Researchers have discovered that insects--especially bush flies and blowflies--are the primary cause of the rapid, long-distance spread of rabbit hemorrhagic disease (RHD), the virus decimating Australia's feral rabbit population. The government began deliberate release of the virus 2 years ago after it escaped confinement during a government-sponsored trial--before scientists had figured out just how it spread.

"We've always been mystified at how the disease spread so fast and so completely in arid, lightly populated areas," says Brian Cooke, principal ecologist with the Commonwealth Scientific and Industrial Research Organization. In some dry regions it has killed off 95% of the population; elsewhere, fatality rates are closer to 65%.

At a 23 September seminar on RHD held in Adelaide, scientists reported that of 10 insects now identified as disease vectors, common bushflies, brown and green blowflies, and rabbit fleas are the main ones in arid areas. The flies can be carried long distances by winds, says Peter Janssens, deputy director of the Vertebrate Biocontrol Centre in Canberra. That explains how "the virus would hit one area hard ... then pop up in another place virtually overnight." It also clears up the mystery of how the virus escaped from the island test site.

As RHD continues to rage unabated, some fear that the rabbits' predators will switch to endangered native wildlife. But so far, it appears that predator populations have declined and survivors are seeking other food such as insects and carrion. But there has been one undesirable result, says environmental scientist Rob Morrison from Flinders University: As the country's endangered wedge-tailed eagles are forced to eat road kill instead of rabbits, more eagles are being hit by cars.