A dramatic drop in populations of monarch butterflies throughout Eastern North America is being caused by the ongoing loss of their main food source, milkweed, in U.S. breeding areas, a Canadian study has found.
Researchers at the University of Guelph in Ontario, studying the steady decline in the migrating butterflies, confirmed a loss of milkweed is the main factor decimating the species.
A 21 percent decline between 1995 and 2013 has been seen in milkweed, which the orange and black butterflies both feed and lay their eggs on.
The tall and leafy plant, the only one the monarch caterpillars feed on as they develop into butterflies, is steadily being wiped out by herbicides in agricultural regions, the researchers said.
The impact is most profound in the U.S. Corn Belt the butterflies pass through on their way south to their winter grounds in Mexico, they said.
"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 Guelph professor of integrative biology Ryan Norris.
The mathematical models used in their study did not support a previous belief that drops in butterfly numbers was because of deforestation in their Mexican wintering grounds, he said.
"We have essentially ignored what's been happening in the breeding grounds and focusing our efforts on the wintering grounds and this is kind of the situation we're in," he said.
Estimated of monarch populations seen in Mexico during the winter season have fallen to a record low of around 33.5 million from the 350-million annual average.
In February the three countries that lie under the monarch butterflies' migration and wintering routes -- Canada, the United States and Mexico -- agreed to create a panel to study conservation measures.
Discussions on possible milkweed restoration -- with suggestions of planting it along roadsides or as a garden plant -- will be one issue addressed.
Undeveloped lands in agricultural regions such as prairies and parks should also be preserved as sites for milkweed to flourish, the researchers said.
"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," Norris said.
It's estimated monarch populations suffered a decline of 90 percent in the past year, he said.
The Guelph study has been published in the Journal of Animal Ecology.
