New research suggests the monarch butterfly's stress response can be disrupted by exposure to prolonged periods of highway noise.

Every year, the iconic monarch butterfly migrates south from the United States to Mexico. The insects number in the millions. But each year, there are a few million less found at their wintering grounds. Population numbers have been declining for decades.

Scientists and conservationists have blamed a number of factors, but most point to habitat loss as the primary cause of their decline.

To solve the problem, monarch habitat restoration projects have cropped up across the United States -- patches of milkweed where monarchs can lay their eggs and where caterpillars can eat leaves until it's time for their metamorphosis.

Many of these milkweed patches have been placed along highways.

"It seems like it's a win-win," Andy Davis an ecologist at the University of Georgia, said in a news release. "Not only are they located all across the country, but roadside wildflower plantings are pretty and can reduce maintenance costs at the same time. But one of the things that's been overlooked with this push to develop roadside habitats is the vehicle noise. If you actually step outside and listen at one of these roadside habitat sites, it's deafening."

In a series of experiments, Davis and his colleagues showed monarch caterpillars are physiologically affected by exposure to highway noise. The experiments showed caterpillars exposed to the noise for short periods of time experience elevated heart rates. Their heart rate increased the same amount as humans and other animals when exposed to stress.

When exposed to longer periods of stress, 10 to 14 days, the caterpillars heart rate normalized, suggesting the insect had become desensitized to the stressor. That could be a problem for the monarchs when they attempt to fly south.

"The whole reason heart rates increase when animals perceive a threat is so they can pump more blood to the muscles to help them escape or defend themselves," Davis said. "The monarchs' journey to Mexico could be one of the most stressful journeys that any insect undertakes in the world."

Researchers also found the caterpillars began to exhibit more aggressive behaviors after prolonged exposure to stress, biting and fighting each other. Some of the lab researchers were even bit.

"I was shocked," Davis said. "It was just a little pinch but it was just so surprising. I checked with a number of long-term monarch researchers, who've collectively probably reared over 10,000 monarch larvae, and they said they've never, ever had that happen. But if you look at the literature, there is some research that shows that heightened levels of long-term stress in insects is usually correlated with levels of aggression."

The findings, published in the journal Biology Letters, suggests more research needs to be done to ensure monarch habitat restoration projects placed along highways aren't harming the species' ability to migrate.

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