When trying to curb the the spread of invasive insect species, one of the most important steps is actually being able to catch them. Typical traps, such as those use to catch the tree-ravaging emerald ash borer, use scents such as pheromones to attract males (males are easier to catch because of their mating behavior, which consists of following and landing on females). But scent-based traps aren't ideal: they often require maintenance, and they don't let researchers know exactly how many insects they've has caught.

Fortunately, a group of international researchers appears to have hit upon a brand new way of gaining male beetle's attention: luring them with female decoys. And although the use of a visual cue is interesting on its own, the novelty of their method actually resides in the fact these decoys were biologically accurate, synthetic females (not to mention the fact that the males totally fell for it).

never before have scientists attracted insects "by synthetically replicating a visual signal."

"In the field of entomology this finding is quite novel," said study co-author Michael Domingue, an entomologist at Pennsylvania State University and a co-author of a study published today in Proceedings of the National Academy of Sciences, in an email to The Verge. Most insect researchers are familiar with the chemical attractants that insects produce to attract mates, Domingue explained, and "hundreds of such pheromones are currently used for trapping pests." But never before have scientists been able to attract insects "by synthetically replicating a visual signal from an insect."

In the study, Domingue and his colleagues created three different types of emerald ash borer decoys — two of which were made using molds of actual females, and one using a 3D printer. The 3D printed version was similar in size and shape to the molded ones, but it lacked the surface texture that characterized the other two. Once they were ready, the scientists pinned the decoys onto leaves in a forest in Hungary, along with two dead female emerald ash borer beetles. If the males were attracted to the synthetic decoys, they would land on the decoys as often as they landed on the dead ones, the researchers reasoned.

As expected, the males flew onto the molded decoys as well as the dead ones, whereas male mating flights that were initially directed toward the 3D-printed decoys "were not completed," the researchers write in the study. "The breakthrough we've achieved," Domingue said, "is that we now know that light scattering caused by small bumps and spines on the surface of the bugs are required to obtain a [mating] response."

From left to right, a bioreplicated decoy, an ash borer, a 3D-printed decoy, and an oak borer. Credit: Michael Domingue

The researchers didn’t stop there, however. To ensure that they could use the decoys to catch and kill the beetles, the scientists also made decoys that bore a 4,000-volt charge so when male beetles landed on them, they were zapped and gathered in a cup below — an occurrence that lead to some pretty gruesome damage, the researchers wrote, "such as decapitation or distension of the head or reproductive structures." And interestingly, these zapped specimens also included some females, even though there exists no evidence of females approaching other beetles in nature.

"decapitation or distension of the head or reproductive structures."

"I’m especially interested to see how this method will compare with those currently used for emerald ash borer [...], both in terms of detection and overall cost of the traps," said Dave Jennings, an entomologist at the University of Maryland who didn’t participate in the study, said in an email. Because of North America’s ongoing struggle with the Emerald Ash Borer — an Asian beetle species that was first detected in the US in 2002, and has spread to 24 states and two Canadian provinces since then — developing effective trapping methods has become a priority for entomologists and government officials everywhere.

The "emerald ash borer is a devastating invasive insect pest that has thus far been very successful at avoiding detection methods when present at low population densities," Jennings said. So "being able to find [it] in an area before it’s able to cause significant tree mortality will be very beneficial."

Unfortunately, it’s not yet clear how the males of species will react to the decoys. So far, the traps appear to attract a number of jewel beetle species — beetles of the Buprestidae family — but Domingue’s team isn’t sure if the decoys will be able to target serious pests in a specific way, while excluding less harmful species. The next step in the research will therefore attempt to figure out if varying the decoys’ colorations might increase the traps’ specificity.

The emerald ash borer has spread to 24 states and two Canadian provinces since 2002

Regardless, this study represents a significant advancement for the scientists and government officials who have been fighting to keep the beetle from spreading across North America. Synthetic decoys would require less maintenance than scent-based one, and might bring about new forms of trapping technologies, Domingue said — including "high-tech" systems that could alert researchers wirelessly when males land on decoys. "With that capability, entomologists could obtain information about the pests immediately rather than waiting weeks or months to check trap contents" — a boon when trying to curb the invasion of one of the most tenacious beetle species North America has ever encountered.