Slugging It Out with Caffeine

Anyone who has raised tomatoes in a moist environment knows the tell-tale sign: Overnight, a ripe, juicy orb sustains a huge, oozing wound. If you arrive early, you might catch the dastardly culprit: a slug.

Who would have thought that a defense was as close as your coffee cup?

Federal scientists have discovered that the same chemical that provides the pick-me-up in a cup of java is a deadly turn-off to snails and slugs. Caffeine renders their food unpalatable. Applied to their soil, the stimulant causes snails and slugs to writhe uncontrollably. At the proper dose, these mollusks succumb to the neurotoxin fairly quickly.

The discovery emerged in greenhouse experiments by Agricultural Research Service scientists in Hilo, Hawaii. As the wettest city in the United States, it’s slug heaven, observes Robert G. Hollingsworth, who led a series of caffeine experiments reported in the June 27 Nature. For each trial, he mined 50 to 100 slugs from the field–aka his backyard.

But why caffeine? Earl Campbell, now with the U.S. Fish and Wildlife Service, and his ARS colleagues stumbled on this anti-slug measure while looking for a pesticide to eradicate noisy frogs.

Because there are frogs

The Hawaiian Islands evolved in the absence of amphibians and reptiles. However, some 40 different species of these nonnatives have taken up permanent residency on at least a few of the state’s lush islands. They arrived through trade, stops by tourist vessels, and the deliberate release of pets.

Two of the more recent and troublesome of these aliens are tiny Caribbean frogs from the same genus. Though both are noisy, the species Eleutherodactylus coqui has become especially vexing. Its mating calls, which can go on all night–and year-round in low-lying areas–reach 90 decibels, the volume of barking dogs and vacuum cleaners. The Occupational Safety and Health Administration requires that workers wear hearing protectors when noise averages 85 decibels or higher. At that volume, sustained exposures can produce irreversible hearing loss.

In parts of Hawaii since the mid 1980s, this volume has become typical of backyard coqui choruses. So, while amphibian populations throughout much of the world are declining or becoming extinct, mushrooming populations of Hawaii’s frogs have become not only a public nuisance but also an ecological nightmare. The situation puts state and federal biologists charged with safeguarding Hawaii’s environment in the uncomfortable position of targeting healthy populations of cute frogs for execution.

After working their way through soaps, surfactants, and off-the-shelf pesticides–all without antifrog effects–Campbell’s group started to evaluate products in the grocery store, including acetaminophen (Tylenol) and cigarette nicotine. “We had very poor results with almost all of these,” Campbell told Science News Online. Finally, his team tried a caffeine-rich anti-sleep preparation. “It was the only thing that worked at a legal [label’s recommended] level,” Campbell says.

When field trials established its promise, the researchers petitioned the Environmental Protection Agency for an emergency exemption to use caffeine against Hawaii’s coqui and greenhouse (E. planirostris) frogs. Provisional permission came through last November. Preliminary, targeted eradication and control programs are now set to begin within the next few months.

It was during early evaluation of caffeine’s potential that the researchers applied a dilute concentration of the compound to the soil in greenhouses where many frogs were holed up. At once, Campbell noticed that slugs began surfacing and dying.

That interested Hollingsworth, an entomologist studying pests of ornamental plants, such as potted orchids and anthuriums. Small snails have proven a bane to orchid growers, he notes. Though they don’t hurt the blooms, some of these shelled slugs chew away at roots, loosening the plants’ anchor.

So, Hollingsworth launched tests of various concentrations of dilute caffeine against those orchid snails, known as Zonitoides arboreus, and that local garden denizen, the two-striped slug (Veronicella cubensis). The tests showed what plants around the globe had discovered long ago: Caffeine makes a good all-natural pesticide.

Plants have known, all along

Caffeine, though associated widely with coffee, also appears naturally in tea, cacao–the source of chocolate–and a host of other plants. The reason, according to the 2001 opus The World of Caffeine by Bennett A. Weinberg and Bonnie K. Bealer, flora the world over have found this compound a useful weapon to control predation by bacteria, fungi, and insects.

However, the book notes, exploitation of this natural poison comes at a price, because “the very drug that helps them destroy their enemies ultimately kills them as well.” With coffee, for instance, as branches, leaves, and berries fall to the ground, caffeine leaches out of this litter, eventually enriching soil caffeine concentrations to a point where they become toxic to the parent plant. This is one reason that the productivity of coffee plantations tends to wane with time, the book observes.

In their new field trials, the Hawaiian scientists have also seen evidence of plant toxicity with some of the higher pesticidal concentrations of caffeine. However, they’ve also witnessed responses of the targeted pests to low concentrations.

A 4-ounce solution of 2 percent caffeine applied to the soil of 4-inch greenhouse pots devastated garden slugs, Hollingsworth found. Within 3.5 hours, 75 percent of the slugs emerged from hiding in the soil. Within 2 days, 92 percent of the slugs were dead. When the researchers dropped the concentration of caffeine by half, it took another day to achieve the same body count. When they halved the caffeine level yet again, the kill rate dropped to 55 percent and the time to death extended to 5 days.

Even concentrations of only 0.1 percent caffeine may prove useful. Sprayed onto such slug-prized cuisine as cabbage leaves, those concentrations deterred feeding by 62 percent, respectively, when compared to uncaffeinated salad greens. This suggests that a regular spray of leftover coffee, which tends to have a caffeine content of about 0.1 to 0.05 percent, might control nighttime crop losses in the garden.

Hollingsworth also reports a “contact” repellency of caffeine on garden slugs. In one unpublished experiment, he sprayed half of the soil in a pot with 2 percent caffeine and left the rest untreated. “I put the slugs onto the part that was not sprayed–and watched some of them go right up to the edge of the sprayed part and then turn around,” he notes.

Eighteen years ago, Harvard Medical School scientist James Nathanson reported finding that caterpillars would actively avoid eating garden leaves sprayed with caffeine. Though this led many researchers at the time to hail caffeine as the next all-natural pesticide, commercial pesticide manufacturers passed on any opportunity to exploit the finding. According to Dave Ryan of EPA in Washington, the agency’s pesticide division has “no record of caffeine as an active ingredient in any [registered] pesticide”–besides, that is, the recent temporary permit for the use of dilute caffeine against Hawaiian frogs.

Says Hollingsworth, “I think one reason caffeine never went anywhere as a pesticide for bugs is that most insects have this [water repelling] exoskeleton, making it hard for the caffeine to penetrate.” Not so, slugs and snails. “The mucus, which is the basis for their locomotion, is very high in water content,” he observes, and it permits water-soluble caffeine easy entry. Once inside the critters, the new Hawaiian studies show, the neurotoxic caffeine destabilizes the mollusks’ heart rate.

Now, Hollingsworth says, the trick will be to find ways to package caffeine so that it’s available to kill frogs without posing a risk to plants or untargeted organisms.

For me, an avid tea drinker, I’ll just try spraying vine-ripening tomatoes with my husband’s leftover coffee. It seems the best use of that other brew.