An old folk remedy involving hairy bean leaves strewn around the bedroom may have a new life as a modern bed bug trap, according to new research from the University of California, Irvine and the University of Kentucky. With insecticide resistance on the rise, such a device could be a helpful tool for treating bed bug infestations.
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Although its mechanisms weren't known at the time, the tactic dates back to at least 1678, when the English philosopher John Locke wrote of placing kidney bean leaves under the pillow or around the bed to keep bed bugs from biting as he traveled through Europe.
In the early twentieth century, the approach was also common throughout the Balkans, according to a 1927 report from the Imperial and Royal Austro-Hungarian Army. That report suggested the leaves stunned the bloodsucking bugs as they traveled from hiding places to their sleeping hosts during the night; in the morning, the bug-covered leaves were removed and burned (dense infestations could allegedly amass over two pounds of the buggy leaves in a single room).
American entomologists studying the effect in the 1940s noted the bed bugs "could hardly be induced to move from the leaves," and microscopic images suggested that fine, curved hairs called trichomes on the bottom of the leaves snagged the bugs' feet.
Now, the California-Kentucky team has zoomed in even closer to reveal that the leaves' sharp trichomes actually pierce the bugs' feet like meat hooks, immobilizing them.
"It was astonishing to me that it worked at all," says Catherine Loudon, a physical biologist at UC-Irvine and lead researcher of the new study, "You see this big muscular bug vigorously struggling, and it's astonishing to me that the little tiny microscopic hairs don't snap."
Loudon's team tipped single male bed bugs from a glass vial onto the bottom surface of kidney bean leaves, which usually captured the bugs within seconds (they used males, rather than a mix of both sexes, to avoid making baby bed bugs).
A low-vacuum scanning electron microscope (LV-SEM) allowed the researchers to examine the bugs while they were still trapped on the leaves. The images revealed that the trichosomes sometimes hooked the bugs' feet like Velcro, but more often went right through. Some bugs were able to rip themselves free by breaking the trichome or rending their own flesh, but they were usually recaptured.
While there is no evolutionary connection between bed bugs and bean leaves, similar trichomes on other plants are known to capture ants, aphids, bees, flies, and leafhoppers, among other species. Scientists hypothesize that the structures first evolved for other reasons, possibly to retain water, with the defensive role coming later.
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