A sweeping Canadian-led study of environmental influences on monarch butterflies has thrown into sharp focus what appears to be the most crucial factor affecting the migrating insect's survival: loss of milkweed in the U.S. Midwest due to a change in farming practices.

Monarchs lay their eggs on milkweed plants every spring and summer as successive generations migrate northward from Mexico as far as Canada. At the end of the breeding season a single "super generation" heads back south, travelling thousands of kilometres so that the cycle can begin anew.

In recent years the overwintering population in Mexico has been on a sharp downward trend, with lowest numbers ever recorded last December.

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The new study, published today in the Journal of Animal Ecology, draws on 30 years of earlier work including information about milkweed prevalence, logging in Mexico, climate change effects and monarch migration timing. Researchers used the wealth of data to assemble a computer model that allowed them to simulate the butterflies` yearly cycle.

"The model replicates, in our opinion, what's happening on an annual basis," said Tyler Flockhart, a post-doctoral researcher at the University of Guelph and lead author of the study.

"We provide the first direct evidence that the population decline is being driven by milkweed loss."

In the model, milkweed loss is estimated separately for different regions of Eastern North America, allowing researchers to "determine not only what is causing the decline of the monarch, but also where," said Ryan Norris, a professor of biology at Guelph.

The evidence points to the U.S. corn belt, where increased cultivation of genetically modified corn and soybean crops comes with a devastating side effect for milkweed.

When GM crops are planted, fields are sprayed with herbicides to wipe out any wild plants that don`t share the crops' genetically engineered protection. In the past, herbicides would typically be applied early in the growing season, when milkweed seeds are still underground. With GM crops, the spraying happens later, and any milkweed growing adjacent to the crops is hit hard.

Even before GM crops were adopted, milkweed was never overly abundant. Farmers found only "30 or 40 stems per acre," said Chip Taylor, an insect ecologist at the University of Kansas who was not involved in the study.

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Despite the modest number plants, a survey done in 2000 found that "corn and soybean fields were producing more monarchs per acre than anything else," said Dr. Taylor, who is also the director of Monarch Watch, a conservation and outreach group.

The study reaffirms that milkweed protection is essential to monarch recovery but officials have a lot of catching up to do.

In Ontario, for example, milkweed has been listed as a noxious weed for more than 60 years – a designation that only just changed on May 9 after a period of public consultation.

Under the Weed Control Act, "every person in possession of land is obligated to destroy all noxious weeds on it," said Mike Cowbrough, the province's weed specialist.

In the U.S. designations vary by state. Monarch Watch runs a program to encourage the planting milkweed across the U.S. and as of last week had shipped more than 30,000 'plugs' – immature plants that are about 10 centimetres tall – to schools and non-profits.

Researchers have also called for an international plan to co-ordinate monarch habitat protection.

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During trade talks last February Prime Minister Steven Harper, U.S. President Barack Obama and Mexican President Enrique Peña Nieto, agreed to such a plan in principle but so far, "there's no money on the table," Dr. Taylor said.