It's a beekeeper's worse nightmare: to find their bee colonies decimated by Varroa mites. Hopping into brood cells just before the pupae are sealed in during metamorphosis, the mites reproduce and feed on the developing bees before hitching a lift on their emerging cellmates when development is complete. Initially a natural parasite of Asian honeybees, Varroa destructor began wreaking havoc among European bees during the last century. And the situation escalated when the parasites picked up naturally benign viruses, which had previously lived in harmony with their European hosts, transmitting them back in a sometimes-lethal form to the vulnerable bees. Fortunately though, all was not lost. Elke Genersch from the Institute for Bee Research, Germany, explains that nurse bees naturally weed out and dispose of damaged or deformed pupae while developing in the brood comb to protect the colony from infection.

Yet, attentive nurse bees face a dilemma when presented with mite-infested brood. According to Genersch, honeybee viruses – such as deformed wing virus – mutate continually, so some mite populations carry virulent and crippling forms of the disease whereas the viruses transmitted by other mites are practically harmless. However, it was not clear whether the Varroa mites carrying harmless virus or no virus might also damage the developing pupae sufficiently to trigger removal. So, would bee nurses dispose of all developing pupae that are infected with Varroa mites, or dispose only of those pupae where the mites are carrying damaging forms of deformed wing virus (p. 264)?

Teaming up with Sebastian Gisder, Genersch screened honeybee colonies infested with Varroa mites to identify colonies living with mites carrying virulent deformed wing virus and other colonies infested with mites carrying less-virulent – or even no – virus. Having isolated two mite-source colonies, the team then looked for colonies that were completely free of Varroa infestation – to provide clean brood – and identified hygienic colonies with good housekeepers, which had been selectively bred by Kaspar Bienefeld as part of a Varroa control strategy. Then Caspar Schöning and Ivonne Kretschmann tested how the good housekeepers would respond to virulent and non-virulent Varroa infestations to find out whether the bees respond to mite-induced damage or deformed wing disease.

Taking clean brood combs from the uninfested colonies, the duo added virulent and less-virulent mites from the mite-source colonies to the comb's clean cells. Then, the team transferred the newly infested combs to the hygienic colonies and waited to see how the good housekeepers dealt with the contaminated combs. Would they dispose of all Varroa-infected pupae, regardless of deformed wing virus virulence, or would the nurses select out the pupae at risk from the most virulent virus, leaving the pupae infested with the less-virulent mites to develop?

Amazingly, the bees homed in on the pupae that were infested with mites carrying the most virulent deformed wing virus, leaving the pupae that had been infested with less-virulent mites to develop to maturity. And when Sven Geiselhardt and Monika Hilker analysed the odours produced by pupae infested with virulent mites and pupae infested with the less-virulent mites, they found distinct differences between the odours emanating from both groups.

‘The bees reacted towards the damage done to the brood by the virus rather than to the Varroa mite parasitisation per se’, says Genersch. She adds, ‘There is a certain combination of deformed wing virus and Varroa mite that is fatal for the bees and beekeepers need to treat against mites or breed for hygienic bees to get rid of this virulent combination.’

Evidence for damage-dependent hygienic behaviour towards Varroa destructor-parasitised brood in the Western honey bee, Apis mellifera
J. Exp. Biol.