Here’s the good news about monarch butterflies, a species whose arrival in Canada has long heralded that spring has finally arrived: the area of its wintering grounds in Mexico in December 2014 increased by 69 per cent compared to December 2013.

Now, here’s the not-so-good news: that 69 per cent increase still represents the second smallest area the iconic arthropod has occupied in Mexico since 1993, the year monitoring began.

In a report published by the World Wildlife Fund (WWF) based on surveys and research from Mexico, experts concluded that monarch butterflies continue to be threatened by three main factors: the decline of milkweed available due to increased herbicide use, which reduces the amount of the insect’s breeding habitat in the United States; deforestation and the illegal logging of its Mexican overwintering sites; and “extreme weather conditions” in North America that affect its migration pattern.

In 2008, experts from Canada, the United States and Mexico collaborated to produce the North American Monarch Conservation Plan (NAMCP). In a press conference in Toluca, Mexico, last month, President Enrique Peña Nieto “agreed to work on the preservation” with Prime Minister Stephen Harper and President Barack Obama to protect what he called one of North America’s “landmark species.”

Karen Oberhauser, an associate professor from the University of Minnesota and the main author of the NAMCP, and a team of researchers are currently updating the plan, a new version of which will accommodate details about more specific factors that are causing the decline of the monarch butterfly population. “A lot has changed since then [2008],” she said. “Here’s one example: the plan was written before neonicotinoids were important. This is a relatively new class of insecticides that I think have only been around in wide use for less than a decade. That’s something that’s important to include.”

The use of herbicides and insecticides such as neonicotinoids in the cultivation of corn and soybeans has led to a 58 per cent decrease in available milkweed, according to the WWF report. Milkweed is an important plant for monarch butterflies because it is where the insect lays its eggs, and is the only food that the caterpillars eat.

The new research has also allowed Oberhauser to measure the rate at which milkweed is disappearing, which was not previously possible. “At the time the plan was written, we hadn’t really quantified the impacts of herbicide-tolerant corn and soybeans,” she said. “What we knew is that we were losing milkweed from rural crop fields. We really didn’t know the extent at which that was happening.”

She also emphasized the continuing destruction of the butterfly’s habitat by human activity as a main factor of its dwindling population. “We have lots and lots of things that are affecting monarchs,” she said. “Most people would agree the biggest one is habitat loss. We think of them as having a migration period and an overwintering period and a breeding period. They need habitat and they have different habitat requirements during each of the stages.”

Oberhauser was reluctant to say whether she was optimistic about the species’ outlook, but that she was glad to see an increase in its population. “It didn’t quite double — [the area occupied] went from 0.67 hectares to 1.13 hectares,” she said. “You’ve got to be happy about that. But we’ve had much bigger increases in the past. We were kind of hoping for a bigger jump, but it’s certainly better than declining.”

Prime Minister Stephen Harper has not yet announced a conservation plan for Canada, but the David Suzuki Foundation says that Canadians can help by planting milkweed.

“We simply cannot afford to wait,” said Faisal Moola, the foundation’s director, in a press release. “It’s time for Canada to step up conservation efforts.”