Bed bugs have re-emerged as an urban blight in the past several years, forcing people out of homes, resisting chemical pesticides and evading other removal tactics. But researchers are building bug-catchers inspired by an age-old folk remedy to this "ancient scourge": kidney bean leaves.
Their experiments, described in the Journal of the Royal Society Interface, tested the home-grown solution and even made synthetic leaves that could help scientists devise an easy, environmentally friendly method of trapping bugs before they establish a full invasion.
Bean leaves have historically been used in Eastern Europe as low-tech bug traps: bed bugs wandering a floor strewn with the foliage would get stuck on their surfaces, struggling in vain even as the leaves were collected and burned.
Curious about this simple remedy, study co-author Michael Potter of the University of Kentucky teamed up with lead author Catherine Loudon, a UC Irvine biologist, to test out the practice.
"I was entranced," Loudon said. "It seemed so bizarre and unlikely that bean leaves would be capable of actually capturing bed bugs."
Bedbugs and bean leaves do not usually interact, but the leaves possess a natural defense against hungry insect predators, stopping leaf-eaters in their tracks.
The leaves sport tiny, sharp-hooked hairs called trichomes across their surface. These microscopic hooks, some 10 micrometers in diameter and some 50 to 100 micrometers high, jab into the insects' bodies, trapping them before they get very far. Even those that break free are often trapped again a few steps later.
The scientists made molds of the leaf surface and poured epoxy into them to create synthetic leaves. Some of the natural leaf hooks unexpectedly broke inside the molds and became part of some synthetic leaves when they poured epoxy in.
At first, this was frustrating – until the scientists realized they could use the half-natural, half-man-made leaves to test the pests, too.
The scientists had male bedbugs run across the three types of leaf – real, synthetic and hybrid – and tracked their performance.
The real kidney leaves captured every bug that ran across the surface – typically within nine steps. The synthetic leaves were another matter: They seemed to snag the bugs rather than jab into their bodies, and so the bugs managed to break free and skitter to safety.
The hybrid leaves — the synthetic stalks with the real-leaf hook points -- performed poorly as well; the plant points weren't able to puncture the bugs' shell when attached to a man-made stalk.
"So that tells us there's something very cool and subtle going on mechanically in these tiny microscopic hairs," Loudon said.
The researchers plan to experiment with other materials. Learning the unassuming bean leaf's secrets could help them create a bio-inspired reusable bug trap that would avoid chemical solutions -- and, unlike a real leaf, that wouldn't dry out after a few days.