New ‘Detective Work’ on Butterfly Declines Reveals a Prime Suspect

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.