Second Expedition Studying Impact of Climate Change on Orange Sulphur Butterflies

Hello again! Thank you everyone for your help with my first expedition entering label data for the Orange Sulphur (and if you’re new to the project, welcome!). The first expedition went really well, and I’ve photographed a lot of specimens that need label data, so I’m starting a second expedition. For the second expedition, I’m using specimens from the University of Florida’s McGuire Center (It turns out they have a lot from all over the US).

The research focus remains the same as our first expedition: Orange Sulphurs have many generations each year, and their wing patterns vary seasonally across generations. The underside of their wings are dark during spring and fall, helping them warm themselves, and then lighter during summer to avoid overheating. Climate change, however, is altering the relationship between day length—which the butterflies use to determine their adult wing pattern—and the actual temperatures the butterflies will encounter. I want to know if evolution has helped the butterflies adjust to these changes (see my previous post for more details).

For this expedition, I want to share some more details on our predictions for how these butterflies might adapt to climate change by altering the seasonal variation in wing pattern. We have three main hypotheses:

1. Producing the summer form on shorter days: The warming world effectively makes summer longer. To adjust, the butterflies could change how they respond to daylength. Normally, the butterflies produce the summer form if they experience long days as a caterpillar. In response to climate change, evolution could decrease the hours of daylight required to switch to the summer form, resulting in summer-form butterflies appearing earlier in spring and lasting later into fall.
2. Lighter winter coloration: Instead of changing when their wing pattern changes, instead the wing patterns themselves could change, becoming lighter to be more appropriate in a warming climate. While the summer form can’t get much lighter, the spring/fall form can, so we expect to see a greater change then.
3. Respond to temperature: Daylength indicates time of year, which was historically a better predictor of the weather weeks from now than the current temperature in most temperate climates. With climate change, however, daylength isn’t as good a predictor any more. Instead, temperature may now be a (relatively) better cue and the butterflies could have switched to using it. Some related species already use temperature instead of daylength to control similar changes. It will be hard to test this hypothesis using natural history specimens, but I’m planning additional lab experiments to test it by raising caterpillars with different temperature and light conditions to see which forms the produce.

Right: Colias eurytheme summer form (underside),

Left: Colias eurytheme spring/fall form (underside)

We could also find a combination of these predictions has occurred. Regardless, the world is warming extremely fast, so the butterflies may not be able to keep up. As another part of this project, I’m working with my advisor (Dr. Joel Kingsolver at University of North Carolina) to create a mathematical model to predict the ideal wing pattern for an Orange Sulphur butterfly depending on climate, accounting for the effect of both seasons and climate change. We plan to use this model to determine if the changes you are helping us find using museum collections are enough to keep up with climate change or if the butterflies are falling behind.

— Matthew Nielsen, University of North Carolina at Chapel Hill