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Varroa mite threshold calculator

Enter the mites you counted and the size of your sample. The calculator gives mites per 100 bees and compares it with the threshold the Honey Bee Health Coalition publishes for that point in the colony’s year.

Close-up of an adult female varroa mite
Photo: Gilles San Martin from Namur, Belgium, CC BY-SA 2.0, via Wikimedia Commons
Count every mite that falls out of the wash or roll.
300 bees ≈ ½ cup, lightly packed (HBHC).
Sampling method
Colony phase
Mites per 100 bees—
—

mites ÷ bees × 100

Thresholds: Honey Bee Health Coalition, Tools for Varroa Management, 9th edition (June 11, 2026), Table 1. Treatments: only EPA-registered varroa products, used exactly as the label says.

Treat only with EPA-registered products, exactly as the label says. The label is the law: it sets the dose, the temperature range and whether honey supers may stay on. Industrial or “wood bleach” oxalic acid is not a registered hive treatment. Thresholds can vary by region, so check with your state apiarist or local association.

How the calculator works

Mites per 100 bees is simply the mites you counted divided by the bees in the sample, times 100. HBHC’s own example: 12 mites in a 300-bee sample is 0.04 mites per bee, or 4 mites per 100 bees.

The threshold you compare against depends on the colony phase. HBHC’s 9th edition (June 11, 2026) sets a lower action level in the dormant and population-increase phases, when the colony is small and every mite does more damage.

Varroa thresholds by colony phase (mites per 100 adult bees), HBHC 9th edition

Colony phaseImmediate control not neededPromptly control
Dormant
Little or no brood; winter cluster (sample just before clustering)		✓ below 1%✗ 1% or more
Population increase
Late winter to spring build-up, brood expanding		✓ below 1%✗ 1% or more
Peak population
Summer, colony at full size, supers often on		✓ below 2%✗ 2% or more
Population decrease
Late summer to fall, brood rearing slowing		✓ below 2%✗ 2% or more

Source: Honey Bee Health Coalition, Tools for Varroa Management, 9th edition (June 11, 2026), Table 1. Thresholds can vary by region; ask your state apiarist.

What changed from the 8th editionThe 2022 guide used roughly 2–3% as the action level during population increase and decrease, and 3% at peak. Many websites and older extension calendars still quote “3%”. The 9th edition lowered the levels to 1% (dormant, population increase) and 2% (peak, population decrease). The calculator shows the older number in small print so you can compare.Source: HBHC Tools for Varroa Management, 9th edition, Table 1

Taking a sample that means something

  1. Pick the right frame

    Take bees from one to three frames in the brood nest, where mites concentrate. Find the queen first, or at least make sure she is not on the frame you shake.

  2. Measure about 300 bees

    HBHC equates 300 adult bees to about ½ cup, lightly packed. Fewer than 300 bees will not give an accurate result, and the calculator warns you when the sample is small.

  3. Wash or roll

    An alcohol or soap wash kills the sample but dislodges mites most reliably; HBHC lists it as the recommended method. A powdered-sugar roll returns the bees to the hive. Shake for the time your kit’s instructions give, then count every mite.

  4. Repeat on a schedule

    HBHC advises re-sampling monthly from the population-increase phase, or at least four times between peak population and decline, and more often during decrease.

Hive inspection logRecord each count with the date so you can see the trend, not just one number.

If the count is over the threshold

HBHC’s wording for a result at or above the threshold is “promptly control”. The guide’s product tables sort registered treatments by phase and by whether honey supers are on, because many products cannot be used while you are collecting honey for people to eat.

  • Check the EPA list of registered varroa products and read the label of any product before you buy it.
  • Follow label temperatures; some treatments are ineffective or harm brood outside their range.
  • Wear the protective equipment the label lists. HBHC calls for goggles or a face shield and a respirator for every oxalic acid method.
  • Re-test after the treatment period to confirm it worked.
Not a registered treatmentOxalic acid sold as wood bleach, deck cleaner or “for bees” crystals without an EPA registration number is not a legal hive treatment, and dosing it yourself is off-label use. Use registered oxalic acid products only, per their label.Source: EPA advisory on substances used to control varroa

Questions

How many mites is too many?
By HBHC’s 9th edition, 1 or more mites per 100 bees in the dormant and population-increase phases, and 2 or more during peak population and population decrease, call for prompt control.
Is 3% still the varroa threshold?
Not in the current HBHC guide. The 3% figure comes from older guidance, including the 8th edition’s peak-population level. The 9th edition (June 2026) lowered thresholds to 1–2% depending on phase.
Can I use fewer than 300 bees?
You can count them, but HBHC says fewer than 300 bees will not give an accurate sample. Half a cup of lightly packed bees is about 300.
Is a sugar roll as good as an alcohol wash?
HBHC lists the alcohol or soap wash as the recommended method. A sugar roll keeps the bees alive and is still useful; if the result is close to the threshold, confirm with a wash.
When should I test for mites?
Monthly from spring build-up through fall, and just before the colony clusters for winter. Our seasonal calendar marks mite checks month by month for each region.

Sources

  1. Honey Bee Health Coalition: Tools for Varroa Management, 9th edition (June 11, 2026)
  2. Honey Bee Health Coalition: Varroa management resources
  3. Honey Bee Health Coalition: Tools for Varroa Management, 8th edition (2022)
  4. EPA: Registered pesticide products approved for use against varroa mites in bee hives
  5. EPA: Advisory on FIFRA and FFDCA applicability to substances used to control varroa mites
  6. Mississippi State University Extension P4049: Sampling for varroa mites using an alcohol wash