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MANHATTAN, Kan. – As the weather warms this spring and gardeners sit outside, sipping their morning coffee, they may face a new temptation -- to share that caffeine with their plants.
USDA researchers in Hawaii have discovered caffeine solutions aren’t just a wake-me-up for tired humans. Depending on strength, caffeine can repel or kill two major garden/greenhouse pests: snails and their slug kin.
“That’s been exciting news. But, gardeners should wait for more results before trying caffeine as a pesticide,” cautioned Ward Upham, K-State Research and Extension horticulturist.
The researchers’ results underline the point that caffeine can sometimes be a fairly effective poison. The challenge now is to determine safe, but effective application levels for particular plants and soils, Upham said.
“If you’ve been around many school science fair projects, you understand why,” he said. “Sometimes treating plants with caffeine hurts the plant. Sometimes it seems to help the plant. Sometimes it has no effect. All kinds of factors could explain such mixed results, but you have to suspect plant species is one.”
Soil absorption and runoff rates could be important, too. Historically, coffee plantations start poisoning themselves when years of decomposing plant litter raise soil caffeine levels beyond a certain point.
“So far, all that gardeners need to remember is this: Drenching soil or spraying plants with a caffeine solution is simply not the same as the better-known practice of spreading around used coffee grounds,” Upham said.
For example, recent research at the University of Aberdeen, Scotland, found low-dose caffeine solutions killed all three earthworm species being studied. In contrast, gardeners who currently raise earthworms often add used coffee grounds to their vermiculture soil – “with mixed, but apparently no negative results,” he said.
The Pest That Persists
Experienced gardeners are naturally attracted to caffeine’s potential, Upham said. Land snails and slugs are destructive plant pests, particularly in irrigated gardens and wet weather. They’re also difficult to control.
“You never get rid of them completely. Although they mate, they’re hermaphrodites – each one can lay eggs. Those eggs can lie dormant in the ground for years, waiting for favorable weather,” he said. “Besides, the adults spend lots of time underground, too. They feed at night and then hide in dark, damp places. Unless you know what to look for, you could have an invasion without seeing anything but damage.”
The bigger they are, the more damage the pests can do, Upham said. The nation’s widely varying array of snail and slug species fills a range that extends from teeny-tiny to 8-plus inches long.
The creatures can ravage bulbs, chew on roots and make seedlings disappear. They’re often the culprit behind irregular holes in hosta, dahlia, ivy, cabbage, viburnum and other leaves. They create gaping wounds that invite bacteria into such fruits and vegetables as tomatoes, strawberries, potatoes and green beans.
FACTS: The colorful and sometimes patterned banana slugs found in North America’s cool coastal regions can reach 10 inches long. That makes them No.2 on the slug size chart, behind Europe’s keelback species (up to 11.8 inches in length).
The Giant African snail – now invasive in south Florida – established the world’s record for snails with a Goliath measuring 12 inches long and weighing 2 pounds. Florida is working hard to eradicate its examples, which probably are descendants of a pet or pets let loose in the wild. So far, the state’s giant, crop-destroying aliens are maturing at about the size of a human fist.
Entomologist Robert Hollingsworth is heading the current USDA-ARS caffeine studies with gastropod garden pests. He’s said that as the wettest city in the United States, Hilo (Hawaii) is the ideal place for researching slug controls. He can gather dozens of test subjects, just strolling through his back yard.
Snails must live where they can get a nutritional supply of calcium for their shells. They eat limestone, as well as fungal and plant material (living, dead, or digested and deposited). They’ll even eat chalk from rocks. |
“If you find some under a board or they come out after a rain, you learn they also have a ‘yuk’ factor,” he said.
Snails carry their shell on the outside. Slugs may have an inner, vestigial shell. However, most slugs have a mantle (extra fold of skin) on their back, positioned like a woman’s shrug or shawl.
Other than that, snails and slugs are basically the same naked-looking, pulsing cigar of stretchy flesh. They’re covered in a sticky, sometimes drippy slime that soap can’t clean from hands or clothes.
“If you step on one … well, they’re boneless and 70 to 80 percent fluid. It’s a definite yuk,” Upham said. “They probably seem so disgusting, though, because they’re more closely related to squids and clams than to insects or other land animals. The long eyestalks and two sensory ‘fingers’ on their head are tentacles.”
Despite snails’ shell, neither pest has an exoskeleton, as insects do. Both can easily dry out and die – which is the basis for some of their environmental controls, the horticulturist said. That lack of body armor may also be why snails and slugs are vulnerable to caffeine solutions.
Still, the only real clue the pests are at work can be the slime they leave behind. Slugs and snails slowly ripple along by flexing a muscular “foot.” That foot deposits traction-easing mucus – which dries into a silvery trail.
Caffeine on Your Cabbage?
“Before now, caffeine has never received much attention as a potential organic pesticide,” Upham said. “Perhaps that’s because we humans have consumed so much for so long. We almost automatically cut back when we develop overdose symptoms – dizziness, twitching, irregular heartbeat, trouble sleeping.”
Caffeine is a natural substance, he said. Common sources for human consumption include coffee beans, tea leaves, cacao beans (cocoa, chocolate) and kola nuts (cola).
Other plants produce caffeine, too. It helps protect vegetation from predators, he said. Caffeine is actually a bitter-tasting alkaloid (and the reason pet experts warn humans to keep chocolate away from cats and dogs).
The USDA Agricultural Research Service team who discovered caffeine’s slug-disturbing effects was hunting for something different: a natural or easy-access way to control tiny tree frogs (invasive from Puerto Rico). The frogs can breed year-round. Their night-long mating calls can reach the sound level of a back hoe.
The scientists had tried such off-the-shelf products as soaps, Tylenol, pesticides and nicotine. They finally got good results when they tested a soil-applied, diluted caffeine solution in greenhouses where the frogs gathered.
A faster and totally unexpected outcome, however, was that slugs began to surface and die.
“In the Hilo studies, just treating cabbage leaves with a 0.01 percent solution led to a significant drop in slug feeding,” Upham said. “To put that in perspective, instant coffee is about 0.05 percent caffeine. Brewed coffee is somewhat stronger … which could make applying leftover coffee a tempting idea, if we only knew more.”
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