The Sting: Entomologists Have Canny Plans to Rob a Likely Invader of Its Chance to Ravage Our Pollinator, the European Honeybee
Pyper, Wendy, Ecos
AUSTRALIA is the final frontier for a tiny mite that parasitises honeybees and threatens our $1.2 billion pollination industry. The mite is poised to enter the country after incursions in New Zealand and Indonesia. Scientists warn it's just a question of when.
Dr Denis Anderson from CSIRO Entomology in Canberra has been studying the varroa mite since 1989, to better understand its biology and behaviour, and identify effective control strategies. His research has taken him on a rollercoaster ride across 32 countries, culminating in a recent discovery that could end the mite's reign of destruction.
The varroa mite was first described on its native host, the Asian hive bee (Apis cerana), in Java in 1904. The mite, formally named Varroa jacobsoni, begins its parasitic relationship with the Asian bee when the female mite lays up to six eggs in a drone brood cell.
When the young mites hatch, they feed on the developing drone, weakening it and reducing its adult life span. During adulthood, the unfortunate drone acts as a living food source for the mites, which slip under its intersegmental membranes, where they can avoid detection and feed on the bee's soft tissue.
While the mites also feed on female worker bees, they cannot reproduce inside worker bee brood cells. According to Anderson, this is because they do not receive a recognisable chemical signal from the developing worker bee to stimulate egg laying. Developing drones, however, do release a signal the mites recognise. This means that although some drones die prematurely, enough survive to mate with new queens, and the driving force of the colony -- the worker bees -- remains strong.
DNA analysis by Anderson has shown that V. jacobsoni and the Asian hive bee have co-evolved for thousands of years. During this time the bees have developed a range of behavioural traits that reduce the harmful effects of file mite. Some of these traits, such as a tendency to swarm and a willingness to abandon their hives, may have effectively countered the mite, but they have produced a bee that is not the most reliable or productive honey gatherer or pollinator.
A sweeter host
To overcome these problems, a second, more reliable and productive bee was introduced into Asia about 30 years ago. The European honeybee, Apis mellifera, is the `bees knees' in terms of honey production and pollination. Not long after the bee's introduction however, the varroa mite jumped hosts. Mite-infested A. mellifera were subsequently transported around the world via quarantine incursions and the normal practice of shipping live bees between countries.
Unfortunately for the European honeybee and its keepers, the varroa mite proved to be more virulent on its new host. In mainland Asia, America and Europe, European honeybee colonies were dying, as the mite was capable of reproducing on both drones and workers.
Anderson began investigating the mite following its introduction into New Guinea on imported Asian bees. Initial observations indicated that only the European honeybees in New Guinea and Java appeared able to resist the parasite. A closer look showed the mites to be incapable of reproducing on these bees, something never observed elsewhere.
Anderson hypothesised three likely causes for this discrepancy: the European bees in New Guinea and Java were resistant to the mite, environmental factors impaired the mite's ability to reproduce, or the mites were genetically different from those affecting European bees in other parts of the world. To test these possibilities, he raised 40 genetically identical European queen bees and shipped half to Germany and half to New Guinea.
`We found the offspring of the bees were susceptible to the mite in Germany, while those in New Guinea were not affected,' Anderson says. `So we realised it was not resistance by the bees, but something to do with the environment or the mite. …