The Associated Press reports that large quantities of some defoliants and insecticides, which have long been banned in the U.S., are being smuggled into the remote farms.

Officials say they have found that rat poison has been used to keep animals away from the pot crops, and there is evidence that plant growth hormones have been dumped into streams and then pumped to the pot fields through networks of PVC pipes.

Several hundred illicit growing sites were found on U.S. Forest Service land in California alone during 2006 and 2007, while many other growing operations were uncovered in West Virginia, Tennessee and Kentucky.

Scott Wanek, the western regional chief ranger for the National Park Service, told the AP he believes the eradication efforts have touched only a small portion of the marijuana farms, and that the environmental impact from pot cultivation is much greater than anyone knows.

Photo: Richard Stamper - iStockphoto