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Treating Honey BeeColonies for Varroa
with Oxalic Acid Drip
(Drizzle Method)

Note: This page is inserted automatically several places on the site and as a stand-alone page
which I update from time to time.  That will explain future events appearing in the past record
Click on pictures to enlarge


The 'Optimiser"from Medivet


The Gun

Gun Setting Indicator
(mL per stroke)

Medivet's
Instruction
Sheet

Drip Method
Instructions

In 2008, to treat 10 colonies, I measured 250mL of sugar and 250mL of water.  They weigh about the same.  When I mixed them, I got a little over 400 mL of syrup. That was the most uncertain part of the task. 

I then calculated the correct amount of dihydrate for 400 mL of syrup, mixed it up, and applied it in fall.  It did not seem to harm the bees and they did well in 2009.

From that mixing experience, I conclude that to get around a litre of syrup, 20% more, or 600mL of each sugar and water must be used.

To the resulting one litre of 50/50 syrup, add 35 grams of  oxalic acid dihydrate.  Measuring carefully is important and the acid should be weighed, since volume measurement is inaccurate and depends on how packed the powder is.

Pre-dissolving the acid crystals in a small amount of warm water after weighing and before mixing into syrup is recommended as it does not dissolve easily.

Here is the complete Canadian oxalic label.  (relevant excerpt below).

SOLUTION METHOD

NOTE: To completely dissolve oxalic acid dihydrate, use warm syrup. Dissolve 35 g of oxalic acid dihydrate in 1 litre of syrup made from a 1:1 sugar : water (weight:volume) mixture of sugar and water. 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 dose is 50 mL per colony whether bees are in nucs, single, or multiple brood chambers. Under certain unfavourable conditions, e.g., weak colonies, unfavourable overwintering conditions, this application method may cause some bee mortality or overwintering bee loss.

Other resources:

An email rec'd Nov 29...

> I've been working on reconciling various OA formulas and was using your site (Nov 18-19 2009 diary) to figure the weight of 1:1 syrup. So I had to figure out the cause behind your statement: "Well the best-laid plans... I mixed as above and came up with 1,600 mL, not the expected amount,..." on Nov 19. The cause was your calculation 4x400=1800 behind your statements in the previous paragraph.

Thanks. You are right. That explains it.

> I had been concerned that you were not getting consistent syrup weights from mixing weighed ingredients. The underlying physics appears consistent, so I am comfortable now.

> Another surprise is that the Canadian label produces a much weaker solution than what Randy Oliver is recommending on his web site. Randy's 1:10:10 by weight looks to me to be nearly twice the strength you used.

There tends to be confusion between the acid concentration in water and the amount of dihydrate required to achieve it. The dihydrate already has two water molecules attached, so a considerably greater weight of dihydrate is required than would be the case if pure acid were used. I am not aware of sources of the pure acid, and there is no need to use pure acid if the fact that the dihydrate contains water is taken into consideration in the mixing.

Follow the label and all should go well.


Footnote

A Persistent Confusion
(note: The info below was corrected Dec 1/09)

There tends to be a persistent confusion confusion between the actual resulting acid concentration in water and the amount of the dihydrate required to achieve it.  This confusion is likely due to the fact that the process has been developed by people with some chemical training and is practiced by people who may be less conversant with chemical matters, or who have been out of school a long time.

From Wikipedia

Oxalic acid is the chemical compound with the formula C2O2(OH)2 or HOOCCOOH. This colourless solid is a relatively strong carboxylic acid, being about 3,000 times stronger than acetic acid... Typically oxalic acid is obtained as the dihydrate. (emphasis added)

As can be seen in the figure below, the dihydrate contains two molecules of water and weighs much more than the actual oxalic acid it contains.  The calculations below simply add up the weights of the atoms in each and show how much of the weight in each 126 grams of the dihydrate crystals we buy is actually acid (90 g) and how much is water (36g).

Molecular formula C2H2O4 (anhydrous)
Molar mass 90.03 g/mol (anhydrous)

Molecular formula C2H2O42H2O (dihydrate)
Molar mass  126.07 g/mol (dihydrate)

From the above, we can see that to get the same 90 grams of oxalic acid, 126 grams of the dihydrate crystals must be used, compared to 90 g of the pure stuff -- thus the confusion.

Since water is almost half the weight of the crystals, it must be considered if we are trying to figure the actual amount of acid in a given sample of powder.  However, the amount of acid used is very small compared to the water in the syrup, so the 'water of hydration' may be ignored when calculating the water in the total solution and the water in the crystals is insignificant compared to the total water in the syrup.

Still confused?  Some very intelligent people get mixed up by this problem. It is a good thing that there is a label that spells it out in simple terms. 

Follow the label and all should go well.

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