Millions of bees on Great Mercury Island are giving a good buzz for beekeepers and New Zealand's horticultural and agricultural industries.
The imported bees are involved in a HortResearch experiment to see whether the varroa mite can be controlled by breeding bee resistance.
Monitoring showed two strains of bees were controlling the varroa mites' rate of reproduction. One genetic line had not had to be treated for 18 months and the other for a year.
Scientist Michelle Taylor said it was a fantastic result.
"It means the honey bees are keeping varroa at a level they can live with."
Uncontrolled, varroa can kill a bee colony within a year. Since arriving in New Zealand eight years ago, the varroa mites have spread throughout the North Island and into parts of the South Island. In the past few weeks they have been confirmed in Canterbury.
Control by miticides costs the beekeeping industry more than $1.5 million a year and there were signs overseas that varroa was developing resistance to them. Left unchecked, the disease could destroy the horticulture and beekeeping industries and impact severely on the pastoral industry.
The National Beekeepers' Association and kiwifruit marketer Zespri International are backing the research with funding. Association executive adviser Jane Lorrimer said individual beekeepers were also putting money in.
Ms Taylor said the breeding project aimed to give beekeepers another tool to combat the disease, although at this stage scientists did not know exactly what the bees did to control the mites.
HortResearch staff at Ruakura spent three years selecting bees from around New Zealand and interbreeding them to raise the levels of a genetically inherited trait which inhibits the ability of varroa mites to reproduce.
Forty colonies, each with about 80,000 bees, were shipped to the island about a year ago. Out of six genetic lines, two have shown enduring resistance. Scientists will breed naturally from these lines with the aim of producing a strong enough population for introduction to the mainland in two to three years.
Ms Taylor said resistance-breeding was tried in the United States but the bees were released too early.
"The bees weren't vigorous enough and the programme wasn't very successful. We're not going to release early."
The isolation of the island was vital to the success of the programme.
"It's exciting that we have something that's remote enough for a closed mating population but so convenient. The owners (Sir Michael Fay and David Richwhite) have been so supportive. And the bees love it."
HortResearch was also working on a biological fungal treatment, which Ms Taylor said could be a third tool in the toolbox for combating the disease. Beekeepers also use organic treatments.