Don’t blame monarch butterfly losses on milkweed, scientists say

The plight of the monarch butterfly has become a touchstone environmental issue in recent years, with conservationist groups, citizens and governments all across North America now taking steps to save the iconic species.

For the most part, these efforts have focused on replenishing milkweed supplies, the monarch’s only larval food source, yet a new study from the University of Illinois Urbana-Champaign suggests that declines in monarch butterfly populations may not be solely attributable to a loss of milkweed.

For the monarch butterfly, a continent-wide population estimated at 700 million in 1997 plummeted to a low of 42 million in 2015, prompting scientists and environmental groups to call for policy changes and citizen-led campaigns around the protection and proliferation of the milkweed plant. Found in ditches and fields and commonly considered a scourge by farmers and, until recently, governments as well, milkweed losses due to herbicide use have been targeted as the main culprit for the disappearance of the monarch.

In 2014, Ontario’s Ministry of Agriculture, Food and Rural Affairs took milkweed off its list of noxious weeds, citing the importance of the plant to the monarch species, and in 2015, the US Department of Agriculture launched a 10-state effort to encourage farmers and conservation partners to plant more milkweed along field borders and wetland buffers. In Canada, the David Suzuki Foundation took up the cause, urging citizens in its Got Milkweed campaign to plant more milkweed in yards and gardens across the country.

But the connection between monarch losses and the destruction of milkweed has been challenged in a recent study by researchers at the University of Illinois, who reviewed decades of studies on monarchs and analyses of milkweed losses and found that the cause of monarch population declines is more multifaceted than previously assumed.

Researchers looked at milkweed crop losses in the state of Illinois, habitat for a substantial portion of the US’s monarch population, and determined that in agricultural land across the state, milkweed declined by about 95 per cent over the last 20 years. But the team also found that in natural areas, milkweed had dropped by a lesser amount, 50 per cent, a trend which showed that the milkweed in natural areas was buffering the loss of the plant in agricultural zones.

Further, the researchers concluded that monarch populations, once they returned to Illinois from their over-wintering in Mexico, were doing very well and ballooned in numbers, indicating that being unable to find milkweed in the state was not a determining feature for the species.

“Previous studies have found that even when small numbers of monarchs leave Mexico, they’re able to rebuild their populations within a couple of generations of reproduction in the summer in the Midwest,” says plant ecologist Greg Spyreas at the University of Illinois at Urbana-Champaign. “That suggests that the supply of milkweed plants here is not the primary problem.”

Instead of being a one-issue problem, the researchers say that many elements — habitat loss, disease, parasites and climate change — are likely in play. The study’s authors suggest that since much of the population losses seem to occur during the monarch migration down to Mexico, a lack of late-flowering nectar sources along their route should be considered a primary contributor.

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