Rare good news for butterflies — but pesticides still a big worry

A Monarch butterfly is in a flower
The Monarch is famous for its southward migration and northward return in summer in the Americas which spans the life of three to four generations of the butterfly.

Neonicotinoid insecticides are widely used in agriculture and in residential areas. When insect pests feed on treated plants they get a dose of the insecticide and die. But the chemicals also can harm beneficial insects, and Monarch butterflies were thought to be among that group.

However, when University of Minnesota Researcher Cody Prouty dosed adult monarchs with neonicotinoid, he discovered that wasn't the case.

“Monarchs are actually much more tolerant to neonicotinoids than was previously suggested, which is an interesting finding and sort of goes against some of the earlier work on the effects on adult monarch butterflies,” said Prouty.

While adult monarchs might tolerate the insecticide, the larval or caterpillar stage of the monarch is harmed by the chemical according to previous studies.

“You know, the important thing is that adult monarchs at least are probably not impacted by what's in the field,” said Prouty, referring to the level of insecticide commonly found in ag plants.

There might be a couple of reasons why adult monarchs are tolerant of neonicotinoids according to University of Minnesota professor Vera Krischik, a coauthor of this study and author of several previous studies examining the effects of neonicotinoid insecticides on beneficial insects.

Monarch caterpillar
This monarch caterpillar was photographed in Brookings County, S.D., by USDA entomologist Jonathan Lundgren in 2014.
Courtesy Jonathan Lundgren

First, it appears butterflies do not absorb the chemical in their digestive system, said Krischik, and they have fewer of the central nervous system receptors blocked by the chemical than other insects like bees.

Krischik says while the U of M study shows little harm to adult monarchs, it's important to focus on the insects most at risk from neonicotinoids.

“The canary in the mine is the bees because they're so sensitive. The bees have neonicotinoid receptors. The butterflies don't have as many. So it's really quite obvious that's the main physiological reason,” she said.

While the research shows neonicotinoid insecticides do not harm adult monarchs, Krischik said there are other widely used chemicals such as pyrethroids which pose a serious risk to the butterflies.

The study was published last month in the journal Ecological Entomology.

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