The tracheal mite is a microscopic mite living inside the breathing tubes of adult bees. It was once a serious problem in some regions; today its economic importance has declined in many places.

Where it lives

The bee has no lungs. Air enters through openings along the body and reaches the tissues through a network of fine tubes.

The mite lives inside those tubes, feeds there and reproduces there. Because it never leaves the bee it cannot be seen without dissection.

The critical detail: bee age

The mite can only enter very young bees, within the first few days after emergence. After that the openings narrow and entry is no longer possible.

This single fact explains the disease's dynamics: infestation depends on how many young bees are available, and the population rises through winter when the colony is not rearing brood.

The damage

• Breathing is impaired and flight capacity falls
• Lifespan is shortened
• Overwintering suffers; the colony dwindles in late winter
• Bees crawl in front of the hive, unable to fly

Signs — and their limits

The signs are not distinguishing. Crawling bees unable to fly occur in chronic bee paralysis, in poisoning and in nosema as well.

Wings held at odd angles are sometimes cited as a sign but are not reliable.

Diagnosis requires microscopy. Around thirty bees are sampled and the thorax dissected; the tubes of an infested bee are darkened rather than clear.

Why it declined

Two reasons are generally given. Selection: colonies with resistance survived and passed it on; grooming behaviour appears to play a part. And treatment: substances used against Varroa, formic acid in particular, also act on this mite.

The second means many beekeepers are treating it without knowing they are.

What to do

• Formic acid is effective, being a vapour that reaches the tubes
• Requeening: Differences in resistance between lines are reported
• Keep colonies strong; reduce stress and improve nutrition
• Sample and send for analysis where a colony weakens unexplainably in winter

Practical position

In most regions no separate treatment is required. If colonies weaken persistently in late winter with no other explanation, it is worth having a sample examined.