The wings of a butterfly can give clues to the changes happening in their environments and, in turn, ours. At Butler University, Associate Professor of Biology Andrew Stoehr is using those clues to figure out if these wings can serve as early indicators to climate change. The wing patterns could serve as a warning flag for the overall health of the environment.
By measuring changes in the colors and patterns on the wings of the invasive cabbage white butterfly, Stoehr and his students are able to see how changes in temperature affect the butterflies’ health.
The work measures the invasive butterfly’s phenotypic plasticity, which is when environmental factors influence how an organism looks or behaves. Changes in the butterflies’ wing coloration and patterns over time reveal how they are responding to temperature changes that took place while they were still caterpillars. The darker the wings, the colder the temperatures, Stoehr says, and the simple white wings with small flecks of black make the cabbage white butterfly an ideal test subject. Even just a short period of temperature change during development can have a noticeable effect on wing patterns: Just 48 hours of abnormally cool or warm weather, if it occurs at the right time for a caterpillar, can affect the wing pattern of the eventual adult.
Stoehr is an ongoing collaborator in the Pieris Project, a global effort to understand the spread of the cabbage white butterfly and, potentially, its reactions to increasing temperatures. Citizen scientists from as far as Russia, New Zealand, and Korea have shipped the butterflies to scientists involved in this project.
Much to the chagrin of farmers and gardeners of leafy greens, the caterpillars of cabbage white butterflies feast on kale, bok choy, and cabbage. But their prevalence is better for researchers than it is for farmers, and Stoehr has studied butterflies from as close as The CUE Farm on Butler’s campus to as far away as Australia.
“They’re widespread and easy to study,” Stoehr says. “The butterfly’s life is very dependent on temperature. Temperature affects what they look like, and temperature affects what they’re able to do as butterflies, essentially controlling their own temperatures. Can they warm up enough to fly? They’re good ecological models for understanding the role of temperature and changing temperature in basic animal biology.”
With 90-degree heat in October, these little butterflies and their white wings are early subjects for animal behavior in unseasonal heat. If the wing development of these fluttering insects doesn’t match the weather outside, resulting in unregulated body heat, how would other animals react?
An ideal subject
The cabbage white butterfly is not only well-traveled—it can also be found around your garden as early as March and as late as November. The insect’s lifespan is short—probably no more than a week or two as a butterfly. Throughout the summer, each generation of butterflies has lighter wings as the weather gets hotter.
“The population’s wings will change over the course of the year,” Stoehr says. “It takes many days for their wings to develop so they are trying to predict the weather weeks in advance. During those caterpillar stages, they’re receiving information about the temperature.”
These predictions give the butterflies an easier three-week life. As ectotherms, they rely on sunlight and temperatures to function. As a caterpillar and chrysalis, the insect is monitoring the weather so it can develop the most comfortable pair of wings, which are designed to soak in the preferred amount of heat.
Stoehr seeks anomalies in wing patterns — the amount of tiny black wing scales on the white wing background — to reveal unusual weather in a region. What’s a caterpillar to do if it’s 85 degrees one day but then plummets to 55 degrees a few days later?
“In Indiana, there are seasonal patterns of predictability, but they’re not perfectly predictable,” he says. “Do the caterpillars ignore the temperature change and come out mismatched?
This is important knowledge, Stoehr adds, because it tells us that weather fluctuations might be enough to cause a butterfly to emerge mismatched to the temperatures it is likely to encounter. It may be that a cold snap or warm snap is enough to make a butterfly emerge with wing patterns that are not optimally suited for its ability to use those wing patterns to regulate temperature to the conditions it will be facing, compared to what it would look like if it had not gone through that cold or warm snap.
In Stoehr’s research, each insect is photographed before the wing markings are analyzed through software that has collected more than 10,000 data points from the total butterfly wings, which include variations in areas of the wings that change with temperature. Each area is circled and analyzed with the lab’s computer software. The project’s findings will be finalized in 2020.
Initially, the local specimens were studied separately from the samples sent from abroad. However, combining the data could give clues to how the species will endure climate change.
“Do butterflies from different parts of the world develop in the same way in response to temperature and day length variation?” Stoehr asks. “In other words, how do butterflies from northern climates — like Canada and Finland — where the days are longer but also cooler, compare to butterflies from more southern places — like Mexico — where summer days are hotter but not as long?
To add further dimension, Stoehr hopes to eventually explore the use of museum collections of preserved butterflies from decades ago. How do butterflies collected in May 2019 compared to butterflies collected in May 1969?
“Given the way temperature and day length together affect the wing patterns,” Stoehr says, “we might be able to make predictions about how the butterflies look in the future as those two factors become uncoupled from each other. In other words, the temperature is changing but day length does not.”
Out in the field
“The cabbage whites are pretty easy to catch, and they’re very plentiful, especially by the Prairie,” says Makenzie Kurtz, a junior Biology major who has worked in Stoehr’s lab since January. “There’s usually five or six around in one small area.”
Kurtz’s role includes catching butterflies, freezing them, and preparing them for photos before logging each insect. It’s a mix that fortifies her pursuit of a career in research.
“It’s been an overall great experience getting in the field and helping with data analysis,” says Kurtz, who plans on pursuing entomology in graduate school. “It’s interesting to see it all come together.”
Stoehr’s upcoming spring sabbatical will be spent analyzing data and writing his findings from the white cabbage butterfly work. Each wing tells a story about the state of our environment, but just how cautionary will the tales be?
“Since we know something about how their appearance affects their ability to thermoregulate,” Stoehr says, “we might be able to eventually make predictions about whether climate change will increase or decrease populations in different places. It could make them pests in more places than they are now, or it might have the opposite effect.”
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