What Varroa Mites Do to a Colony

Varroa destructor is an external parasite that feeds on the fat bodies of both adult bees and developing pupae. The damage is twofold: the physical wound provides an entry point for viruses, and the deformed wing virus (DWV) transmitted during feeding is directly responsible for the shrunken, flightless bees that appear when mite loads are high.

A colony can tolerate a low mite population through the summer. The problem compounds in late summer and autumn when the bee population declines but the mite population does not shrink at the same rate. By the time a colony enters winter, a high mite load means the overwintering bees were reared under heavy viral pressure — and those bees won't survive long enough to raise spring brood.

How to Count Mites: The Alcohol Wash Method

The alcohol wash is the most reliable method for quantifying mite levels in a colony. It requires about 300 adult bees (roughly half a cup), 70% isopropyl alcohol, and a jar with a mesh lid or two-part lid.

  1. Find a frame with capped worker brood and nurse bees clustered on it. The nurse bees are more likely to be hosting mites than foragers.
  2. Scoop approximately 300 bees into the jar. You do not need an exact count — a half-cup measure is a close enough standard.
  3. Add enough 70% isopropyl alcohol to cover the bees. Seal the jar and shake firmly for 60 seconds.
  4. Pour the liquid through the mesh lid into a white container. Count the mites in the strained liquid.
  5. Divide the mite count by 3 (since you used 300 bees) to get the percentage of infestation.
Interpreting results: A count of 6 mites from 300 bees equals 2% infestation. The generally accepted Canadian threshold for treatment during the brood season is 2–3%. Above 3%, treatment is considered urgent. Below 1% during peak summer may not require immediate action but should be re-checked in 3–4 weeks.

The Sticky Board Method

A sticky board placed under a screened bottom board measures natural mite fall over 24–72 hours. While less precise than an alcohol wash, it's useful for establishing trends over time and requires no bee sacrifice.

A rough rule of thumb used by many Canadian beekeepers: a daily mite drop above 10 during the brood season suggests a count worth acting on. However, sticky board counts are sensitive to colony size, foraging activity, and whether there is capped brood, so they work best as part of a pattern of observations rather than a one-time reading.

When to Count

Most experienced beekeepers count mites three times per season:

  • May: Establishes a spring baseline before the colony builds rapidly. Mite populations are typically low at this point.
  • Late July: The most critical count. If the colony is going to be treated before the overwintering bee population is reared, this is the window. Treatment by early August gives the colony time to raise a healthy fall generation under low mite pressure.
  • September: A final check to confirm the fall population is heading into winter under acceptable mite levels.

Treatments Registered in Canada

Health Canada's Pest Management Regulatory Agency (PMRA) regulates which varroa treatments are available in Canada. The landscape shifts periodically, so checking current registration status with your provincial apiarist or the CFIA is worthwhile before purchasing. As of 2026, the most commonly used treatments in Canadian backyard apiaries are:

Oxalic Acid (OA)

Oxalic acid is a naturally occurring compound found in rhubarb and many common plants. It is effective against phoretic mites — mites on adult bees — but does not penetrate capped brood cells. This makes it most powerful when used during broodless periods: the late autumn application after colonies have stopped rearing brood, or a brief broodless window created by caging the queen in early spring.

Two application methods are available in Canada:

  • Dribble: A 3.5% OA solution in 1:1 sugar syrup is dribbled directly onto bees in each occupied interframe space. Effective but stressful if the cluster is large.
  • Vaporization: Sublimated OA vapour is introduced into the hive via a specialized vaporizer. Results in better contact with the entire cluster and is the preferred method for treating colonies with large populations. Requires respiratory protection.

Oxalic acid treatment in late October or early November — when brood is minimal — consistently achieves 90%+ efficacy in published trials. A single treatment is generally sufficient if applied during a true broodless period.

Formic Acid (MAQS, Mite Away Quick Strips)

Formic acid is one of the few treatments that penetrates capped brood cells, reaching mites in the reproductive phase. This makes it valuable during the active season when brood is always present. MAQS (Mite Away Quick Strips) is the registered Canadian product delivering formic acid vapour over a 7-day period.

Temperature constraints are significant. MAQS should not be applied when ambient temperatures are consistently below 10°C or above 29°C. In practice, this means the optimal application windows in most of Canada are May–June and August–September. Applying MAQS during a heat wave risks queen mortality and significant brood loss.

MAQS causes some brood and adult bee mortality even under ideal conditions — this is expected and generally tolerable. The net effect is still a significant reduction in mite population.

Amitraz (Apivar)

Apivar strips contain amitraz, a synthetic acaricide. They are effective across a wider temperature range than formic acid and require no special applicator equipment. Strips are placed between frames and left for 6–8 weeks. Miticide resistance is a documented concern with amitraz, so rotating treatment classes over time is advised. Honey supers must be removed before application.

Integrated Pest Management: Beyond Chemicals

Treatment alone rarely results in a long-term low mite load without supporting practices. Several management approaches reduce varroa population growth without relying on chemicals:

  • Drone brood removal: Varroa mites preferentially reproduce in drone cells. Removing capped drone brood on a frame (by placing a shallow drone-comb frame in the brood box and freezing it when capped) removes a disproportionate number of mites from the colony.
  • Brood breaks: Splitting a colony to create a temporary queenless period halts brood production and allows an OA treatment to reach all phoretic mites.
  • Screened bottom boards: Allow a portion of falling mites to exit the hive permanently. The effect is modest but cumulative over a season.
  • Hygienic behaviour: Selecting locally adapted stock with documented hygienic behaviour (the ability to detect and remove mite-infested pupae) reduces mite reproductive success over time.

Record Keeping

Keeping a simple log of mite counts by colony and date makes it considerably easier to spot trends, identify problem colonies early, and assess whether a treatment worked as expected. A spreadsheet with date, colony ID, method, mite count, and action taken is sufficient for most backyard operations.

"A single mite count in July tells you where you are. A count in August tells you whether your July decision was right."

Further Reading

The Canadian Honey Council publishes provincial guidelines on varroa management. Your provincial apiarist's office is the best source for current product registration and local resistance patterns.

Related articles: Seasonal hive management covers when in the calendar these monitoring steps fit. Honey extraction addresses timing the harvest around treatment windows.