How amitraz works
Apivar strips deliver the active ingredient by contact. The amitraz sits on the strip surface, and bees pick it up as they walk across it and carry it through the colony. It does not vaporize and it does not pass through the brood cappings, so it cannot hit mites sealed in brood directly. Instead it stays in the hive long enough to catch those mites as they emerge with the new bees. That is why the treatment runs for weeks.
The long window is the mechanism
Apivar goes on for about 42 to 56 days, six to eight weeks. Because it works by contact over time rather than in one hit, it needs to span a full brood cycle so every mite that was hiding under a capping eventually walks across a strip. Pull the strips early and you leave survivors. Leave them in past the window and you feed low-dose exposure that speeds resistance. Place two strips in the brood nest where the bees are thickest, and set a reminder to remove them on time.
Temperature and supers
Amitraz tolerates a wider temperature range than formic or thymol, which is a big part of its appeal in the fall. It works best around 65 to 85 F (18 to 30 C) and falls off in the cold, below roughly 45 F (7 C), where the bees stop moving across the strips. It is not a treatment for honey supers. Take the supers off first, do not use it if you will super within about eight weeks, and wait the labeled interval, commonly two weeks, after pulling the strips before you put supers back on. Amitraz leaves residue in wax, so keeping it clear of honey comb is the whole reason for those rules.
Resistance is the catch
Amitraz has carried commercial beekeeping for twenty years, and that success is now its weakness. In areas where it has been used alone season after season, mites that survive it are becoming common, and a treatment that once cleared the great majority of mites can suddenly underperform. You will not see it coming without a count. This is the strongest argument in all of varroa management for rotating treatment families and for testing after every treatment.
Where amitraz fits
Its home is the post-harvest treatment in late summer or early fall. Supers are off, the temperatures still suit it, and the long window carries the colony through the tail of mite season while the winter bees are raised. Pair it across the year with a broodless oxalic acid treatment in late fall and you have covered two seasons with two different modes of action, which is exactly the rotation that keeps amitraz working. If you would rather stay off synthetics, the essential-oil and acid routes in the which treatment guide cover the same slot.
Confirm it worked
Because resistance is the real risk with amitraz, the follow-up count matters even more here than usual. A mite wash a couple of weeks after the strips come out tells you whether the treatment did its job or whether your mites are shrugging it off. Finding resistance in September gives you time to hit the colony with something else. Finding it as a dead-out in February does not.
Sources and further reading
Follow your product label and local regulations. The guidance here follows the sources below.
- Honey Bee Health Coalition, Tools for Varroa Management (9th edition), for timing and super rules.
- Randy Oliver, Scientific Beekeeping: Questions on Amitraz, on how it works and the resistance picture.
- University of Guelph Honey Bee Research Centre, for method demonstrations.
Record the treatment and both counts in your hive records. A rising after-count over successive years is the early warning of resistance in your yard.