Use of caffeine
on pests perks up
gardeners worldwide

Big Island scientists have found
caffeine kills slugs and snails

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By Helen Altonn
haltonn@starbulletin.com

Homeowners and gardeners plagued with slugs and snails might find the solution in a can of coffee.

Scientists on the Big Island have found caffeine repels and kills the annoying critters.

Their study, reported in NATURE magazine Thursday, has attracted worldwide attention.

Returning home Friday from a trip, investigator Robert Hollingsworth, with the Pacific Basin Agricultural Research Center of the U.S. Department of Agriculture in Hilo, was besieged with e-mail and calls from gardeners, garden groups and people who write letters for organic gardeners.

"I had a lot of e-mail from people around the world," he said.

Interest is particularly high in the United Kingdom, which has serious snail and slug problems because of its wet climate, he said. Five or six BBC reporters contacted him, he said.

In a telephone interview, Hollingsworth said he began testing caffeine on a small orchid snail after scientists noticed slugs were also in tests against the coqui frog.

He was interested in the frog work as a researcher on ornamental plant pests because growers are concerned that frogs will hide in exported potted plants and hurt the industry, he said.

"I found that caffeine solutions did a good job of killing the little orchid snail, which is fairly hard to control with conventional pesticides that kill mollusks, including slugs and snails," Hollingsworth said.

Caffeine also killed large garden snails when tested, he said.

Working with Hollingsworth were John Armstrong, a colleague in his organization, and Earl Campbell, who was doing the frog tests for the U.S. Department of Agriculture's Animal, Plant and Health Inspection Service.

The researchers used caffeine powder, which is sold as a pure product for adding to cola-type drinks. They mixed it with water and drenched or sprayed it on potted plants, Hollingsworth said.

He said the strength of caffeine that repelled the slugs was about the same as that in strong coffee. A cup of instant coffee has about 0.05 percent caffeine and brewed coffee would be stronger, he said.

A caffeine solution doubling the strength of instant coffee affected slugs' feeding behavior, he said. Nearly all slugs and snails died within two days after a 1 percent to 2 percent caffeine solution.

If they had a choice, slugs generally shunned cabbage leaves dipped in caffeine solutions and went for untreated cabbage, Hollingsworth said.

Coffee grounds mixed with soil also turned slugs away, but a caffeine solution is more effective, he said.

He believes people are interested in the study because they're familiar with caffeine and the Food and Drug Administration recognizes it as a safe, natural product to add to food.

"But it should be remembered the pure product, which is what we started off with, is poison," he said. "You have to be very careful with it. You don't want to inhale it and you wouldn't want to drink a solution made from it. ... It has to be handled with care."

Hollingsworth said he's making no recommendations on the use of caffeine to kill or drive away snails because pesticides must go through the Environmental Protection Agency registration process.

But, he added, "Who knows? Maybe some company will pick up on this and develop and register products for crops."