Acaricide Resistance

Acaricide resistance arises when a medicine with the same mode of action is used for years, so that susceptible mites die and resistant ones multiply. Its sign is that the medicine does not bring about the expected drop, and you can only see this by measuring before and after treatment. The way to guard against it is rotation by mode of action, the correct dose and duration, organic acids and non-chemical methods.

How does resistance develop?

In any mite population there are always a few individuals that are more tolerant of a medicine. Each time the medicine is applied, the susceptible mites die and the tolerant ones survive and reproduce. When the same active substance is used for several seasons in a row, most of the population becomes resistant and the product stops working.

This is not a product defect but the result of selection. Underdosing and expired strips speed up the process, because they leave partly resistant mites alive.

In which groups is resistance seen?

What matters is not the product name but the mode of action of the active substance. Switching between two different products that work by the same mechanism does not count as rotation.

The risk of resistance to organic acids and thymol is considered low; but this does not mean “never”. These substances too must be used at the correct dose and at the right time.

How can you tell resistance?

The only reliable sign is that the treatment does not have the expected effect. To see this:

1. Measure the colony by a wash method (alcohol or icing sugar) just before treatment.
2. Once the treatment period is over, measure again with the same method.
3. If you cannot measure, monitor the mites falling onto the floor board throughout the treatment.
4. If you want to see the efficacy for certain, after the treatment is over apply a “control treatment” with a different mode of action (for example oxalic acid in the broodless period) and count the mites that fall. If many mites fall during the control treatment, the first treatment did not kill most of the mites.

Failure is not always resistance

If the mite count is still high after measuring, rule out the following possibilities in turn:

• Wrong dose or duration: Was the number of strips right for the colony's strength, were the strips in the brood area, was the full period completed?
• Wrong timing: If a substance that does not act in capped brood, such as oxalic acid, was applied when there was a lot of brood, the effect remains low.
• Reinfestation: If there are collapsing colonies with high mite loads in the area, robbing and drifting bees carry mites. Consider this if the count rises again shortly after treatment.
• Product problem: Products past their expiry date, or stored in sun or heat, can lose their effect.
• Resistance: If all of the above were correct and the same group of medicines has been used for years, resistance is a strong possibility.

Some research institutes and laboratories can carry out susceptibility tests on mites. If you have doubts, consult the official veterinary service / local agriculture authority or your beekeepers' association.

Mistakes that speed up resistance

• Using the same active substance (or substances from the same group) for years. This is the main cause of resistance.
• Underdosing: Putting too few strips in a strong colony, hanging the strip away from the brood area.
• Leaving the strip in longer than its period: A strip whose effect has declined exposes the mites to a low dose for a long time; this is the ideal environment for resistance. Remove the strip when the period is over.
• Unauthorised, home-made treatments: With methods such as medicine-soaked cardboard, wooden slats or fumigation the dose is unknown; they both fuel resistance and lead to residues in honey and wax. Such treatments are also illegal.
• Treating without measuring: Every unnecessary treatment increases selection pressure.

Resistance management: what to do

1. Rotate by mode of action. If you used one synthetic group for a season, choose a substance from a different group or an organic acid for the next treatment. Changing brand is not enough.
2. Include organic acids in your programme. Especially in the broodless period (early winter or after a brood break), oxalic acid is very effective and carries a low risk of resistance.
3. Use non-chemical methods. Drone brood removal, brood breaks, splits and artificial swarms bring down mite numbers without medicine; less medicine is then needed.
4. Only authorised products, at the dose and duration stated in the instructions. Too little is as harmful as too much.
5. Follow the honey harvest rules. Check the product instructions to see whether it can be applied with the honey super on; carry out treatments that pose a residue risk after the harvest.
6. Measure and record the effect of every treatment. Write down which hive received which substance and when, and the result.

Brief summary

• Do not treat without measuring; always measure again after treatment.
• Change the mode of action, not the product.
• Tau-fluvalinate and flumethrin are in the same group; using one instead of the other is not rotation.
• Organic acids and non-chemical methods are the foundation of resistance management.
• Do not use unauthorised mixtures; remove the strip when its period is over.