While they have their backs turned, fighting wars in Asia and the Middle East, it seems that US troops have allowed a counter-invasion of their country by marauding legions.

An infestation of the Turkistan cockroach, which can carry diseases such as typhoid and dysentery, can be blamed on a handful of the insects hitching a ride with soldiers returning from the war zone, experts say. Now the fast-breeding cockroaches, which are adept at dodging pesticides, have spread from California to New Mexico, threatening mayhem further afield as they seek new territory.

"It started with soldiers and equipment coming back from the first Gulf war in the early 1990s, but it's still happening and they're spreading to other areas," said Philip Koehler, a professor of entomology at the University of Florida, Gainesville.

Experts say that the Turkistan cockroach, native to many countries including Iraq, Afghanistan and Iran, is particularly troublesome. Its natural habitat is the desert, of which there are large areas in California, Arizona, New Mexico and Texas. It breeds quickly at room temperature and, according to Koehler, is not timid. "They like to live around the outside of houses, but also to invade them," he said.

While the spread of the Turkistan is his biggest worry, Koehler warned that warmer areas of the US were also under threat from other newly imported species such as the Madagascar hissing roach, the lobster roach and the orange spotted roach. "We have 69 species of cockroaches in the US and 29 of them were brought in from other countries," he said.