They might be only tiny, but the Varroa mite has the potential to bring large parts of western agriculture to its knees. Infesting and destroying honeybee hives, the pernicious arachnid is already causing untold damage by disabling the trusty insects that pollinate much of our staple diet. However, having brought a hive to the verge of collapse, the mites are confronted with a dilemma: stay put and perish, or – like all good rats – abandon the sinking ship ready to infest another pristine hive. ‘Halting mite transmission among beehives is of primary importance’, say Rita Cervo and colleagues from the Università degli Studi Firenze, Italy. According to Cervo, mites prefer to hitch rides on nurse bees when the colony is healthy, in order to infect new brood at home. However, the team wondered whether the mites could switch preferences when conditions become overcrowded, choosing instead to thumb a lift from a non-nest mate forager that could transport them to uninfected hives when their current home is about to expire (p. 2998).
Testing the mites' preferences for hitching rides on hive-bound nurses or roving foragers, the team found that mites from hives with low rates of infection preferred to hop aboard nurses. However, as the rates of mite infestation in hives climbed, the mites became less choosy; they seemed equally content to ride on foragers and nurses alike. Intrigued, the team analysed the blend of waxy substances coating the bees' surfaces and found that the mites could probably distinguish between the nurses and foragers in hives with low rates of mite infection, because the nurses' waxy blend was very different from the blend of the foragers. However, the wax mixtures on nurses and foragers from the hives with the highest levels of Varroa mite infection were more similar, making it harder for the mites to distinguish between nurses that would keep them in the dying hive and foragers that could carry them to safety. The presence of the mites had altered the foragers' waxy coatings.
The team says, ‘These results show that, at low mite abundances, mites stay within the colony where they are born’. However, they explain that by losing the ability to distinguish between nurses and foragers when infection rates are high, mites increase their chances of getting a lift from a forager that happens to be visiting from another hive, improving their chance of survival when their home faces extinction.