How does Oxalic acid kill mites and how does it affect bees?

What is Oxalic Acid?

Oxalic is a natural organic acid found in many plants including its namesake oxalis, also in rhubarb leaves and spinach.  It is a poisonous crystalline powder with a sour taste.

It has been used in Europe as a varroa treatment for many years under the brand Api-Bioxal.   Oxalic Acid is approved in Europe for use when honey supers are on.

Being an acid of Ph 1.3, it is very corrosive so wearing protective clothing is essential when handling it. (I have destroyed a good pair of trousers by spilling it on them.)  Oxalic Acid is also used for removing rust stains and bleaching timber.  Always wear nitrile gloves when handling.  A respirator will be needed if you are vaporising as the vapor can damage your lungs if inhaled.

It is normally sold in powdered crystal form as oxalic acid dihydrate and can have up to 30% moisture content by weight but now is available in tablet form as anhydrous oxalic acid.  These tablets are less than 1% moisture.

Why use Oxalic Acid?

Oxalic acid is suitable for organically treating for mites because it is a natural miticide as opposed to synthetic treatments.

Varroa mites are not known to become resistant to oxalic treatments and it can be used all year round.  It is also an excellent way of ensuring you give your hive the best chance of surviving winter by applying after removing your synthetic strips.  Due to the mites becoming resistant to synthetic treatments it is essential that other, and more environmentally friendly ways of combating varroa are needed.

Oxalic treatment of mites may be more time consuming but the low cost more than makes up for this.  There are many ways to apply oxalic acid so it pays to look at all the options and decide which method will work best for you.  Oxalic acid can be used year round to treat for varroa but is most effective when the hive is broodless  or has low brood numbers as it does not penetrate the capped brood.  Whatever treatment you use it is advantageous to use a different treatment as well as oxalic acid.

How does Oxalic acid kill mites and how does it affect bees?

It is thought that OA vapors enter through the soft pads of the mite’s feet and travels to the blood stream, killing the mite. It is also thought that it destroys parts of the mite’s mouth.  It is 70x more toxic to mites than bees.  Oxalic acid applied with the dribble method can cause uncapped brood death if done too often, but vaporising does not seem to affect the bees in this way.  It is up to 98% effective when used properly.

Ways to treat with Oxalic Acid

1 – Dribble method

This involves making up a solution of oxalic acid and sugar syrup and dribbling a set amount between each frame. This is an effective way of applying oxalic acid but it does mean opening up your hive

to apply it.  The solution coats the bees who are groomed by others, thus distributing the oxalic around the hive.

This method works best when the hive is broodless in winter or during a manufactured brood break. You can then get up to 95% kill rate.  

2 – Vaporising

Or Sublimation, which is the more accurate term.  The advantage of this method is that you can treat your hive even in the middle of winter without having to open it up.

It involves heating the oxalic acid up to 160 deg C at which point it turns into a vapor.  There are different ways of doing this, using gas, 12 volt, or 230 volts as a power source.  The vapour permeates through the hive and coats everything with tiny oxalic crystals – including the mites, causing them to drop off and die.  As oxalic treatments do not kill the mites in the capped brood, vaporising can be used as a spot treatment after synthetic strips are removed or in a broodless period, or as a series of five treatments over a 20-day brood cycle to kill all emerging mites.  If you are using vaporising as your only varroa control you need to do this every 4 months to keep your mite levels as low as possible.

3 – Oxalic/glycerine strips or towels

This involves soaking cardboard strips and laying them over the frames as staples down each side of the frames or as towels laying on the top of the brood frames. The bees will remove them from the hive and in doing so will spread the solution around the hive.  This method of applying oxalic acid means there is no need to regularly treat as you do with the vaporiser or dribbling.  However, if you have a strong active hive you may need to replace the strips after 3 or 4 weeks as the bees may remove them too quickly (they need to be in for at least 8 weeks). 

The Glycerine needs to be heated and the oxalic acid dissolved in it. The strips or towels are soaked in the solution, and dripped off, then placed as required in the hive.