Flumethrin Treatment

Flumethrin is a synthetic acaricide from the pyrethroid group; in beekeeping it is used against varroa in the form of authorised plastic strips hung in the hive. It is effective when applied at the right time and for the period stated on the label; however, repeated use year after year creates resistant mite populations, and the active substance can accumulate in wax. It is not used while supers are on, and flumethrin products made for other animals must never be put into a hive.

What is flumethrin and how does it work?

Flumethrin is a synthetic pyrethroid that affects the mite's nervous system. The substance in the strip hung in the hive is spread through the colony by the bees that touch the strip; the mite is affected when it comes into contact with the bees.

• Long-term release: The strip stays in the hive for the number of weeks stated on the label. As a result, mites emerging from capped brood also come into contact with the substance over time; it can also be used while brood is present.
• It kills the mites on the bees: It does not reach mites inside capped cells directly; its effect appears as the mites emerge from the cells.
• A substance from the same group: Tau-fluvalinate is also a pyrethroid. There is cross-resistance between these two substances; a mite resistant to one is usually resistant to the other too.

Common errors in older information

• "Liquid flumethrin is poured into the hive and vaporised": Flumethrin products authorised for beekeeping come in strip form. Using pour-on or dip flumethrin products made for cattle, sheep and goats in the hive is prohibited; the dose is uncontrolled, it can kill the bees and it leaves heavy residues in honey and wax.
• "It is repeated every 1–2 months": Wrong. The strip is applied once for the period stated on the label and then removed. Frequent repetition is the fastest route to resistance.
• "Leaving it in longer gives better protection": A strip whose active period has ended keeps releasing the substance at a low dose and so selects for resistant mites. When the period is up, the strip must be removed.

When is it used?

• Late summer, immediately after the honey harvest: Once the supers have been removed, to reduce the mite load before the winter bees are reared. This is the main treatment period of the year.
• In spring, before the supers go on: If a count shows a high mite load and the label permits use in this period.
• Based on counts: The decision to treat should be based on the result of an alcohol wash, icing sugar shake or mite drop tray.

Do not use the same active substance every year. Rotating flumethrin from year to year with a medicine from a different group (e.g. amitraz) and with organic acids delays resistance.

Step-by-step application

1. Remove the supers. Strips are not hung during a nectar flow or while supers are on.
2. Check the product: Make sure it is authorised for bees in your country, that it is within its expiry date and that the packaging is intact.
3. Put on gloves. Do not handle the strip with bare hands.
4. Decide on the number of strips: Use the number of strips stated on the label for the strength of the colony; do not cut the strips or split them in two.
5. Hang them in the right place: The strips are hung between two frames in the brood area, where the bees are most concentrated. A strip hung in the empty spaces between outer frames does not have enough contact with the bees.
6. Note the date and write the day the strip is to be removed on the hive or in your record book.
7. Remove it when the period is up. Do not reuse used strips; dispose of them together with their packaging as stated on the label.

Resistance: how to tell

Resistance to pyrethroids is widespread in many parts of the world; there are also studies reporting resistance from Türkiye. In a resistant population the mite load does not fall even though the strip is in place.

• Put a mite drop tray in place when you hang the strip; a marked mite drop is expected in the first few days.
• After removing the strip, count again with an alcohol wash or icing sugar shake.
• If the load remains high despite treatment and there has been no application error, resistance should be suspected. In that case do not try the same group (flumethrin, tau-fluvalinate) again; switch to a method from a different group.

Residues and honey safety

• Flumethrin is fat-soluble; it can accumulate in beeswax. Regularly renewing old, dark combs reduces this build-up.
• Combs that were in the hive during treatment are not used for table honey; observe the withdrawal period on the label.
• More strips than recommended, excessive treatment periods and unauthorised products are the main causes of residue problems.

Common mistakes

• Hanging the same strip every year out of habit, without counting.
• Cutting the strip to "make it go further", or re-hanging a used strip.
• Hanging the strip on the outer frames, where there are few bees.
• Forgetting the strip in the hive.
• Not measuring the result after treatment.