Deadly mite infestations considered a leading cause of the continuing collapse of the global commercial honey-bee industry are being abetted by modern bee-keeping practices, new research suggests.
The research, published in the journal Environmental Entomology, points the finger at the practices of siting commercial hives too close to each other, and of thwarting the bees’ swarming behavior, for creating conditions ideal for the rapid growth and spread of the parasitic Varroa mite.
The mite (Varroa destructor) is a text-book example of zoonosis – a predatory or parasitic species that has “spilled over” from its traditional host into a new species which, not being adapted to it, suffers catastrophic consequences.
Varroa’s natural host in the Asian honey-bee (Apis cerana). Co-evolution has resulted in the two species being able to live in balance.
However, some time after 1905, commercial apiarists introduced the European honey-bee (Apis mellifera) into Asia. DNA sequencing has revealed that the mite spilled over from Asian to European bees on at least two occasions. Migration and trade then led to the mite spreading to European bee colonies around the world, to the point that it now represents an existential threat to commercial apiaries used for honey production and pollination.
One aspect of Varroa’s relentless spread that has puzzled researchers is how it is so successful in moving from one hive to another, given it is not itself a notably mobile or agile insect.
This new research, led by Gloria DeGrandi-Hoffman from the US Department of Agriculture’s Carl Hayden Bee Research Centre in Tucson, Arizona, appears to solve that puzzle.
The mite, the researchers report, has adapted and co-opted several honey-bee behaviours. In particular, it exploits the predilection of individual bees to visit other hives, either to forage or because the insect has become lost and disorientated; the mite effectively hitchhikes from one hive to the next.
DeGrandi-Hoffman’s team spent 11 months monitoring a single US commercial bee-keeping operation comprised of 120 hives.
Some of the hives were treated with a mite-killing spray in spring, and others in autumn, but timing, in the end, made no difference: about half the colonies collapsed.
With the field evidence supplemented by computer modelling, the scientists conclude that the mite populations within the hives were being continually renewed by means of hitchhikers aboard lost, sick or foraging bee visitors.
In the wild, among the Asian honey-bee populations, the mite rarely if ever causes colony collapse. In part this is because the colonies periodically swarm – with a large proportion of a hive leaving to form a new colony – reducing mite density in the process. In part, too, wild honey-bee nests are situated too far apart for hitchhikers to be a problem.
Commercial bee-keeping practices, the researchers note, have thus allowed the Varroa to overcome a critical evolutionary constraint.
In the wild, it is not in the mites’ interest to destroy a bee colony, because that would also kill off the mite population. Where hives are packed tightly together, however, this constraint ceases to exist, because fresh hosts are always only a short distance away.
DeGrandi-Hoffman says her team’s findings mean commercial apiarists and agricultural regulators must completely change their approach to mite control.
“This research provides evidence that the tried and true ways of controlling Varroa are no longer feasible, and that new methods that are designed for control of a migratory pest are required,” she says.
Andrew Masterson is a former editor of Cosmos.
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