Scientists trawled thousands of volunteer surveys over 25 years to understand what imperils the insects.

By Kate Baggaley | Updated Jul 21, 2021 12:00 PM

For the past three decades, monarch butterflies have been dwindling.

The iconic bugs face a number of threats in North America, from weed killers to climate change, but it hasn’t been clear which one has been the most damaging. A new study, however, indicates that the butterflies are especially sensitive to weather conditions in their spring and summer breeding grounds.

Scientists analyzed data from more than 18,000 monarch counts from across the United States, Mexico, and Canada spanning 25 years. They found that over the past 15 years, climate had an influence on the eastern monarch population that was nearly seven times that of other variables such as herbicide use.

Source: Monarch butterflies are beloved—and declining for this sad reason | Popular Science