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Forest Service Scientists Do Some Serious Sleuthing to Detect Illegal Marijuana Grow Sites

Diane E. Banegas
Knowledge Management and Communications

The problem of illegal marijuana farms on national forests is, pardon the pun, growing. In spite of the fact that “pot” is now legal in California and other parts of the U.S. for medicinal and recreational use, illegal marijuana growing is still a billion dollar industry with international tentacles. In fact, the number of illegal grow sites increased dramatically following California’s Proposition 215, which legalized medical marijuana.

“Twenty years ago the people who planted and tended the illegal pot farms were poor farmers from Mexico,” said Craig Thompson, a research wildlife ecologist with the U.S. Forest Service. “Today international drug organizations bring in people from Russia and Asia and other places. They don’t’ have a farming background, it’s strictly mercenary.”

When Thompson was based at the Forest Service’s Pacific Southwest Research Station in Fresno, Calif., he was hardly aware of an illegal marijuana growing problem on national forests and didn’t think it concerned him or the fisher population he was studying. The fisher is a small mammal related to the weasel that serves as an indicator species as the species thrives in healthy forests. When fishers began turning up dead under suspicious circumstances, Thompson sent a specimen to a nearby testing laboratory run by the University of California at Davis.

Forest Service scientist Craig Thompson holds a fisher that was caught and collared for his study on forest carnivores and fire. The cause of death—rodenticide—startled him. Such poison typically is used in urban or agricultural areas to kill rats and other rodents. How in the world did a fisher in the middle of a national forest get into rodenticide? His colleagues in Forest Service law enforcement informed him that illegal pot growers use large amounts of rodenticide to keep mice from destroying their plants. Mice and other species, such as ground squirrels, chew into the plants for their high moisture content, killing the plants in the process. Rodenticide protects the plants and kills the rodents, but it also kills other wildlife.  Species such as the fisher can be poisoned by either eating the poisoned rodents, or by eating the rodenticides directly, which are often bacon or cheese flavored. Besides using rodenticide, the pot growers liberally spray their plants with highly concentrated insecticides such as carbofuran, a chemical that can seep through soil and enter ground water.

Impacts on wildlife

Thompson and his colleagues wanted to know the scope of the problem. Mourad Gabriel, who was then a doctoral student at the University of California at Davis, dug into the archives and tested 58 fisher carcasses that Thompson and others had sent in over the previous 4 years: 79 percent tested positive for rodenticide. “That was ‘the sky is falling’ statistic,’” Thompson said. In 2015 they tested 101 additional fishers carcasses from all over California, 85 percent tested positive. And in 2017 they tested an additional 22 carcasses, this time 100 percent tested positive. Clearly the problem was getting worse. “It’s hard to find any clean animals today,” he said.

Impacts on water

While the impacts of pot farms on wildlife are concerning, the negative impacts of illegal marijuana farms on natural water sources already are devastating.

A marijuana plant needs about 6 gallons of water per day to grow, which translates into 900 gallons per year per plant. In 2016-2017, law enforcement officers eradicated about 1,250,000 plants on California’s national forests. (The officers estimate that they find anywhere between 15-60 percent of the illegal grow sites.) “That is more than 1 billion gallons of water per year for just the plants found, so the true number of gallons is much higher, we just don’t know how high,” Thompson said. The numbers indicate the staggering amount of water the pot farms are diverting from the water balance equation in forest ecosystems—in a time of extended drought. And the real value may be twice that size, given the number pot farms that law enforcement doesn’t find.

A marijuana infographic that says one marijuana plant growing in a national forest uses 900 gallons of water per growing season.

Ironically, the growers plant more marijuana plants in times of drought because they expect to lose more plants under dry conditions, so the negative effects of these trespass sites on forest ecosystems are the worst when national forests are the weakest.

Pot growers build an infrastructure of long hoses and catchment ponds to siphon water away from streams and creeks into the makeshift ponds from which they irrigate their crops. Trees and other vegetation already stressed from drought become further susceptible to pests and diseases. Because wild animals will chew on the hoses for water or drink out of the catchment pools, the pot growers defend this infrastructure by scattering large quantities of rodenticide or mixing concentrated insecticides in tuna or cat food cans.)

“The water lines get gnawed on primarily by rodents, so they scatter rodenticide pellets along the pipe,” Thompson said. “But the water catchment pond and the actual camp are draws for all kinds of critters, so we find rodenticide pellets as well as open cans of tuna and cat food laced with insecticide. The tuna and cat food targets species like foxes, bears, and ravens.”

The killing effects can spread up the food chain, in a process called bioaccumulation, as larger predators feed on the smaller, poisoned animals. In one memorable case of bioaccumulation that Thompson observed, a fox died from consuming insecticide-laced bait. All the fleas, ticks, and flies on the fox died as well, and a vulture that fed on the dead fox also died. A recent study by California State researchers on owls further validates that toxic levels of rodenticides and insecticides are entering the terrestrial food web.

In California alone, these sites also skim more than a billion gallons of water away from sources intended for human consumption in places like San Francisco and Sacramento. These waters, including those that feed municipal water systems, are increasingly at risk of contamination from highly concentrated rodenticide and insecticide.

Pot farming in California also skims more than a billion gallons of water away from sources intended for human consumption in places like San Francisco and Sacramento. Even though Governor Jerry Brown officially declared the state’s historic five-year drought over last year, residents are now adhering to permanent water conservation rules, even in non-drought years. These waters, including those that feed municipal water systems, are increasingly at risk of contamination from highly concentrated rodenticide and he worries right now about the poison entering ground water and seeping into wells in rural areas.

Sounding the alarm

Thompson, who is now based in Missoula, Mont., working on the Forest Service’s Blackfoot Swan Landscape Restoration Project, said that given the scope and immediate dangers of illegal marijuana farming on national forests, he and his colleagues in the Forest Service and scientists in other federal, state, and nonprofit organizations are assisting law enforcement personnel with the detection, documentation, and prosecution of the perpetrators. They also are sounding the alarm in public forums about the immediate threats to the environment and human health.

Fellow scientists are also leveraging their areas of expertise to help law enforcement counter illegal marijuana grow sites on national forests:

Two tools developed by Forest Service scientists could help law enforcement officers detect the presence of illegal marijuana sites on national forests:

  • DIMEC, the Detection and Interdiction of Marijuana to Aid Enforcement and Conservation, is a computer algorithm capable of learning to detect patterns in aerial and satellite imagery consistent with illegal marijuana farms. Once the algorithm is fully trained, it can scan thousands of images in a matter of days to pinpoint a hundred acres of marijuana cultivation from millions of acres of forest land. “Right now law enforcement officers only have a vague estimate of how many trespass marijuana sites really exist,” said Forest Service postdoctoral researcher Adam Cummings. “DIMEC could give them a much more accurate picture of the true scope of the problem. It will also precisely pinpoint grow site locations so they can allocate their finite resources more effectively.” Cummings is working at the agency’s Pacific Southwest Research Station on a fellowship through the Oak Ridge Institute of Science and Education.
  • Environmental DNA, or eDNA, is a highly sensitive method of detecting low concentrations of organisms or substances in low abundance from a cup of water taken from a stream or other water source. The method has detected the presence of cannabis in streams. Forest Service scientist Michael Schwartz, who directs the agency’s National Genomics Center for Wildlife and Fish Conservation, believes the technology could be useful in detecting the presence of marijuana grow sites but no formal studies have been conducted.

A nationwide map of grow sites: Forest Service scientist Frank Koch and his colleagues at the agency’s Southern Research Station developed predictive models that reveal how drug markets, policies, and environmental conditions influence growers’ decisions for the location of grow sites. Their paper titled Predicting cannabis cultivation on national forests using a rational choice framework identifies marijuana street price and variables associated with grow site productivity, such as elevation and proximity to water, production costs, and risk of discovery as significant predictors. They are using their models to construct regional maps of grow site likelihood.

The map shows the likelihood of illegal grow site establishment (red=high, blue=low) based on multiple factors that illegal marijuana growers take into consideration when deciding where to locate their farms. The blue and black lines depict streams and ro

According to Forest Service Special-Agent-in-Charge Don Hoang, scientists can provide the Department of Justice with credible scientific evidence in support of prosecution for damage to government property and damage to timber value, both of which are federal violations. Documentation on types, quantities, and distribution of unauthorized pesticides, which violates California state laws and regulations is also of value in building a strong case. Scientific evidence provides documentation on unlawfully harvested wildlife in violation of state and federal regulations. The science also supports values claimed for overall reclamation and restoration of an illegal marijuana grow site.

Public Opinion:
The public generally is less concerned about illegal marijuana sites on national forests than they are about the negative effects of such sites on the environment and their potential threats to human safety, Thompson said. He and his colleagues are working to raise public awareness about the problem. It will take a shift in public opinion to solve the problem because it’s too big for one agency to conquer. The tremendous amount of water used to irrigate illegal grow sites, especially in a time of drought, is a message he wants the public to hear. When people understand that the illegal activity may cause water rationing, or that natural treasures like Giant Sequoias may be put at risk, it puts the problem of illegal grow sites in perspective.


Note: An abridged version of this article appeared July 11, 2018, as a USDA blog:


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