Oxalic acid may be used in both the liquid form, and as crystals that can be evaporated by electric heater pans, for treatment of honey bees with varroa infestation…. Each of the methods of treatment are described below.

Spray method

Dilute oxalic acid is sprayed on to bees directly frame by frame.

Dribble method

A solution of oxalic acid, sucrose and water is dribbled along the seems of clustering bees during cold conditions that cause the bees to cluster.

Evaporation method

Solid crystals of oxalic acid are heated so that they evaporate. The resulting vapour treats the bees by fumigation. This is by far the most popular method here in the United States.

Oxalic acid, also called Ethanedioic Acid, as purchased is usually in the form of oxalic acid dihydrate, which is a crystalline form with two water molecules attached to each oxalic acid molecule.
Basic properties of Oxalic acid…

Colourless, crystalline, water soluble, solid, toxic, organic compound with specific gravity of 1.6 – 1.7

If heated the water of hydration boils off at 101/102 °C leaving anhydrous oxalic acid crystals.

On further heating to between 149 and 160 °C, the oxalic acid sublimes from a solid to a gas)

At 189 °C the oxalic acid decomposes into formic acid and carbon monoxide.