Like Moths to a Flame? We May Need a New Phrase.

It used to be that you could put a black light at the edge of a cornfield at night and expect a bountiful harvest of moths the next morning. For entomologists, such light traps have provided an invaluable record of moth numbers. But in recent decades, light traps have shown dwindling catches of insects of all kinds. Some have interpreted these empty traps as evidence of a documented decline in insect numbers around the world.

But there might be other factors at play. In a paper published on Friday in the Journal of Insect Conservation, researchers report that while some light traps show declining numbers of corn earworm moths, a well-known agricultural pest, their catches in another kind of trap are as healthy as ever. The results suggest that something has changed in the moths’ attraction to light.

From the very beginning of the study of evolution, entomologists have wondered about moths’ tendency to fling themselves at light sources. That included Roland Trimen, who wrote to Charles Darwin to ask how he explained moths’ unhealthy obsession with flames.

“Darwin was like, ‘Very true, maybe it’s because lights are quite new and moths haven’t quite figured it out yet,’” recounted Avalon Owens, an entomologist at Harvard and an author of the new paper. “But you might expect that over time they will stop doing this. He literally put that out there 150 years ago, and everybody just sort of forgot.”

The fact that natural selection might punish the very behavior scientists used to track moths made Dr. Owens wonder: Were there any tracking programs that used more than one kind of trap?

She discovered that a handful of farms in the United States used both light traps and pheromone traps, baited with a hormone produced by female moths, to keep an eye on levels of the corn earworm moth. She and her colleagues analyzed the number of moths that were caught in each kind of trap over the years, with the longest records, from Delaware, stretching back 25 years.

“We asked, are they telling us the same thing?” she said. “And the answer is, not at all.”

In Delaware, at first, black light traps reliably caught about 30 percent of the number of moths that pheromone traps caught. Then that percentage began to decline. In recent years, the light traps caught only 4.6 percent of the amount the pheromone traps did. A model based on the pheromone traps suggests that population levels have not declined compared to 25 years ago; a model based on the light traps suggests that moth numbers have plummeted. The data over 10 years of monitoring in New Jersey showed a similar trend.

Why the difference? It might be, as Darwin suggested, that evolution has removed moths with an attraction to light from the gene pool, so that today’s corn earworm moth is no longer as drawn to light.

But another explanation for the decline in light trap effectiveness might be that it’s a consequence of the world surrounding those light traps growing much brighter. With streetlights and spotlights and everything else lighting up the night, moths may not be noticing the light traps as much as they notice other glowing things.

The findings are an important first step toward adjusting the way scientists approach insect monitoring, and the paper raises issues that the field is just starting to discuss, said Jolyon Troscianko, an ecologist at University of Exeter in England.

“This is very much a hot topic,” he said.

Do the findings mean that reports of insects’ decline may have been overblown? Unfortunately, Dr. Owens said, there is enough evidence from other sources to conclude that the “insect apocalypse” is real, even if the corn earworm moth is doing fine. But if scientists are to understand what is causing these declines, they will need to find more reliable means of measurement, and adjust their expectations of historical data.

The results suggest that moth scientists may need to branch out into other kinds of traps, said Yash Sondhi, an entomologist at the Florida Museum of Natural History. Other options beyond light and pheromone traps include suction traps, which vacuum moths out of the night air, and bait traps, which use substances like fruit, beer and honey to lure moths.

“The butterfly people have been doing that for ages,” he said.

Dr. Owens worries about the status of other moth species.

“It should concern us because moths are out pollinating our plants nightly. They get no credit for it,” she said, adding that moth caterpillars are a major source of food for many creatures.

“If you enjoy birds in your backyard,” she said, “you should be worried about the moths.”