New Zealand researchers say they have bred honeybees which are not only resistant to varroa mites, but fight back by making the mite larvae sterile.

Using bees selected from around New Zealand, HortResearch honeybee scientist Mark Goodwin and his team have been crossing breeding lines of bees to increase the levels of a natural resistance in the population.

"It's a tricky business, requiring the careful artificial insemination of queen bees in the lab," Dr Goodwin said. "We've been able to breed bees whose hives render up to 80 per cent of varroa sterile," he said.

"The problem is that the process is expensive and time-consuming. We could never rely on artificial insemination to supply the amount of bees needed to continuously replenish the resistance genetics of New Zealand's 300,000 commercial beehives."

The idea to breed "resistant" or "tolerant" bees came from overseas research which showed that varroa mites emerging from certain hives were unable to reproduce. This is due to a genetic trait found in a few bees called "delayed suppression of mite reproduction" or SMRD.

While the mechanism is not yet fully understood, it appears to render the mites sterile.

If varroa populations do not grow to damaging levels in a hive because most of the mites are unable to reproduce, the bees may be able to tolerate their presence.

Non-reproducing mites either die before laying eggs, stop laying eggs or produce only a male mite.

Normally, the "resistance" levels reach only around 20 per cent because the "resistance" genes are constantly watered down through interbreeding.

But Dr Goodwin's team hope that they can establish a self-sustaining "stud" population of high-resistance bees by keeping them isolated as "closed hives" as crossbreeding from any remaining wild bees or ordinary hives would effectively dilute the resistance.

Their stud bees are being bred up on Great Mercury, an island billionaire entrepreneurs Sir Michael Fay and David Richwhite own off the coast of the Coromandel Peninsula.

The research team have now transported 50 high-resistance hives to the island, containing 500,000 bees.

"Over the next few months those bees will breed and the population will reach around 2.5 million," Dr Goodwin said. "We'll be watching with interest to see if isolation allows these hives and their offspring to maintain a high degree of varroa resistance."

If the experiment is successful the island could provide a model for other offshore breeding centres for large numbers of highly resistant queens. These could then form the basis of a continuous flow of new genetics to the mainland, helping to maintain high levels of resistance in commercial hives.

Since its arrival in New Zealand seven years ago - reportedly on a queen bee smuggled into the country - the varroa mite species has spread rapidly, laying its eggs inside the brood cells of a beehive, where the mites develop to maturity by feeding on bee larvae.

Uncontrolled, varroa will usually "kill" a bee colony within a year - an expensive loss to beekeepers as well as a major threat to honey producers and the billion-dollar fruit export industry, which relies on bees for pollinating crops such as kiwifruit.

Beekeepers can used chemical miticides - at an annual cost to industry of more than $1.5 million - but many do not like using chemical pesticides in the hives.

Dr Goodwin said HortResearch was seeking to control the mites with methods which were cheaper and more sustainable in environmental and food safety terms.

- NZPA

The problem

* The varroa mite is a major threat to New Zealand's billion-dollar fruit export industry, which relies on bees for pollinating crops such as kiwifruit.

* The mite has spread rapidly around the country since it got here seven years ago.

* Now scientists believe they have bred mite-resistant honeybees which can make varroa sterile.