Uber-tidy bees defend their hives from disease

For some, cleanliness is next to godliness. For honeybees, which are suffering a major decline, it's a way of saving the hive from disaster. A naturally-occurring cleaning behaviour protects colonies from harmful parasitic mites, and breeding bees to be more hygienic could protect hives from the viruses they spread.
The varroa mite sucks the blood of pupae of worker bees, reducing their immunity to disease and transmitting viruses. Mites sneak into the honeycombs, then lay their eggs and feed on the larvae in the safety of the wax-capped cells. When the cells are uncapped to release the adult bees the mites are also released – ready to parasitise other members of the hive.
Bees that have been trapped with mites are often smaller, and can show signs of infection with a disease called deformed wing virus. If present together, varroa mites and this virus can kill off a whole hive, particularly during the difficult winter months.

Saviours of the colony

But hygienic worker bees can save the day. They sniff out dead or diseased larvae, uncap their cells and dispose of the contents. Although any adult mites inside may survive this upheaval, their young offspring are killed. This cleaning behaviour, if done intensively, is highly effective at controlling mite numbers and protects the hive against deformed wing virus.

Francis Ratnieks of the University of Sussex in Brighton and colleagues studied this behaviour in 42 honeybee colonies. In each hive they placed a section of honeycomb that had been frozen to kill the pupae inside. Within a day, most colonies had uncapped and removed up to half of the dead pupae, but some particularly hygienic colonies managed to remove more than 95 per cent.
Ratnieks' team found that these super-hygienic colonies had less than half the varroa mite levels of the less hygienic colonies. Also, while around a third of less hygienic colonies had individuals with shrivelled wings – a sign of deformed wing virus – the 14 colonies that threw out at least 80 per cent of the pupae had none, suggesting that hygienic behaviour can protect hives against viruses too.
"This finding could be very useful for beekeepers as it would reduce the harmful effects of varroa in a natural way," says Ratnieks.

Heritable habit

What's more, studies have shown that the tidying behaviour is a heritable trait, suggesting that it has a genetic component. "You can breed for this behaviour by screening colonies for hygiene levels and the breeding the most hygienic," says Ratnieks.
His team has confirmed this in a separate study, which found that when queens from hygienic colonies mate with drones from another hygienic colony, cleaning levels in new colonies populated by the offspring remain high.
Varroa has been a pest of US honeybee colonies since 1987, and arrived in the UK in 1992. "Varroa is the number one health problem for the beekeeping industry worldwide," says Ernesto Guzman of the University of Guelph in Ontario, Canada. He estimates that in most countries, more than half of honeybee colonies are infested by the mite.

To control this pest, beekeepers use a range of synthetic and organic pesticides. While synthetic compounds are effective, the mites usually develop resistance to them quickly. Breeding more hygienic hives could keep varroa numbers low enough that it becomes harder for them to evolve resistance to synthetic pesticides and allow the use of organic pesticides, which are not as effective so are less attractive to beekeepers at high infection levels.
But to avoid reinfection, all beekeepers must be using hygienic colonies, warns Marla Spivak of the University of Minnesota in St Paul. "When some beekeepers have susceptible stock with colonies full of mites, healthy colonies with low levels of mites try to rob honey out of these weakened colonies and get re-infested," she says.
Journal reference: Journal of Apicultural Research, DOI: 10.3896/IBRA.1.53.5.10 and 10.3896/IBRA.1.53.5.07

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