KREFELD, Germany — In a nature preserve in western Germany, an elderly gentleman approached a tent-like structure that was in fact a large trap for flying insects. Peering through thick eyeglasses, the 75-year-old retired chemist checked the plastic bottle attached at the top, filled with alcohol and bugs.

Then, with a glance at the clear, late-autumn sky, the man, Heinz Schwan, recalled comparing a 2013 haul from a trap like this one to samples taken in the same place some 20 years earlier. The drop was huge: “75 percent,” Mr. Schwan, a caterpillar lover, said.

Alarmed, the group of local insect enthusiasts Mr. Schwan is co-chairman of ran similar tests in different locations the next year. And the next year. And the next.

Now their findings have been made public. The news shot around the world, eliciting headlines about “insect Armageddon.” The insect populations they tested had declined by more than 75 percent over the last three decades, explaining why, these days, car trippers no longer need to clean bugs from the windshield.