Oxalic acid is a naturally occurring organic acid found in various plants and vegetables that are part of our daily diet, including honey. Its abundance in the environment means we are exposed to it regularly, and it is considered safe to consume at low levels.

Oxalic acid has many applications outside of beekeeping. It is commonly used as a cleaning agent for surfaces such as buildings, swimming pools, and rust stains. It is also used for bleaching wood and as an ingredient in tooth-whitening products.

As beekeepers, we use oxalic acid as a "soft" acaricide to combat our primary pest, Varroa destructor. Oxalic acid is an effective treatment for varroa mites due to the way it interacts with them. Unlike some other treatments, it is unlikely that varroa mites will develop a resistance to oxalic acid. However, it is always advisable to rotate treatments as part of an integrated pest management strategy to avoid the overuse of any single treatment, including oxalic acid.

Treating the varroa mite is challenging because the mites take advantage of the honey bee's natural behaviour. Honey bees reproduce by swarming, and they may also rob from other colonies at certain times of the year and drift from hive to hive within an apiary. This means that any female varroa mites attached to adult honey bees will be transported to a new colony along with the bee, making it difficult to control the spread of the mites.

When beekeepers manipulate colonies, they can inadvertently move varroa mites between hives, especially when they move bees or broods. This can happen during swarm control, making nuclei or boosting other colonies with brood. Varroa mites are especially problematic during their reproductive phase, which occurs in sealed brood cells. Therefore, beekeepers need to take steps to prevent the spread of varroa mites, such as using treatments like oxalic acid and implementing integrated pest management strategies.

The varroa mite enters the reproductive phase when it moves into a brood cell after the dispersal phase. This typically occurs around day 5 of the honey bee egg hatching into a larva. The mite waits until the bees cap the cell before laying its first egg. This strategic move ensures its young are kept safe and well-fed. Each mature female lays around 5-6 eggs, with the first being male and the rest female. The females mature and become reproductive in just seven days. This reproductive phase of the mite's life cycle is particularly devastating as it can rapidly increase the mite population within a colony.

As the varroa mite population grows under the brood cap, it can be difficult for beekeepers to access these mites. There are specific times of the year when varroa treatment has the most impact on the population. Winter is one of the best times to treat oxalic acid, which does not penetrate the brood cap. The colony is at its smallest, and there is little to no sealed brood around the winter equinox. The varroa population is also small when the colony is small, making it easier to manage. The oxalic acid will impact the varroa population when they are sapping the winter bees of their vital fat body tissues, which sustain them through the winter. Another ideal time to use oxalic acid is after a colony has swarmed. All the varroa mites taken by the hive will be on the bees in the dispersal phase, so an oxalic acid spray here will significantly impact all the mites.

Due to the close relationship between the varroa mite and its honey bee host, it is highly challenging to eliminate varroa from a hive. Furthermore, there is a constant risk of reinfestation from other hives with varroa, even if a colony has been treated and appears to have little to no mite infestation. Thus, beekeepers need to have a proactive approach to varroa management and implement integrated pest management strategies to maintain healthy colonies.

Like honey bees, varroa mites have a lifespan of 2-3 months during the active season and up to 5 months in the winter. In the winter, adult varroa mites live on the honey bee and primarily feed on the bee's fat body tissue, not the haemolymph as previously believed. Samuel Ramsey made this discovery in 2019, revealing that the varroa mite targets the honey bee's "liver," a crucial organ that performs essential functions. When choosing between fat body tissue and haemolymph, varroa mites prefer the former. They live longer and lay more eggs when they consume fat, which suggests that fat is an essential part of the varroa mite's diet.

Varroa mite infestation poses a significant challenge for beekeepers, and if left unmanaged, it can lead to colony collapse and death within 2-3 years. The risk of colony mortality increases with higher varroa infestations, underscoring the importance of treating varroa to avoid the risk of losing your colony over winter.

Treating varroa mites with oxalic acid

Oxalic acid is a potent killer of the varroa mite, a parasitic mite that infests honey bees. The acid works by crystallising on the adhesive lobes, the mite's feet, and its feeding mouthparts, penetrating the mite's body. The varroa mite's adhesive lobes contain a watery secretion that binds to the oxalic acid crystals, causing them to grow. This process causes severe damage to the mite's vital organs and eventually results in its death.

Winter Trickle Method

The winter trickle method is the most widely used method for treating varroa with oxalic acid. This involves mixing oxalic acid with sugar syrup and dribbling the mixture over the bees in the hive during winter when the colony is bloodless. The acid kills the mites, which are attached to the adult bees and hiding in the cracks and crevices of the hive.

Oxalic Acid Spray

Another popular way to apply oxalic acid is through a spray. This method treats varroa when the colony has a brood because the acid can penetrate the cells and kill the mites hiding there. To apply the spray, beekeepers mix oxalic acid with sugar syrup and spray it over the bees in the hive. It's important to note that the spray can only be used without honey supers to avoid contaminating the honey.

Vaporization Method

The vaporisation method is another way to use oxalic acid to treat varroa. This method involves heating oxalic acid crystals to produce a vapour, which is pumped into the hive. The vapour penetrates the cracks and crevices of the hive and kills the mites hiding there. This method is effective when the colony has a brood, but it requires specialised equipment and safety measures to prevent accidental exposure to the vapour.

Oxalic acid safety when applied to hives

Oxalic acid is an irritant; exposure to any solution or vapour can irritate the eyes and respiratory system. Always wearing protective clothing, including acid-resistant gloves, safety glasses, and a respiratory mask when vaporising, is essential to minimise the risk of inhalation. Additionally, always vaporise your hives with oxalic acid upwind to further reduce the risk of inhalation.

Oxalic acid proves to be a valuable asset in the arsenal of a beekeeper. It is an economical alternative that can easily be applied to the hive through spraying or trickling. Although the initial cost of oxalic acid vaporisation may be higher, it becomes more affordable if beekeepers pool their resources and share a vaporiser. Long-term usage favours vaporisation, especially if one has numerous hives to treat. Alternatively, a successful honey season could warrant the purchase of a new and exciting gadget.

The benefit of the colony's tolerance to oxalic acid is that it can be distributed throughout the hive, enabling the bees to counter mites efficiently. However, unlike formic acid, its effectiveness is limited as it cannot penetrate the brood cap. Thus, only phoretic mites are impacted. However, if oxalic acid is administered when the colony is bloodless, its efficacy is notably impressive.

Traditionally, oxalic acid is used in the winter, particularly during the Christmas season. However, employing oxalic acid in the spring or summer, particularly during swarming or when colonies experience a brood break, is another opportune time to administer this remedy. Incorporating oxalic acid into an Integrated Pest Management (IPM) strategy is an excellent approach to controlling the varroa population in hives.

It is important to note that using oxalic acid in beekeeping requires careful consideration and execution. Inappropriate usage could result in detrimental effects on the hive and its inhabitants. Therefore, beekeepers should equip themselves with comprehensive knowledge before treating.