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Bean hairs (green) impale bed bug foot (yellow). Szyndler et al. 2013. Interface, Royal Society

Can We Fake Leaves That Stab Bed Bugs in the Feet?

Bed bugs have been sucking human blood for thousands of years and they’re enjoying a new resurgence. They are among the most difficult pests to control, and infestations have risen in the last few decades. Old defences like insecticides are failing us, leaving scientists racing to find new solutions. But in the Balkan countries of southeastern Europe, there’s a old folk remedy that might be the insects’ undoing.

Before nightfall, people would scatter the leaves of bean plants on the floor by their beds. In the morning, the leaves would be full of immobilised bed bugs, which could then be taken outside and burned.

In 1944, a scientist called H. H. Richardson realised that this works because of tiny hooked hairs called trichomes dotting the surface of the bean leaves. Each is just a tenth as wide as a human hair and Richardson thought that they tangle the claws of walking bed bugs. Velcro, which was conceived at roughly the same time, works along similar principles.

But no one followed up on Richardson’s discovery. There was the small matter of a distracting world war, and there was little impetus to research other forms of control when insecticides like DDT were so good at killing bed bugs. But since their post-war nadir, the bugs are on the rise again and many have evolved resistance to the pesticides. The bean leaves are looking pretty tempting again.

Mike Potter from the University of Kentucky heard about Richardson’s work and decided to create a synthetic version of the leaves. After a few failed attempts, he turned to Catherine Loudon from the University of California, Irvine, who specialises in insect movements.

Loudon and former student Megan Szyndler discovered that Richardson got one crucial detail wrong about the bean leaves. By photographing and filming them under a microscope, they saw that the hairs aren’t merely tangling the bugs’ feet—they are actually impaling them, stabbing through at their weakest spots. (Bed bugs, of course, are no stranger to awkward stabbings.)

When a bed bug walks onto a bean leaf, it strides across a lethal minefield. The hooked hairs are so dense, that it takes just a few seconds (and around 50 footsteps) for the insect to become inescapably trapped. Sometimes, the hairs just wrap around their claws as Richardson envisioned, in which case they always managed to pull free. But often, they became irreversibly stuck, unable to break away.

When Szyndler scanned the trapped feet using a powerful electron microscope, she saw that the hairs had actually pierced the bugs’ feet. They typically penetrated the softest parts, like the underside of the claws, or the thin membranes between their segments. They’re less like Velcro, and more like butchers’ hooks. Even if the bugs could pull themselves free—which involved either breaking the trichome or their own shells—they would immediately get recaptured.

The results will be unsurprising to some, since bean trichomes are known to puncture the feet of other insects, including agricultural pests like leafhoppers and aphids, and (unfortunately) guardian insects like ladybirds. In fact, these are probably the victims that the beans’ stabby hairs have evolved to entrap. The fact that they also capture bed bugs is (for us) a happy offshoot of a different evolutionary conflict.

But few people have studied the hairs in detail—how many are needed, how sharp they need to be, whether insects can pull free, and so on. Those details could point the way to new ways of controlling bed bugs. After all, people have tried to trap the insects with glue or double-sided sticky tape with little success.

The team returned to the original goal of creating materials that mimic the penetrating bean hooks. You could just scatter bean leaves, but these dry out fairly rapidly and they can only be applied to horizontal surfaces—they’d do nothing to trap bed bugs that are already living in a mattress.

Szyndler first made a mould of a bean leaf’s surface and then used this to create replicas. They looked indistinguishable. They had the same density of trichomes, which came in the same size and had the same sharp, hooked tips. But there was one big difference—they were rubbish at capturing bed bugs. Disappointingly, the fake leaves completely failed to pierce the bugs’ feet.

It’s not clear why. In some cases, the tips of the real trichomes broke off during the moulding process and became incorporated into the synthetic hairs. But even these hybrid trichomes didn’t pierce bed bugs like their natural cousins. Clearly, the real and fake hairs differ in some subtle physical property.

The team thinks that stiffness might be the key one. The natural trichomes are solid at their tips but hollow down their length, while the synthetic ones are completely solid. That makes them stiffer. When a bed bug walks on a natural trichome, Szyndler suspects that the hair would bend and twist, allowing the tip to skit the surface of the bug’s foot until it ‘finds’ a weak spot. By contrast, an artificial trichome would just bend out of the way. The next step is to create synthetic trichomes that mimic the full properties of the real ones, and that’s exactly what the team is working on.

Reference: Szyndler, Haynes, Potter, Corn & Loudon. 2013. Entrapment of bed bugs by leaf trichomes inspires microfabrication. Interface