Flumethrin is a synthetic acaricide used against Varroa, applied as impregnated strips hung between the frames.

How it works

The strips release the active substance slowly. Bees walking over them distribute it through the colony by contact.

Because the release continues for weeks, the treatment spans several brood cycles and reaches mites as they emerge from cells.

Advantages

• Effective where no resistance has developed
• Simple to apply
• Not temperature-dependent, unlike formic acid
• Works with brood present, because of its duration

The two serious drawbacks

Resistance

Resistance to synthetic acaricides has developed in mite populations in many regions. Once present, it does not go away.

The commonest causes are underdosing and leaving strips in too long: towards the end the release falls to a sublethal level, which is exactly the condition that selects for resistance.

Residues in wax

Flumethrin is fat-soluble and accumulates in wax. It returns to the hive through foundation made from collected wax — affecting even beekeepers who treat carefully themselves.

Keeping cappings wax separate and having your own foundation made from it limits this.

Rules that must not be bent

• Remove the strips at the prescribed time. This is the single most important rule
• Use the prescribed number of strips for the colony size
• Do not treat with supers on
• Observe the withdrawal period
• Record product, date, dose and colonies treated

Rotation

Rotate the mode of action, not the brand name. Two products working the same way are not an alternation.

Building non-chemical methods into the programme reduces how often any product is needed, which is what slows resistance.

Measuring afterwards

This is how resistance is detected early. A treatment that once worked and now does not is the first signal — and without measurement it goes unnoticed until it costs a season.