Oxalic vapour mid-summer with brood in the hive — why did one round do nothing?
Quebec, five years, six colonies. Alcohol wash on my worst hive came back at 11 per 300 last week — way over threshold, can't wait for fall. I vaporised oxalic once, waited five days, washed again: 9 per 300. Barely moved. I thought OA was supposed to hammer them?
Reading afterwards (yes, wrong order, I know) it seems the problem is brood: OA only kills mites riding on adult bees, and with brood present something like 70–80% of the mites are sealed under cappings where the vapour can't touch them. So a single round mostly kills the phoretic minority and the nursery keeps restocking.
So what's the actual play in July? I keep seeing 'three rounds five days apart' to catch mites as they emerge — does that genuinely work, or am I better off with formic (which does penetrate cappings) or caging the queen to force a broodless window?
#oxalic-acid#varroa#brood#treatment
▲ 18 votes💬 4 replies
4 replies
MieleMarco · Jul 24, 2026
You diagnosed it exactly — the capped brood is a mite bunker and one puff of OA cannot reach it. The 3x5-day series works on the logic that mites emerging between rounds get caught before they re-enter cells, but with 12 days of capped worker brood always present, 3 rounds still leaves gaps. Some of us run 4–5 rounds at 4–5 day intervals for that reason. It knocks mites down; it does not knock them out.
ImkerinLena · Jul 25, 2026
At 11 per 300 in July I would not rely on repeated OA alone. Formic is the only common treatment that reaches mites under cappings — Formic Pro works with supers on, but mind the temperature limits (daily highs under about 29C) and expect some brood loss. Alternatively caging the queen 24–25 days then a single OA once all brood has emerged is extremely effective, at the price of a brood break. In your position, with your temperatures, I take the formic.
DrentseBij · Jul 25, 2026
Just to defend the OA series for others reading: for a colony NEAR threshold it's a fine summer holding action, cheap and gentle on the queen. QuebecBees's hive is past that. Also check your rig actually sublimates fully — a scorched pan or leaky seal quietly halves the dose and people blame the chemistry.
QuebecBees · Jul 25, 2026
Thanks all — forecast shows highs of 24–26C for the next two weeks so Formic Pro is going on this weekend, and I'll re-wash 2 weeks after it comes off. Keeping queen caging in the back pocket for a future year, it appeals to the part of me that likes elegant solutions and hates buying things.