Coffee grounds keep molluscs at bay. Credit: © A.Bargery

Humans have a new weapon in the eternal battle against slugs and snails - the double espresso.

Slugs and snails hate caffeine, researchers have discovered. The chemical could become an environmentally acceptable pesticide.

Robert Hollingsworth of the United States Department of Agriculture's Agricultural Research Service in Hilo, Hawaii, and his colleagues were testing caffeine sprays against the coqui frog, an introduced species that infests potted plants.

They noticed that a 1-2% caffeine solution killed nearly all the slugs and snails within two days. Concentrations as low as 0.01% put the pests off their dinner. A cup of instant coffee contains about 0.05% caffeine, and brewed coffee has more.

Coffee grounds are already recommended as a home remedy for keeping slugs and snails at bay. Grounds repel slugs, Hollingsworth found, but a caffeine solution is much more effective, he says: "Slugs turn back immediately after contacting the [caffeinated soil]."

Caffeine is more effective against snails than the current commercial standard, metaldehyde. The United States bans metaldehyde residues in food, but classifies caffeine as safe. It may even qualify as organic, adds Hollingsworth.

Caffeine is likely to have an effect on beneficial insects Peter Usherwood , University of Nottingham, UK

"I would expect caffeine applications to kill small snails and slugs, and repel the larger ones," says Hollingsworth. He envisages it being used in orchid greenhouses and on fruit and vegetable crops.

Toxicologist Peter Usherwood, of the University of Nottingham, UK, thinks that caffeine is best suited for use in domestic gardens. But he cautions that the chemical's toxic effects are not limited to slugs and snails. "It's likely to have an effect on beneficial insects," he says.

It's not known how caffeine kills the molluscs. Hollingsworth's team suggests that the chemical may cause damage to the nervous system - they report that caffeine-sprayed snails develop weak and irregular heartbeats, and slugs fall to "uncoordinated writhing" before dying.