SLUGS, the bane of every gardener’s life, devouring carefully nurtured seedlings and the precious buds of flowers, are under threat: death by latte or espresso.
Coffee, that powerful stimulant that jolts human beings from sluggishness to being wide awake first thing in the morning, appears to have the reverse effect on molluscs. Give the voracious army a shot of caffeine during its onward march through your herbaceous border and the chances are you will put it to sleep forever.
Scientists have discovered that slugs and snails cannot stand caffeine. Research in the United States has shown that the stimulant is lethal to the pests and repels them even at concentrations lower than those found in a cup of watery American coffee.
The findings, by the US Department of Agriculture (USDA), suggest that brewing a pot of coffee and spraying it on the garden could offer a safe and environmentally friendly means of controlling the pests that eat up to five times their bodyweight in plants every day.
Unlike slug pellets, which are made from highly toxic metaldehyde or methiocarb, caffeine is generally accepted as safe for human consumption, and it is also unlikely to affect beneficial insects.
The horticultural side effects of throwing espresso on the garden, however, are less certain. Trials have shown that caffeine solutions do not damage plants such as palms, orchids, Dracaena and anthuriums, but that they can cause leaf yellowing in ferns, bromeliads and lettuce.
In the long term, scientists are investigating polymers that could be mixed with caffeine in a commercial slug spray, which would stop it from causing any harm to garden plants. At present, however, it is impossible to say whether strong or weak coffee is less likely to damage plants.
The slug-repellent properties of caffeine were discovered by chance, during a study at the USDA’s Agricultural Research Service in Hilo, Hawaii.
A research team led by Robert Hollingsworth was field-testing caffeine as a potential means of controlling a frog pest, and noticed that large slugs were dying when the stimulant was sprayed.
To confirm the effect, they wetted the soil of potted plants infested with a type of slug called Veronicella cubensis with a 2 per cent caffeine solution. After 3½ hours, 75 per cent of the slugs had left the soil, while after 48 hours, all the slugs had slithered away and 92 per cent had died.
Further tests on orchids infested with the snail Zonitoides arboreus found that a 2 per cent caffeine solution killed 95 per cent of the molluscs, and that a 1 per cent solution killed 60 per cent. At much lower concentrations — of the sort found in a cup of coffee — caffeine still works well as a repellent, according to the study, details of which are published today in the journal Nature.
Dr Hollingsworth’s team dipped leaves of Napa cabbage in solutions at 0.01 per cent, 0.1 per cent, 0.5 per cent and 2 per cent, and found that slugs were much less likely to eat the leaves at every value.
When slugs were given no alternative to eating leaves sprayed with caffeine, they ate between 9 and 39 per cent less cabbage, according to the strength of the solution. When given a choice between treated and untreated leaves, their consumption of sprayed cabbage fell by between 11 and 77 per cent. A typical cup of instant coffee contains 0.05 per cent caffeine — more than enough to have a significant repellent effect.
Filter or cafetiere coffee contains a little more, while espresso coffees have the highest caffeine content. “We have discovered that solutions of caffeine are effective in killing or repelling slugs and snails when applied to foliage or the growing medium of plants,” Dr Hollingsworth said.
“Because caffeine is a natural product and is classified by the US Food and Drug Administration as a ‘generally recognised as safe’ compound, it has potential as an environmentally acceptable alternative toxicant for control of slugs and snails.” The mechanism by which caffeine kills slugs and snails is unknown, but the researchers suggested it is a powerful neurotoxin that disables the molluscs’ nervous systems. Slugs sprayed directly with caffeine respond with “unco-ordinate writhing”, they found.
The average garden is estimated to have more than 30,000 slugs and snails. Each can live for up to ten years, and lay 3,000 eggs in its lifetime. The creatures have been named as “pest of the year” by the Royal Horticultural Society. So the solution is simple: just give them a cup of coffee and they will never eat lunch in your garden again.
Five other ways to defeat the pests
Pellets: Effective poison, usually metaldehyde or methiocarb. Pellets should be scattered around plants. Blue colour and bittering agents are meant to deter other animals and birds from eating them and being poisoned. Not for the green gardener
Salt: Sprinkling salt on slugs kills them by dehydration, absorbing water and making them shrivel up. Extremely difficult method of control on a large scale, however. Not for the squeamish
Beer traps: Margarine tubs set into the ground and filled with a little beer attract the slugs, which cannot get out once they fall in. They can then be collected and killed, with the comforting thought that they died happily
Predators and parasites: Natural slug parasites such as nematode worms can be introduced to the garden. Some gardeners bring in hedgehogs, which eat slugs, to control numbers. You also get a happy hedgehog in the garden
Midnight patrols: The simplest but most time-consuming method — picking them up by hand and dropping them in a bucket. What you do next is between your conscience and your neighbouring gardener