Oxalic acid (OA) is used by beekeepers to treat the parasitic mite, Varroa destructor. Varroa mites pose the greatest threat to honeybee colony health worldwide. The effectiveness of OA relies on understanding its interaction with the mite and the specific methods used for its delivery into the hive environment.
The Target: Understanding Varroa Mites
Varroa destructor is an external parasite that feeds directly on honeybee fat body tissue. This feeding weakens both adult bees and developing brood, transmitting debilitating viruses that lead to colony decline.
Mites exist in two phases: the phoretic phase, where they cling to and feed on adult bees, and the reproductive phase. During the reproductive phase, the female mite enters a brood cell just before it is capped. Once sealed, the mite reproduces, and her offspring emerge with the young adult bee.
Oxalic acid treatment is primarily effective against phoretic mites exposed on adult bees. The waxy capping of the brood cells protects mites reproducing inside. Therefore, beekeepers aim to treat colonies when there is little to no sealed brood, forcing most mites into the vulnerable phoretic stage.
Mechanism of Action: How Oxalic Acid Kills
Oxalic acid functions as a contact toxicant. The primary mechanism involves the crystalline structure of the acid damaging the mite’s exterior upon contact. When applied, the acid rapidly re-crystallizes into fine particles that adhere to the bees and inner hive surfaces.
These sharp crystals physically abrade the mite’s delicate cuticle and footpads (tarsi) as it moves across treated surfaces. The acid’s low pH then penetrates the mite’s exoskeleton, causing corrosive damage. This topical penetration disrupts the mite’s internal cellular metabolism and severely lowers the pH of its hemolymph, a condition known as acidosis.
OA also chelates, or binds, with calcium molecules inside the mite’s body. This results in the formation of calcium oxalate crystals within the mite’s tissues, disrupting internal homeostasis. The combined physical damage and internal poisoning lead to severe dehydration, organ disruption, and the eventual death of the mite, which typically falls off the bee within hours.
Common Application Methods and Effectiveness
Beekeepers employ two main methods for delivering oxalic acid: vaporization and the dribble method.
Vaporization
Vaporization, or sublimation, involves heating crystalline oxalic acid dihydrate in a specialized device. The solid acid turns directly into a gas without becoming a liquid, requiring the colony to be sealed briefly. The vapor permeates the hive space, cools, and settles on all surfaces, including the bees, as fine acid crystals. This method requires minimal hive opening and is effective even in colder temperatures when bees are clustered. Vaporization generally results in high efficacy, often achieving mite kill rates above 90% when applied to a brood-free colony.
Dribble Method
The dribble method requires mixing oxalic acid into a sugar syrup solution, typically a 3.2% concentration (e.g., 35 grams per liter of 1:1 syrup). This solution is trickled directly onto the seams of bees between the frames using an applicator. Approximately 5 milliliters are applied per seam of bees, with a maximum total volume applied per colony, such as 50 milliliters. The dribble method is simpler as it does not require specialized heating equipment, but it introduces liquid into the hive and is best suited for use when ambient temperatures are warmer than freezing.
Safety and Handling Considerations
Oxalic acid is a corrosive chemical that requires careful handling to ensure the safety of the user and the honeybee colony. When using the vaporization method, inhaling the toxic acid fumes can cause severe respiratory damage. Beekeepers must wear full Personal Protective Equipment (PPE) to prevent exposure.
Users must apply the treatment upwind and avoid standing near the hive entrance where the vapor escapes. PPE includes:
- A respirator equipped with acid gas cartridges
- Chemical splash goggles that form a seal around the eyes
- Acid-resistant gloves, such as nitrile or neoprene
For the bees, OA is relatively safe when applied correctly, especially during a broodless period. However, overdosing the colony with the dribble solution or applying it too frequently can cause increased bee mortality and stress. Repeated applications of the sugar syrup solution can also damage the bees’ midguts and shorten their lifespan.