Challenges Using Oxalic Acid Against Summer Varroa Mite Infestations for Panhandle Beekeepers

One the greatest threats to the western honey bee, Apis mellifera Linnaeus, is a mite species known as Varroa destructor. Unfortunately, the Florida Panhandle is within the range of this hive devastating pest. Oxalic acid treatment has been shown to work well against Varroa infestations. However, the success of some products designed to combat pests vary seasonally. Honey bee and Varroa populations peak in the warmer months (late spring to mid-summer).

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Is oxalic acid a good management measure to use this time of year?

The varroa mite is technically an ectoparasite that feeds on both immature and adult honey bees. Varroa mites enter a hive and slowly weakens the colony until the mite’s reproduction advances past that of the hive, eventually causing a complete hive kill. Oxalic acid is a naturally occurring chemical compound found in many plants but is toxic to Varroa mites. Oxalic acid reacts forming crystals on the mite’s mouthparts and leg lobes, therefore disrupting their ability to feed or move, so they essentially starve.

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How is oxalic acid applied to hives and what is the efficacy of oxalic acid in the summer months?

Oxalic acid can be applied by dribble (solution in sugar syrup) or vaporization. Oxalic acid is most effective Varroa mite treatment, especially if the mites are not protected within capped brood cells. During the summer, a significant portion of Varroa mites (around 80-85%) may be located inside capped brood, making them unreachable by oxalic acid treatment. With the summer population boom, mid-summer is often considered a less-than-optimal time for oxalic acid application, that is if the goal is to target a large percentage of the total mite population. With that being said, oxalic acid can still be an effective treatment, especially during periods of broodlessness or low brood presence. A good example of this is when a new swarm has settled, but well before the capped brood appears.

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What are the recommended dosing protocols for oxalic acid treatments?

A recent UF/IFAS research study (Jack Et al., 2020) on oxalic acid dosage and efficacy found that colonies treated with the highest dose (4 grams) of oxalic acid vaporization had lower mite infestations compared to those treated with lower doses. Repeated applications of 2-4g oxalic acid vaporization every 5-7 days significantly decreased mite load. No significant negative impacts on colony health were detected at the tested dosages, including repeated 4g applications. However, research suggested that the current label rate of 1g oxalic acid per brood chamber was ineffective at adequately reducing Varroa populations.

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What practices can enhance the success rate of oxalic acid?

Creating a forced or artificial brood break can increase the efficacy of oxalic acid treatment during the season (including summer) by making more mites accessible. A study indicated that combining oxalic acid vaporization with a brood break resulted in a five-fold increase in mite mortality. However, brood interruption is not a recommended practice in late summer in Florida, due to the potential stress and difficulty for colonies to recover from the loss of brood.

Important considerations:  It is crucial to follow label directions and local regulations regarding the use of oxalic acid in beehives. Using oxalic acid when honey supers are on the hive was previously restricted to avoid honey contamination, however this restriction was lifted in 2021, after research indicated the risk was not significant, given that oxalic acid can occur naturally in honey.

For more information on varroa control, contact your local county extension office.

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Information for this article can be found in the following references:

Varroa, Varroa destructor Anderson and Trueman (Arachnida: Acari: Varroidae)

Determining the dose of oxalic acid applied via vaporization needed for the control of the honey bee (Apis mellifera) pest Varroa destructor

Determining the Dose of Oxalic Acid Applied via Vaporization Needed for the Control of the Honey Bee (Apis mellifera) Pest Varroa destructor

Honey Bee Health Coalition – Varroa Management