BREST, France—Faced with the loss of 35 hives to the invasive Asian giant hornet, French beekeeper Denis Jaffré knew he had to act quickly to save his bees. Known as “murder hornets,” they have no natural predators and can eat through an entire hive in a few hours. They have decimated bee colonies across Europe. “I had to find a solution at all cost. I had been so traumatised by the loss of half of my hives,” Jaffré said. So in 2016 the Brittany beekeeper came up with a device with a one-way mechanism like a lobster pot to catch the hornets, thought to have arrived in France in 2004 in a pottery shipment from China. Originally made from a wooden wine crate and metal mesh, his traps are now 3D-printed in plastic. After receiving a French inventors prize in 2018, Jaffré started making the traps in bulk. Demand is …