Gambian pouch rats, which weigh up to 9lb (4kg) and are the largest in the world, have beaten continued eradication efforts in the Florida Keys and are threatening local crops.

Extensive trapping between 2007 and 2008 was thought to have got rid of them but the vermin has returned, putting farmer's livelihoods at risk.

Scott Hardin, exotic species coordinator for the Florida Fish and Wildlife Conservation Commission [FWC], told "We thought we had them whipped as of 2009.

"In the early part of 2011, a resident e-mailed me and said he saw one of the rats. We were sceptical but went back and talked to people and there were rats that we missed."

Hardin said that more traps containing poison-laced peanut butter were laid in May 2011.

"We trapped about 20 since we started. I would not imagine there's more than another couple of dozen at most. We've caught them all within a half-mile of each other," he said.

"We think they have not moved far but they clearly reproduced. We are surveying the area."

He said that Gambian pouch rats could reproduce within five months of birth and spawn up to 20 young in nine months.

Certain parts of the Florida Keys are home to numerous invasive species, which are kept as exotic pets before being dumped in the wild by bored owners.

Animals such as Burmese pythons, Nile monitor lizards, vervet monkeys, boa constrictors and rhesus monkeys roam the land preying on local species.