It used to be that if you shined a black light on the edge of a cornfield at night, you could expect to harvest a large harvest of moths the next morning. For entomologists, such light traps have provided valuable records of moth populations. However, in recent decades, the catch of all types of insects using light traps has declined. Some interpret these empty traps as evidence that insect populations are declining around the world.
However, other factors may also be at play. In a paper published Friday in the Journal of Insect Conservation, researchers show that some light traps reduce the number of corn moths, a well-known agricultural pest, but others do not. They report that catches in traps of this type are as healthy as ever. This result suggests that something has changed in the moth's attraction to light.
Since the beginning of evolutionary research, entomologists have wondered about moths' tendency to fly toward light sources. Among them was Roland Trimen, who wrote to Charles Darwin to ask how to explain the moth's unhealthy obsession with flames.
“Darwin said, 'That's right. Maybe it's because light is so new that moths don't understand it yet,'” said Avalon Owens, an entomologist at Harvard University and author of the new paper. “I said,” he said. “But over time, you might expect that they would stop doing things like this. He literally put it out 150 years ago and everyone just forgot.”
The fact that the very behavior scientists used to track moths could be punished by natural selection made Dr. Owens wonder. Was there a tracking program that used multiple types of traps?
She found that a small number of farms in the United States monitor corn flea moth levels using both light traps and pheromone traps baited with hormones produced by female moths. She and her colleagues have been analyzing the number of moths caught in each type of trap over the years, and the longest record in Delaware dates back to her 25 years ago.
“Are they telling us the same thing? We asked,” she said. “And the answer is, not at all.”
In Delaware, black light traps initially reliably captured about 30 percent of the moths captured with pheromone traps. After that, the percentage began to decrease. In recent years, light traps captured only 4.6 percent of the amount captured by pheromone traps. Models based on pheromone traps suggest that population levels have not declined compared to 25 years ago. Models based on light traps suggest that moth numbers have declined dramatically. Data from his 10 years of monitoring in New Jersey showed similar trends.
Why is there a difference? Evolution may have removed moths that are attracted to light from the gene pool, as Darwin suggested, and today's corn moths may no longer be attracted to light.
However, another explanation for the reduced effectiveness of light traps may be that they are a result of the world surrounding them becoming brighter. With street lights, spotlights, and everything else illuminating the night, moths may not be as aware of light traps as they are of other shiny objects.
Jolyon Troscianko, an ecologist at the University of Exeter in the UK, said the discovery was an important first step in adjusting the way scientists approach insect monitoring, and the paper added to a field that is just beginning to be debated. He said he was raising an issue.
“This is a very hot topic,” he said.
Does this finding mean that reports of insect declines may have been exaggerated? Unfortunately, Dr. Owens says that even if corn moths are healthy, He said there is enough evidence from other sources to conclude that “Apocalypse” is real. But to understand the causes of these declines, scientists will need to find more reliable measurement tools and adjust their expectations to historical data.
Yash Sondhi, an entomologist at the Florida Museum of Natural History, said the results suggest moth scientists need to expand to other types of traps. Other options besides light traps and pheromone traps include suction traps, which draw moths from the night air, and bait traps, which use substances such as fruit, beer, or honey to lure moths.
“Butterfly people have been doing it for many years,” he said.
Dr. Owens is concerned about the situation for other moth species.
“That's something to worry about for us because the moths come every night to pollinate our plants. They don't give it any credit,” she says, adding that the moth caterpillars It is a major food source for many creatures, he added.
“If you enjoy birds in your backyard, you should be concerned about moths,” she said.