Besides planting milkweed in the garden, people interested in helping monarch butterflies might want to turn off the porch light.

Biologists at the University of Cincinnati say nighttime light pollution can interfere with the remarkable navigational abilities of monarchs, which travel as far as Canada to Mexico and back during their multi-generational migration.

Researchers found that butterflies roosting at night near artificial illumination such as a porch or streetlight can become disoriented the next day because the light interferes with their circadian rhythms. Artificial light can impede the molecular processes responsible for the butterfly’s remarkable navigational ability and trigger the butterfly to take wing when it should be resting.

“We found that even with a single work light that you find at a construction site, monarch butterflies treat that like it’s the sun,” UC assistant professor Patrick Guerra said.

The study was published in the journal iScience.

With their erratic, circuitous movements taking them to and fro across your garden, it might be hard to imagine monarch butterflies sticking to a rigid flight plan. But their migrations take some monarch populations thousands of miles to the same forests in Mexico where they spend the winter.

Monarchs are not unique in their multigenerational migrations, co-author and UC master’s graduate Samuel Stratton said. He is pursuing a doctorate at the University of Michigan.

“Multigenerational migration is a common system in insects. It’s a fairly common but less observed phenomenon,” he said.

For example, the painted lady butterfly can take six generations to travel 9,000 miles (or 14,484 kilometers) from Africa to the Arctic Circle and back. The green darner dragonfly takes three generations to migrate from midwestern and northern states to southern states where their grandchildren overwinter.

Now researchers want to know whether light pollution is impeding this amazing cross-country trek in monarchs.

“It's an important question given that many migrants fly through urban areas,” Stratton said. “Getting some ecological data would be really helpful to seeing what impacts light pollution can have on orientation and migratory outcomes.”


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