Resistance is the loss of sensitivity of a mite population to an active substance. As a chemical is applied, susceptible mites die and hardy ones survive and reproduce. After a few seasons most of the population is resistant and the product stops working.

This is not a product defect but the result of selection. The more we use the same product, the faster we make it useless.

Where it is seen

Resistance has been documented in many parts of the world. The most frequently reported groups:

• Amitraz — reports are increasing with widespread use
• Pyrethroids (flumethrin, tau-fluvalinate) — long-known cases of resistance
• Coumaphos — has lost effectiveness in many regions

By contrast, no established resistance has been reported in organic acids (formic, oxalic, lactic). Because they act on the mite by chemical or physical means, resistance is not expected.

How it is detected

The only reliable sign of resistance is the treatment failing. The only way to see that is to measure.

Measure the mite level before and after treatment. If the expected drop does not occur, there are three possibilities: wrong dose, wrong timing or resistance. Check all three.

A telling sign: very few mites falling onto the floor tray after treatment while the colony clearly appears infested. That means the mites are not dying.

Mistakes that accelerate resistance

• Using the same active substance for years. This is the principal cause
• Underdosing. A low dose kills susceptible mites and leaves partly hardy ones alive, increasing selection pressure
• Leaving strips in beyond their period. A spent strip means a constant low dose — the most favourable condition for resistance
• Preparing unlicensed mixtures. The dose is uncontrolled, residues are a risk and resistance is fed

Resistance management

• Rotate by mode of action. Changing the brand is not enough; the active substance and its mode of action must change
• Include organic acids in the programme. Carrying no resistance risk, they are the solid leg of the rotation
• Use non-chemical methods. Brood interruption and drone brood removal lower mite numbers without chemical pressure
• Measure efficacy after every treatment. There is no other way to catch resistance early
• Apply the dose and duration on the label. Both too little and too much do harm

The regional dimension

Resistance is not the problem of a single apiary. Mites pass between colonies through robbing and drifting; neighbouring apiaries share the same mite pool. If everyone in an area uses the same product, resistance develops regionally.

This is why coordination at association level is valuable: when it is shared which products no longer work in an area, everyone gains.

Summary

An acaricide is a finite resource. The more carefully it is used, the longer it lasts. Treating without measuring spends not only money but the tool itself.