The chemical is intended for use as an insecticide but is so powerful that a quarter of a teaspoon can kill a 300-pound bear.
Researchers and federal authorities are finding what they say is an alarming increase in the use of a powerful pesticide at illegal marijuana farms hidden on public land in California.
The pesticide residue is showing up in about 30 percent of the plants themselves, researcher Mourad Gabriel told The Associated Press.
Most of the illegally grown California pot is destined for Midwestern and Eastern states, federal prosecutors said. Federal and state authorities are announcing Tuesday that they will target the illegal grows with $2.5 million in federal money.
Researchers found the highly toxic pesticide Carbofuran at 72 percent of grow sites last year, up from 15 percent in 2012, said Gabriel, executive director and senior ecologist at Integral Ecology Research Center and one of the few researchers studying the ecological impact of illicit grow sites.
California has long allowed medicinal marijuana, and this year legalized recreational pot. While U.S. Attorney McGregor Scott will continue to enforce federal marijuana laws, he said he is targeting illicit grows on public land with cooperation from California's attorney general and the state's National Guard.
"What is happening here is illegal for all purposes under anybody's law," he said in an interview before Tuesday's announcement.
One of the dangers before legalization was that users could not be sure what was in the product they were smoking or eating, but authorities say the rise in Carbofuran use poses an increased danger.
The chemical is intended for use as an insecticide but is so powerful that a quarter of a teaspoon can kill a 300-pound bear, Gabriel said. Research by Gabriel and colleagues previously showed that the use of pesticides at illegal marijuana farms is poisoning significant numbers of California's few hundred remaining fishers, a threatened carnivore.
Carbofuran can't legally be used in the United States, and every bottle found at the grow sites since 2012 has been labeled in Spanish, Gabriel said.
Scott said it is being smuggled in from Mexico by drug cartels and the itinerant laborers hired to clear forestland and replant it with marijuana. The laborers have to pack in the plants, fertilizer, irrigation hose and camping supplies for the summer growing season.
Laborers apprehended by authorities tell Gabriel the remoteness of the growing sites is one reason highly toxic Carbofuran is so popular.
"What they are saying to us is this is extremely effective — it takes a little amount to kill a deer or a bear — so we don't need to bring a lot of it to last a season," he said.
At normal levels, a typical bottle containing less than 1 liter should be diluted with up to 5,000 gallons of water, he said. But illegal growers are diluting it with just 3 to 5 gallons of water to spray plants, or using the concentrate directly to kill wildlife.
At that concentration, the chemical takes at least 2 ½ years to dissipate instead of roughly a month if it is used at recommended dilutions.