"We're losing milkweed throughout Eastern North America, but what we found out is milkweed loss specifically in the Midwestern U.S. is likely contributing the most to monarch declines," said Ryan Norris, author of a the new study on monarch decline and a professor in Guelph's Department of Integrative Biology.
The migrating orange and black butterfly species lay their eggs on milkweed, a tall and sociable flowering perennial common throughout the U.S. The genus of plants is the only one monarch caterpillars feed on before developing into butterflies.
The study, published this week in the Journal of Animal Ecology, found that when milkweed and monarch decline numbers were compared side by side, sharp dips strongly corresponded.
The most significant decline of milkweed in the Midwest -- thanks to commercial corn growers who spray pesticides to kill the plant -- is largely responsible for the monarch decline, scientists say.
"Reducing the negative effects of milkweed loss in the breeding grounds should be the top conservation priority to slow or halt future population declines of the monarch in North America," said co-author Tyler Flockhart.