Oxalic acid is among the most widely used treatments against Varroa. Its effectiveness depends entirely on one condition: the presence of brood.

The decisive point

Oxalic acid is a contact treatment. It reaches only the mites sitting on the bees.

• In a broodless colony: All mites are on the bees; effectiveness is very high
• With brood present: Most mites are protected inside sealed cells; effectiveness is low

Treating with brood present is therefore largely wasted effort — and it stresses the bees without result.

Timing

The broodless period. Traditionally this falls in winter, but warm autumns shift it from year to year.

Confirm broodlessness by looking or measuring rather than assuming it from a date. Treating on a fixed calendar date is the commonest cause of a failed treatment.

An artificial brood break — after an artificial swarm or a split — creates the same window in summer.

Methods of application

• Trickling: A solution is dribbled between the frames onto the bees. Simple; repeated application stresses the colony
• Sublimation: The acid is vaporised and settles as crystals. Less stressful for the bees but demands strict personal protection

Safety

Oxalic acid is corrosive and the vapour is harmful to the airways.

• Appropriate respiratory protection when sublimating
• Eye protection and gloves
• Do not work in an enclosed space
• Keep bystanders at a distance

Rules

• Only approved preparations and the prescribed dose
• Do not treat with supers on
• Record the application: product, date, dose, colonies treated
• Do not apply more often than prescribed

Afterwards

Measure again. Without a check you do not know whether the treatment worked — and with oxalic acid the brood status is so decisive that this check is indispensable.