Fall vs summer varroa treatment: which one protects the winter bees

Ask a beekeeper when they treat for varroa and most will name a season. The better answer is a date tied to something the colony is doing: raising the bees that have to live through winter. Those bees are made in late summer, just as the mite population reaches its peak, and a treatment that finishes after they are raised protects a colony that is already damaged. That is the whole case for treating in late summer rather than in autumn, and it is why the colonies that die in February were usually decided in August.

Why the date matters more than the product

Varroa breeds inside capped brood, so the mite population grows with the colony through spring and summer. After midsummer the colony starts to shrink while the mites keep multiplying, and the number of mites per bee climbs steeply. At the same time the colony switches to raising winter bees: workers with larger fat bodies that live for months rather than weeks and carry the cluster through to spring.

A larva fed on by mites, and exposed to the deformed wing virus they spread, becomes a shorter-lived adult, and no treatment given afterwards repairs it. In the northern hemisphere most winter bees are raised from August to October; in the southern hemisphere, from February to April. The treatment has to be finished, not started, before most of them are capped.

The summer treatment, during the flow

Treating while honey supers are on is possible, but the choice is narrow. In many countries formic acid products are labelled for use with supers on, within a temperature range, because formic acid already occurs in honey at low levels. Most other treatments, including amitraz strips and thymol, must be used with the supers off. The label and your national regulator decide what is legal where you keep bees; a product approved in one country can be unlicensed in the next.

The other options during the flow are not chemical. Removing drone brood in spring and early summer takes out a share of the mites, which prefer drone cells. A brood break, made by caging the queen for about three weeks or by splitting the colony, leaves the mites nowhere to breed and exposes them to treatments such as oxalic acid that only reach mites on adult bees. Both cost honey, so they earn their place when a midsummer count says a colony will not reach the end of the flow in good shape.

The late-summer treatment: the one that counts

For most colonies the main treatment goes on as soon as the last supers come off. The products commonly used at this point each have a character worth knowing:

• Amitraz strips stay in for six to ten weeks depending on the product, work across several brood cycles and are little affected by temperature.
• Thymol gels and wafers work well in warm weather and poorly in cool weather, so they suit an early treatment rather than a late one.
• Formic acid reaches mites inside capped cells, which makes it fast, but its temperature window is narrow and a hot spell can harm the queen or the brood.
• Oxalic acid with brood present needs repeated applications across a brood cycle, because each dose reaches only the mites riding on adult bees.

Rotating between treatments with different active ingredients slows the resistance that has already made some older products unreliable. The varroa treatment guide covers doses and methods for each; the point here is the timing.

The fall treatment: better late than never

A colony that missed the late-summer window should still be treated in autumn. The mite load drops, and the colony goes into winter carrying fewer mites, which matters. What an autumn treatment cannot undo is the damage to the bees already raised. That is why a late-treated colony often looks strong in October, full of bees and with a good count after treatment, and dwindles to a handful by February: the bees it was relying on were short-lived from the start.

The midwinter oxalic dose

In most temperate climates colonies have a broodless spell in early winter. With no capped brood, every mite is on an adult bee, and a single oxalic acid treatment, trickled between the frames or vaporised, removes the great majority of them. It is the clean-up after both earlier windows, and it gives the colony a low starting count for spring. The dose, the method and the conditions it may be used in are on the label; the timing is a cold, still day when the bees are clustered and there is no brood.

Southern hemisphere dates

Everything above moves by six months. The late-summer treatment falls in February and March, the autumn one in April, and the broodless oxalic dose in June or July. Where varroa has arrived recently, the approved treatments and the rules around them are still changing, so check your state or national bee programme before choosing a product.

Counting instead of guessing

The only way to know whether a colony needs the summer treatment, and whether the late-summer one worked, is to count. An alcohol wash or sugar roll of about three hundred bees, roughly half a cup, taken from a brood frame gives mites per hundred bees. A common action threshold is around three per hundred in late summer and lower, one or two, in spring and early summer, when the colony has months of growth ahead; local guidance may differ.

Count before each window and again a couple of weeks after treating, and write the figures against the hive rather than the apiary: a single colony with a high count reinfests its neighbours as it collapses. Keeping those counts and the treatment dates together is what recurring treatment schedules are for, and it is how the treatment scheduler in BeehiveMind reminds you of the next dose before the window closes.