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Oxalic Acid Vapour as Fall Test & Treatment

Updated: Apr 30

By mid-September, we have finished our hive inspections for the year. We want to limit our disturbance of the hives and allow the bees to set themselves up for winter. BUT, we still want to know whether our August mite treatment was effective. So, we use OAV as both a test and treatment.

If you are looking for information on HOW to use Oxalic Acid Vapour, click here.

In order to use this test & treat technique, you need the following:

1. An oxalic acid vaporizer, crystals, and the required PPE

2. A screened bottom board with pull-out tray

How To:

  1. Clean off the pull-out tray of your bottom board.

  2. Administer OA treatment

  3. Check the tray after 3 days and count mites

  4. Do some mite math (see below)

Mite Math

When we use the mite drop from Oxalic Acid to get an idea of our mite count, we are assuming that nearly 100% of the phoretic mites in the colony have died and dropped. Please note that OA does NOT kill mites under the brood caps, so a single treatment is not going to completely rid your hive of mites. But it does give us a number that we can compare to an alcohol wash.

A benchmark number for mite tests is 3%. A hive with 3% mite load is very unlikely to survive winter.

We like to treat our bees to keep them below 1% mite load.

In order to calculate a percent from your OA mite drop, we need to estimate a hive population. 25,000 is a pretty conservative number for mid-late fall.

If we calculate 3% of 25,000, that would be 750 mites on your bottom board!

At 1%, it would be 250.

If you want to calculate your actual mite drop as a percent, just count up your mites, divide by 25,000, and multiply by 100.

Some Caveats

Two things are very important to keep in mind here.

  1. The mite drop that you see after an OA treatment is a snapshot of your mite situation BEFORE the treatment. You're already better off than you were 3 days ago! Still, if you are seeing above threshold numbers, you might want to repeat this process again until you see a smaller drop.

  2. This mite math only applies when you have done an OAV treatment! If you are seeing high numbers of mites on your bottom board without an OAV treatment, then you need to assume that there are many more living mites still on your bees.

Looking for more?

Looking for more beekeeping education? You can find my virtual, on-demand beginner and intermediate courses at

Or if you'd prefer a more tailored experience, you can join my mentorship group at


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