Using caffeine as pesticide gives mixed results
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.
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.
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.
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.
"Before now, caffeine has never received much attention as a potential organic pesticide," Upham said.