Every year millions of breeding monarch butterflies in the U. S. and southern Canada search for milkweed plants on which to lay their eggs. Concern over shrinking habitat （居住地）has urged conservationists to create monarch-friendly spaces along roadsides, which are more than enough within the butterflies range and usually publicly owned. But traffic noise stresses monarch caterpillars out, a new study finds. They eventually do become desensitized to it—but that might cause trouble to them later on, too.
Noise pollution is known to affect the lives of birds, whales and other creatures. But until recently, scientists had never tested whether it leads to a stress response in insects. When Andy Davis, a conservation physiologist at the University of Georgia, noticed online videos of roadside monarch caterpillars apparently trembling as cars came by, he wondered how the constant noise might affect them. Davis built a custom caterpillar heart monitor, fitting a small sensor into a microscope to precisely measure monarch caterpillars' heart rates as they listened to recordings of traffic sounds in the laboratory.
The hearts of caterpillars exposed to highway noise for two hours beat 17 percent faster than those of caterpillars in a silent room. But the heart rates of the noise-exposed group returned to baseline levels after hearing the traffic sounds nonstop for their entire 12-day development period, Davis and his colleagues reported in May in Biology Letters.
This desensitization could be problematic when the caterpillars become adults, Davis says. A rapid stress response is vital for monarch butterflies on their two-month journey to spend winters in Mexico, as they narrowly escape predators（捕食者）and fight wind currents.
Whether a noisy developmental period reduces monarchs' survival rates remains unknown, notes Ryan Norris, an ecologist at the University of Guelph in Ontario, who was not involved in the study. But in any case, he believes roadside habitat almost certainly drive up the butterflies' death rates as a result of crashes with cars. " There is so much potential road habitat for monarchs and other insects一it would be such a nice thing to capitalize on, " Norris says. " But you just can't get around the traffic. " Davis adds: " I think roads and monarchs just don't mix. "
