The parasitic mite Varroa destructor is the single largest threat to honeybee colonies worldwide, contributing significantly to colony losses. These mites weaken bees by feeding on their fat bodies and serve as vectors for debilitating viruses, such as Deformed Wing Virus (DWV). Effective management is necessary for colony survival. A “natural” approach focuses on methods that avoid synthetic chemical miticides, instead utilizing cultural practices, mechanical manipulations, and naturally derived compounds. This integrated strategy aims to keep mite populations below harmful levels without introducing chemical residues into the hive products.
Assessing Mite Load and Optimal Timing
Monitoring mite populations is necessary because treatment should only occur when mite levels reach a threshold that poses a threat to the colony, ensuring treatments are applied only when they can be most effective. A common action threshold is approximately two to three mites per 100 bees, but this can vary depending on the season and local guidelines.
Two of the most reliable assessment methods are the sugar shake (or powdered sugar roll) and the alcohol wash, both of which sample adult bees to determine the infestation rate. The alcohol wash, involving submerging a half-cup sample of approximately 300 bees in alcohol, is considered the most accurate method, dislodging over 90% of the phoretic mites for a precise count.
The sugar shake is a non-destructive alternative, where powdered sugar is used to dislodge mites, allowing the bees to be returned to the hive, though it may yield a slightly less accurate count.
Timing is a crucial factor for the safe and effective application of natural treatments, as many are temperature-dependent. Treatments are most effective and safest when applied during periods of low or no brood production, such as late autumn or early winter, when the majority of mites are on adult bees and not protected inside capped cells. Applying treatments during a major honey flow should be avoided entirely to prevent contamination of honey intended for human consumption.
Cultural and Mechanical Control Strategies
Cultural and mechanical controls involve physical manipulations of the hive environment or the colony’s reproductive cycle to interrupt the mite’s life cycle. The use of screened bottom boards (SBB) is a popular cultural practice where the hive floor is replaced with a wire mesh screen.
The screen allows mites that fall off adult bees to drop out of the hive and onto the ground or a sticky board, preventing re-infestation. While an SBB alone is not sufficient to control a high mite population, it provides a passive, continuous reduction and is an excellent tool for monitoring mite drop.
Drone brood removal (drone comb trapping) takes advantage of the mite’s preference for drone cells, which offer a longer development time for reproduction. Beekeepers introduce a frame with drone foundation, which the queen fills with drone eggs. Once the cells are capped and the mites are trapped inside, the beekeeper removes and destroys the entire comb before the drones and the new mites emerge. This mechanical removal eliminates a significant number of reproducing mites, reducing population growth.
A temporary brood break disrupts the mite’s reproductive cycle by eliminating capped brood cells. This can be achieved by caging the queen for approximately two to three weeks or by splitting the hive. The temporary absence of brood forces all mites onto the adult bees, known as the phoretic phase, making them highly vulnerable to a subsequent treatment. When timed correctly, a brood break can significantly increase the efficacy of natural treatments like oxalic acid vaporization.
Application of Naturally Derived Treatments
Naturally derived treatments, such as organic acids or essential oils, are effective alternatives to synthetic miticides, but they require careful application and adherence to temperature guidelines. Oxalic acid is a common organic treatment applied using two methods: dribbling and vaporization.
The dribble method involves mixing the acid with sugar syrup and applying the solution directly onto the clustered bees between the frames. It is most effective in cold, broodless conditions because the solution only kills mites on adult bees.
Oxalic acid vaporization (sublimation) involves heating the crystallized acid until it turns into a gas, which then condenses into a fine dust on the bees and hive surfaces. This method is considered less harmful to the bees than the dribble method and can be applied in a series of treatments throughout the active season. However, both oxalic acid methods require the beekeeper to wear appropriate personal protective equipment (PPE), including a respirator and gloves. The concentrated acid is corrosive and the vapor is hazardous to human health.
Formic acid is another naturally occurring compound, often applied via pads or strips (e.g., Formic Pro or Mite-Away Quick Strips) that release the vapor slowly. Formic acid is unique because its vapor can penetrate the wax cappings of the brood cells, killing mites reproducing inside. Application requires careful attention to hive ventilation and temperature, with daytime highs needing to remain between 50°F and 85°F (10°C and 29.5°C) to prevent harm to the queen or developing brood. Ventilation is necessary to prevent the acid vapor concentration from becoming too high, which can cause excessive bee mortality.
Essential oil-based products containing thymol (e.g., Apiguard or ApiLife Var) are used as fumigants, acting by vaporizing the active ingredient. Thymol-based treatments are dependent on ambient temperature, working best in a narrower range between 68°F and 77°F (20°C and 25°C). If temperatures are too low, the thymol does not vaporize sufficiently, and if too high, the rapid vaporization can harm the bees and the brood. These treatments often require multiple applications over several weeks to target mites as they emerge from capped cells and must never be applied while honey supers are on the hive to prevent flavor contamination.
Breeding for Mite Resistance
A long-term, biological solution to mite management involves breeding or selecting honeybees with natural resistance traits. This strategy reduces the beekeeper’s reliance on continuous intervention. The concept centers on hygienic behavior: the colony’s ability to detect and remove diseased or parasitized brood.
Specifically, beekeepers look for Varroa Sensitive Hygiene (VSH) bees, a trait where worker bees selectively detect and uncap cells containing mite-infested pupae. This removal behavior kills the developing mites and their offspring, significantly reducing the mite’s reproductive success.
The VSH trait was initially identified as Suppressed Mite Reproduction (SMR), but it was later determined that the primary mechanism was the bees’ hygienic removal of reproductive mites, leading to the current VSH designation. Purchasing VSH-tested queens is a direct way to introduce these genetic traits into an apiary, building a population that naturally keeps mite numbers lower over time.