Dilute caffeine solutions repel and kill slugs, say researchers in Hawaii. The chance finding could lead to a new environmentally friendly way of controlling the crop pests, the team says.
Robert Hollingsworth and colleagues at the Agricultural Research Service in Hawaii were field-testing caffeine as a toxin against a non-native frog. But they discovered that 95 per cent of large slugs were killed by a spray containing two per cent caffeine.
The team then tested lower concentrations of caffeine on cabbage leaves. At a concentration of just 0.01 per cent, caffeine reduced slugs' feeding by one quarter. A cup of instant coffee contains about 0.05 per cent caffeine.
Bill Symondson, a molecular ecologist at the Pest Management and Ecotoxicology Centre, Cardiff University, says the finding is interesting. "Slug control is extremely important economically - more so than any single insect in the UK. There is very little public tolerance of slugs. People make a huge fuss if they find one in their lettuce."
The researchers say that they do not know why caffeine killed the slugs. But they suggest it may act as a neurotoxin, since sprayed slugs fell to "uncoordinated writhing" before dying.
"Caffeine is present in coffee and cocoa beans to protect the plant against insects, but it is pure chance that it has a toxic effect on molluscs. It could be very useful because coffee is not likely to harm people so it's far better than spreading the traditional products, which also harm insects," Symondson told New Scientist.Luckily enough there is a huge pile of coffee at the allotment for everyone and we have covered the soil around plants most at risk to slugs with some coffee already. With any luck it should work! Fingers crossed!