The United States is in the grip of an alien invasion that is costing the country $123bn each year.

America is now home to some 30,000 animals, plants and microbes that have been introduced from other nations. Some have had beneficial effects, but many have just become a nuisance and require substantial public funds to control or destroy.

"It doesn't take many trouble-makers to cause tremendous damage," Cornell University ecologist David Pimentel told the American Association for the Advancement of Science in Anaheim (AAAS), California.

He has just completed an economic assessment of the impact these organisms have on the USA today. His list of public enemies runs from alien weeds (cost: $35.5bn) and introduced insects ($20bn) to human disease-causing organisms ($6.5bn) and even the mongoose ($50m).

Threatened species

Aside from the economic costs, he added that more than 40% of species on the US Department of the Interior's endangered or threatened species lists are at risk primarily because of non-indigenous species.

Arundo is a fire hazard

Near to the AAAS conference I have seen how the arundo cane is clogging up local rivers. The reed is used in musical instruments but it is a nightmare to control. The plant, originally introduced from the Mediterranean 200 years ago, grows several centimetres each day. It is also a fire hazard and has to be cut down - but this costs a lot of money.

"We just purchased a $95,000 piece of machinery that chews it up - it's monster tractor," Paul Fransome of Riverside County Park told me. "We use chemicals, we use machinery - and it will probably be two more decades before we're done."

Tiny red fire ants are also alien invaders. Their powerful stings are driving people from parks. "They can kill - if you don't get away, they will continue to swarm and continue to sting," Bob Dowell, the state entomologist, said. "They have killed people in the South - they're fairly frightening creatures."

A billion rats

David Pimentel, a professor in Cornell's College of Agriculture and Life Sciences, told the AAAS that the USA had become the land of a billion rats. Most are the introduced "Rattus rattus" (also known as the European, black or tree rat) and "Rattus norvegicus" (variously called the Asiatic, Norway or brown rat).

Rats on poultry farms and other farms number about 1 billion and each destroys grain and other goods worth $15 a year, he said.

"It's too late to send these organisms back," Pimentel observed. While policies and practices to prevent accidental or intentional introduction are improving, he said the US still had a long way to go before the resources devoted to the problem are in proportion to the risks.

"We can only hope that environmental and economic assessments like this one will demonstrate that resources spent on preventing the introduction of potentially harmful non-indigenous species can be returned many times over in safeguarding our environment."