The dosage of oxalic acid (OA) depends entirely on the application method chosen: vaporization (solid) or dribble (liquid solution). OA is a naturally occurring organic compound used by beekeepers worldwide as an effective treatment against the parasitic Varroa destructor mite. The required dosage is carefully calibrated to be toxic to the mites while remaining safe for the honey bee colony. Since the application method dictates the final concentration and volume, beekeepers must follow precise measurements.
Oxalic Acid as a Varroa Mite Control Agent
Oxalic acid works as an acaricide, specifically targeting the Varroa destructor mite. The acid is believed to act as a contact poison, disrupting the mite’s metabolism and homeostasis after penetrating its exoskeleton.
The treatment is highly effective against phoretic mites found on adult bees. However, it cannot penetrate the wax capping of brood cells where mites reproduce. Therefore, OA is most successful when colonies are broodless or have minimal brood, typically during late fall or winter. Beekeepers favor oxalic acid because it is an organic treatment option that leaves minimal long-term residues in the wax or honey.
Calculating the Vaporization Dose
The vaporization method involves heating solid oxalic acid dihydrate crystals, turning the acid directly into a gas via sublimation. The resulting vapor re-crystallizes into fine particles that settle on all interior surfaces of the hive and on the bees. The required dose is based on the volume of the hive cavity.
The standard dosage is typically 1 gram of oxalic acid dihydrate per single-deep brood chamber. For larger colonies, such as a double-deep hive, the dose is increased to 2 grams. Higher dosages, ranging from 2 to 4 grams per colony, may be used if the hive is large or the mite infestation is heavy.
To perform this treatment, the measured crystals are placed onto a vaporizer or sublimator inserted into the hive entrance. All upper hive openings and cracks must be sealed to ensure the vapor is contained. After the acid has sublimated, the vaporizer is removed, and the hive remains sealed for a specified period, allowing the re-crystallized acid to remain in contact with the mites.
Preparing and Applying the Dribble Solution
The dribble method requires preparing a liquid solution applied directly to the bees. This solution combines oxalic acid dihydrate with sugar water, which helps it adhere to the bees and facilitates distribution throughout the cluster. The common preparation involves dissolving 35 grams of oxalic acid dihydrate crystals into 1 liter of 1:1 sugar-to-water syrup, yielding a final concentration of 3.2% to 4.2%.
The mixed solution is applied directly onto the bees clustered in the spaces between the frames, known as bee seams. Application is done using a syringe or similar dispensing tool to ensure precise volume control. The standard application rate is 5 milliliters (ml) of the solution per occupied bee seam.
A single colony should not receive more than a total of 50 ml of the solution, regardless of the colony’s size. If a colony occupies more than ten seams, the 5 ml dose is applied only to the ten seams containing the densest clusters. This volume restriction is necessary because excessive liquid application can negatively affect the bees and their ability to overwinter successfully.
Essential Safety Protocols and Timing
Safety Protocols
Handling oxalic acid demands strict adherence to safety protocols due to its corrosive nature and inhalation hazards. Personal Protective Equipment (PPE) is mandatory. For the vaporization method, which produces acid fumes, a certified respirator rated for organic acids is required to prevent lung damage.
- Long-sleeved shirt and long pants
- Chemical-resistant gloves
- Protective eyewear (goggles or a face shield)
- A certified respirator (for vaporization)
Optimal Timing
Optimal timing for application is when the colony is naturally broodless, typically during late fall or early winter. Applying it during a brood break ensures maximum exposure to phoretic mites, as the treatment cannot penetrate capped brood.
The dribble method is best applied when temperatures are mild, ideally above 40°F (4°C), ensuring the bees are clustered but not frozen. Vaporization can be effective at slightly lower temperatures, generally above 37°F (3°C).