Varroa Threshold & Make the Treat-Now Call

It is seasonal. A common rule is to treat at about 3 mites per 100 bees (3%) in spring and summer, but to act at a lower 2% in late summer and fall, because that is when the colony rears the winter bees that must survive until spring. Above the season's threshold you treat now; below it you keep monitoring.

Take an alcohol wash or sugar roll of a half-cup of bees (about 300 bees), count the mites that drop, and divide by the number of bees, then multiply by 100. For example, 9 mites in 300 bees is 9 ÷ 300 × 100 = 3.0 mites per 100 bees, which is at the spring/summer treat line.

Late summer is when colonies raise the long-lived winter bees that carry the colony through to spring. Mite-borne viruses (especially deformed wing virus) damage these bees most, so a mite load that would be tolerable in spring can doom the winter cluster. Lowering the threshold to about 2% in late summer protects winter survival.

Enter your wash count (mites and bees in the sample) and the season. The tool computes mites per 100 bees and compares it to that season's monitor, treat and critical bands, returning a monitor, treat-now or critical verdict. It also projects how the load grows if you do nothing.

Roughly two to three-fold per month during the active brood-rearing season, because mites reproduce inside capped brood. Starting from 3% in summer at about 2.8× per month, an untreated colony can reach the collapse-risk range within a couple of months — which is why crossing the threshold calls for prompt action, not a wait-and-see approach.

It depends on temperature and whether honey supers are on. Formic acid (Formic Pro) works 10–29 °C and can be used with supers on; thymol (Apiguard) needs 15 °C+ and no supers; amitraz (Apivar) covers a wide temperature range but no supers; and oxalic acid is best in a broodless period since it only kills phoretic mites. The tool flags which options fit your current conditions.

Only with products labelled for it — chiefly formic acid (Formic Pro/MAQS) and hop beta acids (HopGuard) — because most miticides can contaminate the honey crop. Thymol, amitraz strips and oxalic acid are used when supers are off. If your sample shows you must treat during a flow, the calculator highlights the super-safe options.

Oxalic acid only kills the phoretic mites riding on adult bees, not those sealed in capped brood, so it works best when the colony is broodless — typically late fall or early winter, or after a brood break. A single broodless oxalic treatment can knock down a large share of the mite population before the winter cluster forms.

Sustained loads above roughly 5 mites per 100 bees (and the season-specific critical level, which is as low as 3% in fall) put a colony at high risk of collapse over winter from mite-vectored viruses. Many untreated colonies that reach these levels in autumn die before spring even though they look populous in late summer.

At least monthly through the active season, and especially before and after any treatment to confirm the knock-down. Regular alcohol washes give the trend, which matters more than a single reading. Because mites grow exponentially, catching a rising count early lets you treat at a lower, safer level.

An alcohol wash is more accurate and consistent because it dislodges nearly all the mites, but it kills the sample of bees. A sugar roll is non-lethal and useful for frequent checks, though it can undercount. Either way, use a ~300-bee (half-cup) sample and convert to mites per 100 bees for the threshold comparison.

Feeding and profit tools size syrup or value honey; this tool answers the single most important colony-health question — has the varroa load crossed the season-specific treatment threshold, and which treatment fits the conditions. Varroa is the leading cause of winter colony loss, so the treat-now decision is the one that most affects survival.