a special drone frame into the brood nest for the queen to fill with eggs. Once the drone cells are capped (typically around day 18 to 24), the entire frame is removed and frozen to kill the trapped mites and developing drones. This technique removes a substantial number of mites, reducing the overall infestation load.
Brood Break
Another disruptive technique involves inducing a temporary brood break, often by caging the queen for 9 to 14 days. Since Varroa mites reproduce only within capped brood cells, halting egg-laying forces all mites into the phoretic stage on adult bees. This concentrates the entire mite population, making them more vulnerable to subsequent chemical or natural control measures.
Screened Bottom Boards
Passive mechanical control is achieved through the use of screened bottom boards, which replace the traditional solid wooden bottom. When mites are dislodged from the bees, they fall through the mesh and land outside the hive. This prevents them from climbing back to re-infest the colony, offering continuous, low-level suppression.
Breeding and Selecting Mite-Resistant Bees
A long-term, sustainable approach focuses on enhancing the bees’ natural defenses through genetic selection. This strategy centers on sourcing queens that exhibit specific behavioral traits allowing the colony to manage mite populations internally.
One important trait is Hygienic Behavior, which describes the workers’ ability to detect and remove diseased or infested pupae from their cells. A more specialized trait is Varroa Sensitive Hygiene (VSH). Bees exhibiting VSH are adept at identifying mites reproducing within capped brood cells, often by sensing a chemical cue. The worker bees respond by uncapping the infested cells and removing the pupae, which interrupts the mite’s reproductive cycle and reduces the number of fertile offspring.
Beekeepers should prioritize purchasing queens from breeders who actively select for VSH stock. Colonies with high VSH expression can often maintain mite levels below the treatment threshold for extended periods.
Applying Targeted Chemical and Natural Treatments
When monitoring indicates that mite levels have exceeded the established treatment threshold, beekeepers must apply a targeted treatment. Modern management emphasizes rotating treatments to prevent mites from developing resistance to a single compound. Synthetic acaricides, such as amitraz-based strips (e.g., Apivar), are highly effective, but resistance has been reported globally due to overuse.
Organic Acid Treatments
Natural treatments, specifically organic acids, are highly popular.
- Formic acid (e.g., Formic Pro) is unique because its vapor penetrates the wax cappings to kill mites reproducing inside the sealed brood. However, application is highly sensitive to temperature and should be used when air temperatures are between 50 and 85 degrees Fahrenheit (10 to 30 degrees Celsius), as higher temperatures can harm the queen or the colony.
- Oxalic acid is most effective when applied as a vapor or dribble solution in late fall or early winter when the colony is naturally broodless. Since it does not penetrate capped brood, its efficacy is maximized when all mites are in the phoretic stage on adult bees.
Thymol-based products, which utilize essential oils, are also common but require specific temperature ranges to vaporize properly and achieve effective control. All chemical treatments must be timed carefully, typically applied after the honey harvest to prevent contamination. Always follow the manufacturer’s label instructions precisely, including wearing appropriate personal protective equipment.