Varroa Threshold & Make the Treat-Now Call
Decides treat or monitor
Turn an alcohol-wash count into mites per 100 bees and check it against the season's treat-now threshold — about 3% in spring and summer but only 2% in late summer, when the winter bees are reared. See the verdict, projected growth and which treatment fits your conditions.
Enter your alcohol-wash count
Next: treat now with Formic acid (Formic Pro / MAQS) — it suits 24 °C. Re-test 2–4 weeks after treatment to confirm the knock-down; untreated this colony reaches ~192% in six months.
Infestation % = mites ÷ bees × 100 (alcohol wash on 300-bee sample). Seasonal thresholds and ~exponential growth after the Honey Bee Health Coalition "Tools for Varroa Management" and university extension. Treatment temperature/honey-flow limits from product labels. Planning estimate — always read the label.
| Treatment | Temp window | Fits now? |
|---|---|---|
| Formic acid (Formic Pro / MAQS) | 10–29 °C | Yes |
| Thymol (Apiguard / Api Life Var) | 15–40 °C | Yes |
| Oxalic acid (dribble / vapor) | 1–35 °C | only effective when broodless |
| Amitraz (Apivar strips) | 5–40 °C | Yes |
| Hop beta acids (HopGuard) | 10–35 °C | Yes |
Runs entirely in your browser — nothing is uploaded.
Varroa thresholds — key facts
- Infestation
- mites ÷ bees × 100
- Sample
- alcohol wash ~300 bees (½ cup)
- Spring/summer treat
- ≈ 3 mites / 100 bees
- Late summer treat
- ≈ 2 mites / 100 bees
- Critical (fall)
- ≈ 3 mites / 100 bees
- Growth
- ≈ 2–3× per month untreated
- Top winter loss cause
- varroa + viruses
- Privacy
- Runs in your browser; nothing uploaded
Seasonal varroa thresholds (mites per 100 bees)
| Season | Monitor below | Treat at | Critical | Growth/mo |
|---|---|---|---|---|
| Spring (build-up) | 1% | 3% | 5% | 2.5× |
| Summer (peak colony) | 2% | 3% | 5% | 2.8× |
| Late summer (winter-bee) | 1% | 2% | 3% | 2× |
| Fall (pre-winter) | 1% | 2% | 3% | 1.5× |
Source: Honey Bee Health Coalition, Tools for Varroa Management; university extension.
Treatment options and their condition windows
| Treatment | Temp °C | Supers on? | Notes |
|---|---|---|---|
| Formic acid (Formic Pro / MAQS) | 10–29 | Yes | Penetrates capped brood; can be used with supers on, but needs 10–29 °C. |
| Thymol (Apiguard / Api Life Var) | 15–40 | No | Needs warm temps (15 °C+); remove honey supers; can taint honey. |
| Oxalic acid (dribble / vapor) | 1–35 | No | Only kills phoretic mites — most effective when broodless (late fall/winter). |
| Amitraz (Apivar strips) | 5–40 | No | Effective across a wide temp range; 6-week treatment; no supers on. |
| Hop beta acids (HopGuard) | 10–35 | Yes | Contact miticide; best with low brood; usable during flow. |
Source: product labels; Honey Bee Health Coalition. Always read and follow the label.
Varroa is the decision that decides winter survival
Varroa destructor is the single biggest driver of honey-bee colony loss, mostly because the mites vector viruses that cripple the long-lived winter bees. The control decision hinges on one number — mites per 100 bees from an alcohol wash — measured against a threshold that is lower in late summer and fall than in spring. Treat above the line; monitor below it. Because mites grow exponentially, a count that is fine today can be dangerous a month later.
This tool converts your wash count, applies the correct seasonal threshold, projects the untreated growth toward the collapse line, and tells you which treatment actually fits your temperature and honey-flow situation. It closes the gap left by beekeeping feeding and profit tools, which never make the treat-now call. Pair it with the other livestock tools in the hub for whole-apiary and farm management.
How to use it — 5 steps
- 1Wash a sample. Alcohol-wash or sugar-roll a half-cup (~300 bees) and count the mites.
- 2Enter the count. Type the mites counted and the number of bees in the sample.
- 3Set the season. Choose the season so the correct treat-now threshold applies.
- 4Read the verdict. See mites per 100 bees against the treat-now and critical lines, plus growth.
- 5Pick a treatment. Enter temperature and honey-flow status to get a suitable treatment.
Frequently Asked Questions
What is the treatment threshold for varroa mites?+
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.
How do you calculate mites per 100 bees?+
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.
Why is the late-summer threshold lower?+
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.
Has my colony crossed the treat-now threshold?+
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.
How fast do varroa mites multiply?+
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.
Which varroa treatment should I use?+
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.
Can I treat varroa with honey supers on?+
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.
When is oxalic acid most effective?+
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.
What mite level kills a colony?+
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.
How often should I monitor varroa?+
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.
Is an alcohol wash better than a sugar roll?+
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.
How does this differ from a bee feeding or profit tool?+
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.