Why the day you treat matters
Oxalic acid kills varroa riding on adult bees. It does not reach mites sealed under cappings, and while a colony is raising brood most of its mites are in there. A brood break gives you a few days when every mite is out in the open.
| Cell capped | Adult emerges | |
|---|---|---|
| Worker | day 8 to 9 | day 21 |
| Drone | day 10 | day 24 |
| Queen | day 8 | day 16 |
Day 0 is the last day eggs were laid: the day you cage the queen, split, or remove her. The window opens once the last capped brood has emerged and closes the day before the first new brood could be capped. A new queen spends 5 to 8 days in the hive before she flies to mate and starts laying a few days later. The planner uses the earliest likely dates, so a slow queen only gives you more time.
Example: cage the queen on June 1 for 25 days. Worker brood is out by June 22 and drones by June 25. Treat on June 26 and let her out. Her first eggs are capped from about July 4.
How each method breaks the brood
- Caging the queen keeps the colony queenright and you choose the length. A 24 or 25 day cage covers drone brood too. Berry and colleagues (2023) confined the queen for 14 days and treated on day 21.
- The queenless half of a split goes broodless while it raises a queen, usually from about day 21 until a few days later.
- A ripe queen cell added on the day you remove the old queen leaves a day or two at most, sometimes none. Adding it a week or more later opens a real gap.
- After a swarm, the parent hive has a short gap around day 21. The swarm itself has no capped brood for its first week in a new box.
- In late autumn and winter, an inspection with no eggs or open larvae means the capped brood is out within about two weeks.
A break costs a round of brood, so do it while the colony is strong. In a Florida trial, caging queens for 24 days in autumn weakened colonies and hurt their survival (Jack and colleagues, 2020). In summer trials across Europe, caging plus one oxalic acid treatment while broodless removed between about half and 90% of mites, depending on how it was applied (Büchler and colleagues, 2020).
Questions beekeepers ask
How long should I cage the queen for a brood break?
Long enough for the capped brood to emerge: 21 days for workers, 24 if there is drone brood. Many beekeepers cage for 24 or 25 days and treat on release day. A 14-day cage with treatment on day 21 also works for worker brood, because her new eggs are not capped yet.
When do I treat with oxalic acid after caging the queen?
After the last capped brood has emerged and before her new brood is capped. With a 25-day cage, treat on day 25 and release her the same day. If you release her earlier, you have about a week from release before her first eggs are capped.
How do I calculate broodless days after a split?
Count from the day of the split, when the queenless half had its last eggs. Worker brood is out by day 21, drones by day 24. Their new queen emerges around day 12 and usually lays 10 to 14 days later, so the gap runs from about day 21 to day 25 or a little longer.
Which dates should I check against my colony?
Three are typical figures. A virgin queen can lay 8 days after emerging, but 10 to 14 is usual and bad weather adds weeks, so using 8 errs early. A swarm usually leaves as the first cells are capped, so a virgin emerges about day 7. The after wash sits two weeks out because results take that long to show.
How often can I treat with oxalic acid?
The product label decides that, along with your local rules. The planner places one treatment inside the broodless window, which is how the studies cited below used it.
Do I need to wait for the drone brood?
Varroa prefer drone cells, so mites in capped drone brood survive a treatment on day 21. Wait until day 24 or remove the capped drone comb first. Untick drone brood in the planner when there is none, as in most winter colonies.
Should I treat a swarm with oxalic acid?
A newly hived swarm has no capped brood for about a week, which makes it a good time if your label allows it. Give the bees a couple of days to settle first. The hive they left gets a shorter gap around day 21.
Will my mite wash read higher during a brood break?
Yes. With no brood, every mite is on the adult bees, so an alcohol wash or sugar roll finds more per 100 bees than it did before. Compare counts taken at the same stage.
Sources: UF/IFAS Extension, European honey bee; Penn State Extension, queen development; WSU Extension FS403E, oxalic acid options; Büchler et al. 2020; Berry et al. 2023; Jack et al. 2020; Randy Oliver, Scientific Beekeeping.