Research in the US has found that something is attacking the local bees' immune system, rendering the bees vulnerable to any contagion. Australian bees have been coming to the rescue, trying to make up the numbers to pollinate many of America's crops.

But now the new arrivals are also dying, as North America Correspondent Kim Landers reports.

ELEANOR HALL: To a booming Australian export, not minerals but bees.

Australian bee exporters are finding they can't send enough Aussie bees to the United States, where local bees are mysteriously disappearing by the hundreds of thousands.

Research in the US has found that something is attacking the local bees' immune system, rendering the bees vulnerable to any contagion. Australian bees have been coming to the rescue, trying to make up the numbers to pollinate many of America's crops.

But now the new arrivals are also dying, as North America Correspondent Kim Landers reports.

KIM LANDERS: The flight of the bees is proving a mystery. An estimated one quarter of America's 2.5-million bee colonies has been lost. The US Department of Agriculture says bees have been vanishing from 22 states, and no one really knows why.

It means commercial beekeepers, like George Hansen from Oregon, have increased the number of Australian bees they're importing. He says the bees come on packages, put on a flight from Sydney to San Francisco.

GEORGE HANSEN: Somewhere between 25,000 and 50,000 per Spring have been brought into California from Australia and these are three or four pounds each.

KIM LANDERS: How much does a package of bees cost?

GEORGE HANSEN: Three pound package with a queen is somewhere between $110 and $120 and a four-pound package with a queen is somewhere between $130 and $140.

KIM LANDERS: Jeff Anderson is another commercial beekeeper who moves between California and Minnesota. He has used Aussie bees for the past two years, but doesn't think they're immune from the mysterious colony collapse.

JEFF ANDERSON: I'm fairly suspicious that it is directly tied with this colony collapse, because what we noticed on our first shipment in this year, we installed 385 packages and - which is a full shipment - and within three days we probably lost between 75 and 80 per cent of the field bees out of those packages that we installed.

KIM LANDERS: So, what do you think is behind it?

JEFF ANDERSON: Research people out at Penn State said that when they dissect bees that have died from colony collapse, that it appears their immune system has just totally gone to pieces. It's like they just lost their immune system and anything will kill them.

KIM LANDERS: Bees are needed to pollinate about 90 varieties of fruit and vegetables grown in the US, including apples, avocados and blueberries.

Jeff Anderson says the disappearance of the American bees is not the only reason why Australian bees are being used.

JEFF ANDERSON: There's a couple other factors that are creating a need for Australian bees. The almond industry here in the United States, particularly in California I should say, has done a major expansion and we're short on pollinators.

We'll soon be short on pollinators for that crop, so I think the Australian package thing is probably here to stay for a while.

KIM LANDERS: A few years ago, none of this would have been possible. Beekeepers say America has only started to import Australian bees thanks to the Australia-US Free Trade Agreement.

This is Kim Landers reporting from Washington.