Organic Acids in Beekeeping - Part 2 - Formic Acid

Science warning: this post contains actual science. You have been warned...

Formic Acid molecule
(Wikimedia Commons, Public Domain)

In part 1, I covered the basic chemistry of organic acids.  Though you could say that my explanation was the opposite of basic...  (If there are any chemists reading - yes, that was a little in-joke just for you).

Last time we covered oxalic acid, which is used in the winter.  Beekeeping is usually done in the spring, summer and autumn, which means that we need a Varroa treatment that can be used during the warmer months.  Enter formic acid.

Like oxalic acid, formic acid is an organic acid.  But unlike oxalic acid, formic acid contains a carbon-hydrogen bond - which means that it meets the (not entirely accurate) definition of an organic molecule.

And, also like oxalic acid, formic acid exhibits the behaviour of a Brønsted acid.  Formic acid is a carboxylic acid, which means it can donate one proton (positively-charged hydrogen nucleus).  This then makes the formic acid molecule negatively charged, a state known as the conjugate base.  When in this negatively charged state, formic acid is called formate.

If you're still awake, let's move on to where formic acid is found in nature.  One place where you will definitely have encountered it is as a component of the stinging hairs on stinging nettles.  It is also a component of the venom of ant stings.  In evolutionary terms, ants are fairly close cousins of honey bees.  And yet, bee stings don't contain formic acid.  Weird.

So, our interest in formic acid in beekeeping is not in the bees themselves.  Rather, it is because formic acid is useful as a weapon against the Varroa mite.  Formic acid is an acaricide, which is a chemical agent that kills ticks and mites.  The specific mode of action is that it acts on the mitochondria in the cells of the Varroa mite.  Mitochondria are found in the cells of all animals, and they generate energy through aerobic respiration.  There are different processes for doing this (I will spare you the details), but they result in the production of adenosine triphosphate (ATP), which is the molecule that is used for energy transfer throughout the body.

The processes all rely on a very clever bit of biochemistry called an electron transport chain.  This is a sequence of enzymes that receive, and then pass along electrons (negatively charged particles) along the chain.  The end of the chain is negatively charged (because of the electrons), which creates an electrical gradient across a membrane in the mitochondrion.  This causes protons (those positively-charged hydrogen nuclei again) to flow across the membrane from the neutrally charged side to the negatively charged side.  And this proton flow provides the energy for an enzyme called ATP synthase to create ATP.

At the end of the electron transport chain, but before we get to ATP, is an enzyme called cytochrome c oxidase.  As with each of the enzymes in the chain, the job of cytochrome c oxidase is to receive electrons from further up the chain and pass them along.  But formic acid inhibits the action of cytochrome c oxidase.  This interrupts the electron transport chain, which prevents the proton flow and stops the production of ATP.  Without ATP, the Varroa mite has no means of transferring energy round its body, so it dies.

So, does formic acid affect ATP production in honey bees?  Well, actually - yes.  But the effect isn't as severe.  Some bees die in the first few days of treatment, and there is a risk (though less than a 5% chance) of losing the queen.  But these are acceptable losses and risks, when compared the the more significant problems - including colony loss - that can be caused when the number of Varroa mites in the hive gets too high.

I usually apply a formic acid treatment once a year, normally a week after I've harvested the honey.  It comes in the form of a gel which takes about a week to evaporate.  I don't particularly like doing it, but needs must.  And the bees don't like it either, but I suppose taking your medicine is no fun, is it?