Varroa reproduces only inside sealed brood. With no brood the mite cannot reproduce and is left exposed on the bees. Brood interruption uses this simple fact: by leaving the colony temporarily broodless it both stops the mite reproducing and makes it defenceless.

It is the most powerful method of lowering mite numbers without chemicals, and it carries no risk of resistance.

Why it is so effective

In a normal colony most mites are inside sealed brood, where contact treatments cannot reach them. In a broodless period almost all of them are on the bees.

Brood interruption therefore works on its own, but its real power appears when combined with a treatment applied at that moment. A contact substance such as oxalic acid, applied to a broodless colony, reaches nearly the entire population.

Caging the queen

The queen is confined in a cage that prevents her laying. One waits until all existing brood has emerged; since worker development takes about three weeks, the colony is broodless at the end of that period.

Treatment is applied just before the cage is opened. When the queen is released laying resumes.

Advantage: The queen is not lost and the colony is not divided.
Cost: Brood production stops for about a month; colony strength falls back.

Trap comb method

The queen is confined to a single comb with an excluder. When that comb fills and is capped, most of the mites have entered it; the comb is removed and destroyed. The queen is moved to a fresh comb and the process is repeated several times.

It works on the principle of collecting the mites and taking them out.

Advantage: The colony is not left entirely broodless; development is less interrupted.
Cost: The timing must be exact; if the comb is left after capping the mites emerge and the method backfires.

Artificial swarm

The bees are taken from the existing combs and transferred to an empty hive containing only foundation. The brood stays behind, and with it most of the mites.

Treatment is applied immediately afterwards, while the colony is still broodless.

Advantage: A very high mite reduction, and at the same time an opportunity for comb renewal.
Cost: The colony starts from scratch and must be fed. Applied late in the season it cannot recover.

Interruption through splitting

When a colony is split, the queenless part experiences a natural brood gap until a new queen mates. That gap is a favourable window for treatment.

Swarm management and mite control can be combined in a single operation.

Timing

Interruption is usually applied in midsummer, after the main flow has ended. There are two reasons: the harvest is unaffected, and the mite level is brought down before the winter bees are reared.

Left too late, the colony cannot go into winter strong enough. Done too early, the mites build up again before the end of the season.

Points to watch

• Measure the mite level before and after. There is no other way to know whether the method worked
• Do not apply it to weak colonies; they cannot recover
• The colony is open to robbing during interruption. Reduce the entrance
• Make sure the cage is of the right type; the wrong cage costs the queen