Varroa Mite Drop Calculator
Count the mites that fall onto the tray, enter how many days it was in, and see your daily mite fall against the action threshold for the season.
If you monitor varroa with a tray under a screened bottom board instead of a wash, count the mites that fall over several days and this works out the average fall per day. Enter your count and the number of days below to see it against the action threshold for the time of year.
Calculator
Your tray count
Time of year
The action threshold depends on the season. We have picked the one for today; change it if you are in the southern hemisphere or a different phase.
Daily mite fall
Mite fall
1.4/day
10 mites over 7 days
Act above
3/day
Summer
Start watching
At 1.4 mites a day, this colony is in the watch band for summer (1 to 3 a day). Count again in five to seven days and get ready to treat. Natural fall climbs fast once mites outpace the brood.
Natural mite fall is a rougher gauge than a wash. Use a waxed tray, count over five to seven days, and confirm a borderline number with an alcohol wash before treating.
Assumptions & Notes
- Thresholds follow the natural-mite-fall guideline widely taught in the Netherlands, Germany and Belgium (Wageningen University & Research, the Deutscher Imkerbund and Ghent University): act above roughly 1 mite a day in spring, 3 a day in summer, 5 a day in late summer, and 1 a day heading into and out of winter.
- Natural fall is an estimate. Colony size, capped brood, ants and the bees' own hygienic behaviour all move the number, so a hygienic colony can drop more mites at the same real infestation.
- Use a waxed or greased tray and count over five to seven days for a steady daily average.
- Confirm a borderline count with an alcohol wash, and treat only with a registered product that suits the season.
Mite fall thresholds by season
The action line moves through the year because the colony is smallest, and can least afford mites, going into and out of winter. Below the safe figure keep monitoring, in the middle band start preparing to treat, and above the top figure act.
| Season | When | Safe | Watch | Act |
|---|---|---|---|---|
| Spring | March to May | under 0.5/day | 0.5 to 1/day | over 1/day |
| Summer | June to July | under 1/day | 1 to 3/day | over 3/day |
| Late summer | August to September | under 2/day | 2 to 5/day | over 5/day |
| Late fall / winter | October to February | under 0.5/day | 0.5 to 1/day | over 1/day |
These are a widely used rule of thumb, not a legal standard, and they run lower than the North American wash thresholds because they measure a different thing. If you sample with an alcohol wash instead, use the mite wash calculator, which works in mites per 100 bees against the Honey Bee Health Coalition thresholds. Whichever you use, treat the number as a prompt to act early rather than a line to cross.
Frequently Asked Questions
What is a natural mite drop count?
It is the number of varroa mites that fall on their own onto a tray under a screened bottom board over a set period. Divide the count by the number of days and you get the average mite fall per day, which you compare to a seasonal threshold. It does not kill any bees, unlike an alcohol wash.
How many mites per day is too many?
A common European guideline is to act above about 1 mite a day in spring, 3 a day in summer, and 5 a day in late summer, with the winter and early-spring line back down near 1. Below those you keep watching. The thresholds are lower going into and out of winter because the colony is small and can afford far fewer mites.
Is a mite drop count as good as an alcohol wash?
No, it is rougher. The number that falls depends on colony size, how much brood is capped, and whether ants or the bees clear the tray, so two hives at the same real infestation can drop different counts. Use a waxed tray, leave it five to seven days for a steady average, and confirm a borderline count with an alcohol wash before you treat.
How long should I leave the tray in?
Five to seven days gives a steadier daily average than a single day, which can be thrown off by weather or a batch of brood emerging. Wax or lightly grease the tray so mites stick and ants cannot rob them, and clear it before you start counting.
A hygienic colony dropped a lot of mites. Is that bad?
Not necessarily. A colony with strong grooming or hygienic behaviour actively knocks mites down, so it can show a higher natural drop than a less resistant colony at the same infestation. Read the drop alongside what you see in the brood, and confirm with a wash if the number surprises you.