What’s driving ominous declines in insects?

While a growing body of research shows decreases in many insect populations, it has been hard for scientists to disentangle the possible causes. Are insects suffering from habitat loss as natural areas are plowed and paved? Is climate change doing them in? What about pesticides?

The latest insight comes from a study on butterflies in the Midwest, published on Thursday in the journal PLOS ONE. Its results don’t discount the serious effects of climate change and habitat loss on butterflies and other insects, but they indicate that agricultural insecticides exerted the biggest impact on the size and diversity of butterfly populations in the Midwest during the study period, 1998 to 2014.

Especially detrimental, the researchers found, was a class of widely used insecticides called neonicotinoids that are absorbed into the tissues of plants.

“It’s a story about unintended consequences,” said Scott Swinton, a professor of agricultural economics at Michigan State University and one of the study’s authors. “In developing technologies that were very effective at controlling soybean aphid and certain other agricultural pests, non-target species that we care about, butterflies in particular, have been harmed.”

Europe largely banned neonicotinoids in 2018, citing risks to bees. The new findings come as wildlife officials in the United States weigh whether to place monarch butterflies, which range coast to coast, on the endangered species list. (They have already found such protections to be warranted but said they were precluded by higher-priority needs.)

In addition to delighting humans and pollinating plants, butterfly species are a critical food source for other animals, notably birds, during their life stage as caterpillars. In fact, research has linked some bird declines to insect declines.

For the new study, researchers integrated multiple data sets and used statistical analysis to make comparisons between different potential drivers of decline across 81 counties in five states. They found that in the median county over the 17-year study period, pesticides were associated with an 8 percent decline in butterflies when compared with a scenario in which pesticide use remained unchanged over the same period. For monarchs, that comparative drop was a whopping 33 percent.

The authors note that these pesticide-related declines began in 2003, coinciding with the appearance and quick adoption of corn and soybean seeds treated with neonicotinoids throughout the Midwest.

Matt Forister, an insect ecologist at the University of Nevada, Reno, who was not affiliated with the study, praised its authors for their “detective work” and for the number of factors they included in the analysis: six groups of pesticides, climate change and land use changes. The study’s finding about neonicotinoids, he said, could be key to helping tackle butterfly declines.

“We often talk like, well, it’s all stressors of the Anthropocene, everything’s accumulating, it’s all bad,” Dr. Forister said. “But when we see one particular thing being bad, as nasty as that looks in the early 2000s, it’s actually kind of hopeful because it means you can make other choices.”

Earlier research by Dr. Forister found that climate change has played an outsized role in butterfly declines in the American West. The authors of the new study were careful to point out that they were not able to evaluate recent impacts from climate change because they had to end their study period in 2014; after that year, the data on neonicotinoid use was no longer available, so they could no longer make the comparisons.

“The last 10 years have been the hottest 10 years on record,” said Leslie Ries, one of the authors and a professor of ecology at Georgetown University. “So what is the impact in the last 10 years? We need to keep studying that, but it’s hard to study it in total when we don’t have neonicotinoid data.”

The Environmental Protection Agency did not respond to questions seeking comment on the study and asking for an explanation of the status of neonicotinoids in the United States.

Climate change isn’t the only factor that appeared less significant in this research than might be the case more broadly. Another is something that happened before the study period: the momentous shift in land use from natural ecosystems to industrial agriculture.

And in a result that seems surprising, the study did not find declines in monarchs from the use of glyphosate, a herbicide commonly sold under the brand name Roundup. Glyphosate eradicates all kinds of weeds including milkweed, the only food source for monarch caterpillars, and its use is widely considered a cause of overall monarch declines. The authors do not contest that consensus; rather they say that, beginning in the early 2000s, the impact from glyphosate “largely disappeared since the largest decline in milkweed had already occurred.”

“That damage is done, and it’s still anchoring monarchs at lower populations than in the past,” Dr. Ries said. “But it’s not explaining declines or changes during that 17 year period.”