Caffeine shows promise as a slug controller

In a dry year like this one, slugs love to hang out in overly irrigated gardens, or anywhere sprinklers are on often. Artificially moist gardens can keep these slimy little mollusks flourishing when drought conditions would have driven them into early dormancy.

Naturally, one good way to keep slug damage to a minimum is to reduce the amount of irrigation on lawns and in garden beds. Watering less frequently but more deeply and allowing the soil to dry out between waterings is good for soil and plants alike. It's also less encouraging to slugs and snails, which need a regular supply of water in order to get around the garden.

These slippery little buggers make their moves by gliding on homemade slime trails. The lubricating slime contains lots of water, which means slugs can't cope as well in dry gardens.

So what can you do if slugs are still considering your garden as their favorite salad bowl? Try spraying your leftover morning java on beds and borders. Even your coffeepot rinse water could be useful, according to a recent study carried out by USDA scientists.

The USDA tests suggest that even light dilutions of coffee can help keep slugs from destroying plants. Best of all, it probably won't hurt other critters, including frogs. The July issue of Nature (a scientific journal) contains a study demonstrating that coffee sprays that don't harm frogs may even kill slugs under the right conditions.

Ironically, the study that proposed this fascinating new possibility was not intended to be about slug and snail activity at all.

The test results were reported by Robert Hollingsworth of the U.S. Department of Agriculture's Agricultural Research Service in Hilo, Hawaii. He and his team were looking for a safe, non-toxic way to keep plant-harming frogs away from nursery crops of potted plants. While the frogs in question were not especially deterred, the researchers realized instead that slugs and snails also present in the plant nursery test area were dying.

Intrigued, they went on to develop more refined tests using both stronger and weaker solutions of coffee. The team found that spraying a mild concentration of caffeine as low as 0.01 percent (1 part per 10,000) could keep slugs from eating foliage plants. Higher concentrations of 1 to 2 percent (1 to 2 parts per hundred) of caffeine killed both slugs and snails outright.

Since an ordinary cup of instant coffee typically contains about 0.05 percent caffeine, getting the concentration all the way up to 1 or 2 percent is more like showering your garden with a double espresso than using the rinse water from your coffee pot.

As yet, nobody has jumped on the coffee wagon to start making and marketing caffeine-based slug baits. It also is still unclear whether tea-based baits would serve the same purpose. However, since the Food and Drug Administration presently considers caffeine to be a safe substance, it is clearly only a matter of time before slug joe is on every nursery shelf.

Exactly how does coffee affect mollusks? As yet, nobody really knows, but the Hilo research team suspects that caffeine probably is affecting the critters' nervous systems. Coffee jitters might be uncomfortable for people, but for slugs and snails, the condition may prove fatal.

Will coffee control a wider range of pests? That, too, remains to be seen, though further testing certainly will be in the works soon.

The authors of the Hilo study report that mollusks may be especially vulnerable to contact poisoning from caffeine because the chemical is readily water-soluble. As noted, slugs and snails move through the garden by creating a smooth layer of slime or mucus, residual trails of which often mar garden plants.

This locomotive mucus contains a high percentage of water, which makes it a perfect way to introduce a water-soluble substance to slugs. When they crawl over a caffeine-sprayed area, the chemical would be absorbed directly into the mollusks' soft bodies.

The research clearly indicates that frogs are not susceptible to sprays of caffeine. This is very good news, since even environmentally benign slug baits such as Sluggo that are based on iron phosphate are not considered safe for use near water. This is because frog and fish eggs are especially vulnerable to pesticide and herbicide damage.

Indeed, even the nitrogen from grass clippings tossed carelessly into streams, pools, ponds or open ocean water can kill susceptible eggs. Indeed, a few inches of fresh grass clippings can generate enough heat to damage or kill eggs as well.

If coffee proves to be of lasting value as a slug bait, it may spark a boom in the recycling of commercial coffee grounds. A safe, eco-cool slug deterrent that could be used in damper sites would be a boon indeed for water gardeners.

Ann Lovejoy, a free-lance garden and food writer, can be reached via mail at: 9010 Miller Road N.E., Bainbridge Island, WA 98110. Her latest books are "Ann Lovejoy's Organic Design School: A Guide to Creating Your Own Beautiful, Easy-Care Garden" (Rodale, 280 pages, $35) and "The Sage Garden: Flower and Foliage for Health and Beauty" (Chronicle Books, 144 pages, $17.95).

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