Monarch butterflies harmed by common neonic pesticides, study suggests

A recently published study suggests that one of the world’s most common pesticides, previously found to harm bees, may be contributing to the decline of one of its most-loved butterflies.

University of Guelph researcher Ryan Norris conducted one of the first real-world studies on monarch butterflies and so-called neonic pesticides. He says the chemicals seem to reduce the number of eggs that successfully hatch.

“It’s the first field evidence that neonics can have a negative impact on larval survival of monarchs,” Norris said in an interview Wednesday.

Monarchs undergo one of nature’s most remarkable migrations, fluttering all the way from Canada to Mexico and back. But their numbers have declined more that 80 per cent over the last two decades and scientists are trying to find out why.

Neonicotinoid pesticides are widely applied to common crops such as corn, and often drift onto other plants, including milkweed, which monarchs depend on for nesting and food. Monarchs actually prefer milkweed growing alongside or within cultivated fields, Norris said.

“We don’t know why. But that’s where they get hit the hardest (with neonics).”

The research, published in the Journal of Animal Ecology, involved Norris and his colleagues working with a farmer near Halton, Ont. The farmer planted one half of a small plot with corn seed that had only been treated with a fungicide and the other half with corn that had been coated with clothianidin, a common neonic. 

Milkweed was deliberately planted along with the corn to attract monarchs.

Millions of larvae could be dying

Over two years, the scientists found that monarch eggs on the neonic plot had a three per cent less chance of successfully hatching. It sounds small, said Norris, but with the large number of eggs monarchs lay, it adds up to big numbers.

“(That) could easily mean millions of larvae that are dying each year because of the neonics.”

The findings of the study easily mean millions of monarch larvae or caterpillars are dying each year because of the neonics, the researchers say. (Carlo Allegri/Reuters)

Seeds treated with three neonic pesticides, including the one used in the study, have been restricted in Europe since 2013 due to potential harm to honeybees, and are also banned in some U.S. states. Studies suggest they can also harm bumblebees, ground-nesting bees and birds. 

In 2018, Health Canada proposed to tightly restrict the use of neonics, including a ban on all outdoor applications of clothianidin. It has since decided against a ban, but has added new rules to protect pollinators and aquatic invertebrates.

Millions of monarchs migrate each winter to a small area of mountaintop forest in central Mexico, where scientists estimate their population by measuring the area of trees turned orange by the clustering butterflies. That area has shrunk to just over two hectares, down 26 per cent from last year, says the Centre for Biological Diversity.

Monarchs are considered a species of special concern in both Canada and Mexico.

The black-and-orange butterflies face many threats other than pesticides, Norris said, but pesticides seem to be part of the problem.

“This is yet another piece of evidence of how neonics can influence the biodiversity on our landscape,” he said. “They are having a serious negative impact.”

Although monarchs aren’t important pollinators, that’s not the only measure of a species’ value, Norris suggested.

“Monarchs, in many people’s minds, represent butterflies,” he said. “When they think butterflies, they think monarchs.

“They serve a really important role as a connection for people to butterflies and to nature.”

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