The late summer/autumn treatment likely helps to lower virus infection rates in the winter bees that are starting to be produced at that time and which will form the bulk of the colony in early spring. A winter treatment, when the colony is largely broodless, will severely knock-back the phoretic mites and give them a low base from which to multiply in the spring.

Unsuccessful late summer treatment would result in more infected bees, and an unhealthy colony, in the spring regardless of whether winter treatment had reduced phoretic mites. Both treatments are helpful but a successful late summer treatment seems essential. The method of treatment in both cases might be less important, provided it works.

I vaporise rather than trickle OA in winter as it seems both effective and apparently harmless. I have tried multiple OA vaporisations as a late summer treatment but other methods are more time-efficient. I'm pretty sure there is some adverse effect on young brood with multiple applications.
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