It is late fall now, winter is near, and we do not typically think about treating our colonies for varroa as temperatures cool and snow beckons. But the short days of winter can have an advantage for killing varroa, due to the absence, or scarcity, of brood in the hive. Varroa mites reproduce in honey bee brood, and most of their early life is spent there. As much as 80% of a colony’s varroa mites will, during periods of high honey bee and brood production, be contained within the cells of worker and drone cells.
This complicates the methods that beekeepers use to control them. Most of the registered miticides will not kill varroa within the cells, instead depending upon longer treatment periods, over 42 days, to dispatch the mites contained within the cells after they emerge. Likewise, one of the drawbacks of what we call “flash” varroa treatments, miticide formulations that only act for a very short period of time, often only for a few days, is that they do not effectively control mites within capped cells. However, winter, or times when there is no, or little brood in the hive, provides beekeepers with an opportunity to make a flash treatment more effective. One such flash treatment available is oxalic acid, distributed by Véto-pharma as Api-bioxal.
Oxalic acid in not new to the varroa mite wars. It has been used as a miticide in Japan since the 1980s, in Europe since the early 2000s, registered in Canada in 2010 and in the United States in 2015. Oxalic acid is an organic acid like formic acid and commonly found in nature and in the foods that we consume such as carrots, tomatoes, and even peanut butter. However, we need to keep in mind that when beekeepers make use of oxalic acid, they are making use of a very concentrated form of the acid, like formic acid, and FAR from harmless. Not following required safety regulations, such as the wearing of chemical resistant gloves, not exposing uncovered skin, the use of eye protection, and the use of a respirator, can result in burns or permanent lung damage. So, carefully read label directions and safety rules, for your wellbeing.
There are three methods for conducting applications of OA, all approved by the USDA registration label directions. Two of these are intended for hives with bees, and the other for application to honey bees in shipping packages prior to installation. During all applications honey supers should not be on the hives, hence reference to application in “brood chamber or box”. The label also calls for making application when there is little or no brood in the hive, a condition most often encountered in the late fall, or early winter, or when the queen has temporarily stopped laying eggs. This prohibition will be discussed later in this article.
Keep in mind there are advantages and disadvantages to each method. I will describe each method as follows. As with the usage of any chemical in the hive, I urge beekeepers to seek out additional information, educate themselves fully, and again READ THE LABEL.
How to dribble oxalic acid
- Dissolve 35 g of Api-bioxal (oxalic acid) dihydrate powder in 1 liter of lukewarm 1:1 sugar: water (weight: volume) syrup. Smoke bees down from the top bars.
- With a syringe or an applicator, trickle 5 ml of this solution directly onto the bees in each occupied bee space in each brood box. The maximum dosage is 50 ml mixed liquid per colony whether bees are in nucs, single, or multiple brood boxes.
- Vaporization means heating the Api-bioxal (oxalic acid) dihydrate powder in a commercial vaporization heat apparatus until the temperature of the unitcauses the powder to convert to a vapor. There are a number of these devices on the market, with prices starting at less than $100. In addition, a power source is needed, either an automobile battery, or a gasoline powered generator. Follow the manufacturers’ directions for powering, and use of the vaporizer. Vaporization, over trickling, seems to have advantages. However, due to the potential exposure of beekeepers to the acid fumes, extreme caution is needed to avoid the inhalation of the fumes by the beekeeper. I have observed beekeepers conducting OA vaporization WITHOUT wearing respirators. This IS A BAD idea, lung damage may be permanent.
- To conduct oxalic vaporization: Apply only to outdoor colonies with a restricted lower hive entrance. Follow the vaporizer manufacturer’s directions for use. Seal all upper hive entrances and cracks with tape to avoid escape of thevapor. First smoke bees up from the bottom board, place 1 g of Api-bioxal (oxalic acid) dihydrate powder per brood chamber or box (so a one story hive will require 1 gram, a two box hive will get 2 grams) into the pan of the vaporizer. Insert the vaporizer apparatus through the bottom entrance. Apply heat until all Oxalic Acid has sublimated – converted to the gaseous state.
Treating new package bees
- In the spring beekeepers often start new colonies by purchasing a small number of bees (about ten thousand) in a screened shipping package, along with the new queen within in a separate queen cage. Of course, there is no brood, or comb, prior to installing bees into a hive. Any mites present are on the bees. So, this is an advantageous time to eradicate these phoretic mites with oxalic acid by spraying oxalic acid-sugar-syrup mixture onto the screen of the shipping package.
- To use oxalic acid on package bees:
Again, follow the directions carefully. First ensure bees are clustered before applying oxalic acid (for example store bees and package in a cool dark location for 24 hours before application). Mix a 2.8% oxalic acid solution by dissolving 35 g of oxalic acid dihydrate in 1 liter of warm 1:1 sugar: water (weight: volume). Evenly apply (spray) 3.0 mL of 2.8% oxalic acid solution per 1,000 bees using a pump sprayer or battery powered sprayer (for example, a typical 3 lb. package contains approximately 10,000 bees which would require 30 mL of solution). Important: spray broodless package bees with a 1:1 sugar: water solution at least 2 hours before spraying with oxalic acid. This allows bees to fill honey stomachs with sugar water reducing ingestion of oxalic acid, ingesting sugar syrup is harmful to their midgut. Apply solution evenly on both sides of the package. Store bees in a cool darkened room for 72 hours before hiving.
- An alternative to applying oxalic acid by spraying package bees, especially if the beekeeper plans to install the bees upon new foundation, is to first install the bees into the new hive, and then treat that new colony by either trickling, or vaporization. The new colony does not yet possess brood, or even drawn comb if done within a couple of days after placing the bees into the hive.
Some additional thoughts on using oxalic acid
We, beekeepers, are always told when using miticides, to read & follow the label directions. We especially need to always keep this in mind when using oxalic acid. Using oxalic acid is more complicated than many other chemical applications. With others we may just open the package, insert the strips in the proper place in the hive, and make note of the date. Or open a gel container, or place gel containing the active ingredient onto a card and place in the hive. These chemical formulations are relatively safe for the beekeeper making the application. With oxalic acid we are directly exposed to a concentrated acid, and its dangers. So again, read the instructions carefully, and follow them!
The label is also very clear, “Oxalic acid is used to treat colonies during low brood periods, packages, or swarms.” If it is being used when there is appreciable amounts of brood in the hive, this is a violation of the label. I have heard of beekeepers using oxalic acid multiple times, often three or four successive applications – a week apart, to attempt to control varroa as mites emerge with the bees from the brood. Research shows that even repeated applications of oxalic acid by dribbling / trickling up to four times do not necessarily lead to a sufficiently high treatment efficacy. In addition, other publications show that multiple applications of oxalic acid results in reduction of brood in colonies so treated, along with increased queen loss.
Oxalic acid (Api-bioxal) can be an effective varroa control when used properly and there is little, or no brood present. However, safety is an important issue for beekeepers, and proper precautions must be taken to prevent injury by the applicator. If you have questions, feel free to contact me at firstname.lastname@example.org
Photos copyright © Tammy Horn Potter
1- Bacandritsos, Nicolaos, et al. “Efficacy of repeated trickle applications of oxalic acid in syrup for varroosis control in Apis mellifera: Influence of meteorological conditions and presence of brood.” Veterinary parasitology 148.2 (2007): 174-178.
2- Higes, Mariano, et al. “Negative long-term effects on bee colonies treated with oxalic acid against Varroa jacobsoni Oud.” Apidologie 30.4 (1999): 289-292.