Some well-known species had large drops. The red admiral, which is so calm it lands on people, is down 44% and the American lady butterfly, with two large eyespots on its back wings, decreased by 58%, Edwards said.
Even the invasive white cabbage butterfly, "a species that is well adapted to invade the world," according to Haddad, fell by 50%.
"How can that be?", Haddad wondered.
Cornell University butterfly expert Anurag Agrawal said he worries most about the future of a different species: Humans.
"The loss of butterflies, parrots and porpoises is undoubtedly a bad sign for us, the ecosystems we need and the nature we enjoy," Agrawal, who wasn't part of the study, said in an email. "They are telling us that our continent's health is not doing so well ... Butterflies are an ambassador for nature's beauty, fragility and the interdependence of species. They have something to teach us."