Thousands of monarch butterflies are caught and tagged during their fall migration, and just as many caterpillars are used in classroom demonstrations and outreach events. A new study has found that human handling can put a great deal of stress on monarch butterflies when they are in the early stages of life.
Andy Davis is an assistant research scientist in the Odum School of Ecology at the University of Georgia. He set out to investigate how human handling may adversely affect monarch butterflies. Davis looked into whether monarchs had faster heartbeats after being handled by humans. Among insects, elevated heart rate is a sign of stress that is triggered by perceived predators.
“Most people don’t realize that insects get stressed in much the same way as vertebrates do,” said Davis. “People handle monarchs for all of these activities, but if you imagine yourself in their place, what would it be like for us if a 50-foot-tall robot picked us up and passed us around? From the monarch’s perspective, maybe for them this is akin to being attacked by a predator.”
Davis measured the heart rates of the butterflies at all life stages – larval, pupal and adult.
Under a microscope, the heart rates of caterpillars were assessed before and after they were carefully passed from hand to hand for three minutes.
Adult butterflies, who were netted during fall migration, underwent a similar process. Davis measured their heart rates before and after he simulated the procedure for tagging them.
To measure the heartbeats of monarch pupae, Davis used an electronic device that could remotely detect heart movement inside the chrysalis. This process was more complicated, especially due to the unusual cardiac function of monarch pupae.
“At the pupal stage, their hearts beat in a weird way,” said Davis. “The heart will beat for about 15 minutes and then stop for up to two hours; it flatlines.”
The study revealed that human handling significantly impacted heart rates at the two early stages of life. Caterpillars experienced a heart rate increase of approximately 20 percent, while pupal heart rates skyrocketed from zero to 88 beats per minute. This is almost three times higher than the typical active beating rate for pupae, according to previous research.
Meanwhile, the heart rates of adult monarchs showed no significant change after handling. This indicated that either the experiment did not capture the stress response of the adult butterflies or that gentle handling does not cause them stress.
“It kind of makes sense that adult monarchs would be fairly stress-tolerant,” said Davis. “The migration itself is fraught with dangers and perils along the way, and for a monarch to be able to accomplish that, it has to be able to deal with a wide variety of natural stressors.”
Davis said that just because larvae and pupae seem prone to stress reactions from handling, people should not stop working with monarchs.
“These results do not mean people should stop important activities like research and outreach. But the results should at least make us more cognizant about how the monarch is feeling about being handled. In other words, we shouldn’t kid ourselves by thinking the monarchs are OK with it.”
The study is published in the Journal of the Lepidopterists’ Society.