Honey bee pests and diseases: varroa mites, foulbrood and the Asian hornet

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Most colonies that fail do so because of a pest or a disease, and many of these can be spotted early by a beekeeper who knows what to look for. This guide covers the main threats, with notes on reporting duties in the UK. For full identification guides, the UK National Bee Unit and many university extension services publish free illustrated material.

Varroa mites

Varroa destructor is a small reddish-brown mite that feeds on honey bees and breeds inside capped brood cells. On its own it weakens bees; worse, it spreads viruses, especially deformed wing virus, which causes bees with crumpled, useless wings. Left unmanaged, varroa is one of the commonest reasons colonies collapse. Varroa is present almost everywhere honey bees are kept, so the question is not whether you have it but how many.

Monitoring varroa

Do not wait for symptoms. By the time you see bees with deformed wings or mites riding on adult bees, the infestation is usually severe.

Alcohol wash: a sample of roughly 300 bees (about half a cup) from a brood frame, shaken in alcohol to dislodge the mites, which are then counted. Generally considered the most reliable field method, though it kills the sampled bees.
Sugar roll: the same sample shaken in powdered sugar instead; the bees survive. Less reliable in humid weather or during a strong nectar flow.
Mite drop: a sticky board or tray under a mesh floor, counting how many mites fall over a set number of days.

Express the result as mites per 100 bees, and compare it with the treatment thresholds recommended for your region and season. Thresholds differ between sources and are lower in spring than late summer, so ask your association or extension service which figures they use. Mite numbers climb fastest in late summer, just when the colony is raising the bees that will see it through winter, so monitoring then matters most.

Managing varroa

Use an integrated approach: combine monitoring, management techniques such as drone brood removal or brood breaks, and treatments when counts call for them.
Use only treatments authorised for use on bees in your country, and follow the label exactly, including timing, dose and whether supers must be removed. Treatments used wrongly can contaminate honey or harm the colony.
Rotate treatment types to slow resistance.
Record what you used and when.

Foulbrood

American foulbrood (AFB) and European foulbrood (EFB) are bacterial diseases of the brood, the developing larvae. Both spread between colonies on equipment, by robbing and by beekeepers moving frames. Warning signs include:

A patchy, "pepperpot" brood pattern rather than solid capped brood
Sunken, darkened, greasy-looking or perforated cappings
Larvae that are discoloured, twisted or melted down in the cell
An unpleasant smell

AFB larval remains can form a sticky, ropey mass if poked with a matchstick, but do not rely on home tests. If you see anything suspicious, close the hive, do not move frames or equipment, and get it checked. In the UK, both AFB and EFB are notifiable diseases: if you suspect either, you are legally required to report it to the National Bee Unit or your local bee inspector immediately. Inspectors confirm the diagnosis and decide on treatment or destruction. Many other countries also treat foulbrood as notifiable; check with your agriculture authority.

Other pests

Small hive beetle and Tropilaelaps mites: notifiable pests in the UK, not established there at the time of writing. Report any suspicion.
Wax moth: larvae tunnel through comb, mainly in weak colonies and stored frames. Keep colonies strong and store comb carefully.
Chalkbrood: larvae turn into chalky white or grey mummies. Often linked to damp or stressed colonies.
Nosema: a gut parasite of adult bees, associated with weak colonies and, with one species, dysentery-like staining.
Wasps and robbing bees attacking weak colonies late in the season: reduce the entrance.

The Asian hornet in the UK

The Asian hornet (Vespa velutina) is an invasive predator of honey bees and other pollinators. It is darker than the native European hornet, with a mostly dark abdomen with one broad yellow-orange band and yellow tips to its legs. If you think you have seen one in the UK:

Report it using the Asian Hornet Watch app, which is free, or the online reporting form run by the UK Centre for Ecology and Hydrology. Include a photograph if you can do so safely. In Northern Ireland, reports go through the app or the regional recording system.
Never disturb a suspected nest. Report it and leave it to the authorities.

Good habits that prevent most problems

Inspect brood regularly and learn what healthy brood looks like.
Keep records of every inspection.
Never feed bees honey from an unknown source.
Do not buy used comb, and clean second-hand equipment thoroughly.
Register on BeeBase if you are in the UK, so you receive disease alerts.

What do your mite counts look like this season, and what has worked for you? Share your experience or ask for help identifying something in the comments.