I recently read that a 1 to 2 percent caffeine solution is a possible way to kill slugs. I know that coffee grounds themselves repel slugs, but what about the caffeine solution? How exactly does one make a caffeine solution of 1 to 2 percent?

The toxic effect of caffeine on slugs was inadvertently discovered by a biologist in Hawaii, Earl Campbell, now with the U.S. Fish and Wildlife Service, and Robert G. Hollingsworth, an entomologist with the U.S.D.A. Agricultural Research Service. Campbell was seeking a “silver bullet” solution to some tiny nonnative Caribbean frogs that hitched a ride on a ship and disembarked in the Hawaiian Islands.

These frogs are unbelievably noisy. Their mating calls go on all night and year-round, reaching 90 decibels. Note that OSHA requires workers to wear protective hearing devices at 85 decibels. So at this volume, sustained exposure to these irritating vocalists can produce irreversible hearing loss. Since the mid-1980’s, Hawaii’s alien frog populations have been rising exponentially, while the rest of the world populations are declining or becoming extinct. By sheer quantity and volume, the amphibians were now a health hazard and an ecological nightmare.

Campbell’s group began working through various off-the-shelf grocery products, trying to find a legal but readily available product to target the frogs. They found a caffeine-rich, anti-sleep preparation that worked at the product label’s recommended level. He got permission from the EPA to field test a dilute concentration of the compound on soil in greenhouses. As soon as Campbell anointed the soil with the caffeine solution, he unexpectedly discovered that slugs began surfacing and dying.

Hollingsworth became interested from a pest control perspective. Small snails chewing away at the roots ruin tropical ornamentals such as orchids. Hollingsworth tested various concentrations of dilute caffeine against the orchid snails. A 4-ounce solution of 2 percent caffeine devastated most of the garden slugs. Even a .01 percent solution killed a significant number.

The bottom line: caffeine makes a good botanical pesticide. According to Hollingsworth, the reason slugs and snails are so susceptible is that they have no exoskeleton to protect them from coming in contact with a water-based solvent. “The mucus, which is the basis for their locomotion, is very high in water content,” Hollingsworth noted, “and permits the water-soluble caffeine easy entry. Once inside the critters, the neurotoxin destabilizes the mollusks’ heart rate.”

Coffee tends to have a caffeine content of about 0.1 to .05 percent, so it should be somewhat effective at killing slugs in your garden. It won’t hurt to pour coffeepot leftovers on the soil around your hostas. Even the grounds lightly sprinkled over the soil surface will deter them and might also cause a toxic reaction. While it isn’t feasible at this time to make a 1 to 2 percent caffeine solution at home, anti-slug products containing this concentration might be made available in the future.