Winged invasion

For nearly two years, University of Nevada, Reno doctoral student Anne Espeset has been helping collect cabbage white butterflies from all over the globe as part of a citizen science effort called the Pieris Project. Now, with more than 1,400 specimens in hand, she and a team of researchers from across the U.S. are using these samples to learn about how species adapt to changes in their environment.

The cabbage white (Pieris rapae) is a small white butterfly with black wing-tips and one or two black dots on each forewing. Familiar to many vegetable gardeners, these butterflies lay eggs on the leaves of cruciferous vegetables, which are preferred food items for their hungry caterpillars.

“Gardeners do not like them,” Espeset said. “The caterpillars will eat your entire cabbage plant. They’ll eat anything in the Brassicaceae family—so, cabbage, radishes, broccoli, anything of that sort.”

Cabbage white butterflies are native to Europe, but have spread across Asia, North America, Australia, Hawaii and other locations. Small and fairly inconspicuous, they are now one of the most common and widespread species of butterfly in the world.

Although many research projects focus on rare or endangered species, the Pieris Project takes the opposite approach, focusing on how and why this extremely common species is able to do well in so many environments. To do so, they are recruiting citizen scientists from around the world to send in samples of cabbage white butterflies that they catch in their gardens or yards.

Espeset and her colleagues are interested in learning exactly how these butterflies spread from place to place, and about adaptations that they make to live in different environments. They use genetic samples from the dead butterflies that people mail in to determine relationships between various populations.

“These are a pest species,” Espeset said. “They came from Europe sometime in the 1800s, We’re trying to determine if that was just one introduction, or if there were multiple introductions across the United States.”

In her laboratory at UNR, Espeset sorts through thousands of tiny envelopes of butterfly wings. Some come from as close as Lemmon Valley, others from far-off locations like Japan and Australia. Working with advisor Dr. Matt Forister, Espeset’s particular research interest is in discovering whether the physical appearance of the cabbage white butterfly changes in different environments.

To study this, she analyzes each specimen for pigments called pterins, which are rich in nitrogen and responsible for the white color of the butterflies’ wings. In agricultural landscapes, use of nitrogen from fertilizers may make it easier for male butterflies to obtain the nutrients they need to make these wing pigments, says Espeset—a contrast to the conditions these butterflies face in non-agricultural environments.

“In a natural population, only good quality males are able to sequester and allocate these pterins to their wings, so then the female would want to choose those individuals [to mate with]. I’m seeing how human environmental change is affecting that natural process,” Espeset said.