Warmer, Longer Autumns Could Hurt Butterflies

As fall seasons are longer and warmer, it becomes less likely that certain butterflies will survive to emerge in the spring, new research finds.

Scientists in Sweden, Finland, and Germany studied the potential effects of climate change on green-veined white butterflies (Pieris napi). They exposed the chrysalises of these butterflies to conditions of different warmths and lengths to simulate changing autumns.

“We are starting to understand that warm winters can actually be more stressful for overwintering insects than cold ones. Fall conditions are warmer than winter ones (almost by definition), so they have potential to be especially stressful,” Matthew Nielsen at the University of Oulu in Finland, who conducted the research at Stockholm University, tells Treehugger.

“We wanted to know whether autumn conditions actually are stressful for butterflies and whether the length of time they spent dormant during the fall or the temperature during this time was more important. (It turns out it's both).”

Researchers already knew a lot about overwintering and diapause—a hibernation-like dormant state where insect activities and growth stop for a period of time—for this species. That’s why they were a natural fit to find out what happens to them before winter, Nielsen says.

“We raised caterpillars under conditions that would tell them to get ready for winter, and then kept the pupa in different autumn treatments: different temperatures for different amounts of time,” Nielsen says. “During this time we measured their weight and energy use periodically, and at the end of the autumn treatments, we placed all individuals in the same winter treatment and checked how well they developed into adults afterward.”

The chrysalises were exposed to temperatures as warm as 25 degrees Celsius (77 degrees Fahrenheit) for as long as 16 weeks. Although that might seem extreme for fall, the authors point out that the temperatures already occur in some southern parts of the butterflies’ range. Those warmer, longer temperatures could happen in more northern areas in future climate change situations.

They also tested groups of eight to 11 chrysalises in 15-degree C (59 F) and 20-degree C (68 F) scenarios from one to 16 weeks. They then exposed the collection of 459 chrysalises to the same winter conditions for 24 weeks.

During the various simulated autumn scenarios, researchers measured how much energy the chrysalises used and how much weight they lost. They charted their survival to see whether they died or made it to a simulated spring, emerging as healthy adults.

“We saw both immediate and delayed impacts of long, warm autumns. During our autumn treatments, the pupae were using more energy and losing weight, both more when the autumn was warmer,” Nielsen says. “Then, in the spring part of our experiment (which was the same for all of the butterflies), butterflies which had had a long, warm autumn were much less likely to survive and emerge as healthy adults, even if they had survived the autumn itself.”

The results were published in the journal Functional Ecology.

Weight and Survival

Researchers say they know the study doesn’t exactly mirror reality, but the findings are still significant.

“So these lab conditions don't directly mimic real life—we wanted to understand what conditions the butterflies could survive—but they are relevant to real life. When butterflies enter diapause varies in nature, and some enter two months (a whole generation) earlier than others,” Nielsen says. “We studied this species in Sweden, and our warmest treatment (25 C [77F]) would be quite warm by Swedish standards, but this species occurs much further south (as far as Spain) where it is warmer, and of course, temperature will also be increasing with climate change.”

The green-veined white butterfly currently survives well when it’s dormant in the fall in Sweden. But with climate change increasing temperatures, winter is starting later.

“Thus butterflies will likely start losing more weight during autumn and may eventually be at risk of not surviving until adulthood in the spring,” Nielsen says. “That's the other really important result of this study: We show how the seasons are connected and that stress at one time of year can have long-lasting consequences that may not manifest until later seasons.” 

The results likely aren’t limited to just this one species of butterfly.

“Autumn conditions are something any butterfly (or other insects) needs to be able to survive, so the challenge we studied here is widespread and applies to any insect that needs to overwinter. And most places will have warmer autumns because of climate change,” Nielsen says.

But he points out that different species have different strategies for overwintering.

“Some species which eat plants which only grow in the spring have already evolved to survive the entire summer in diapause. Autumn conditions may have very different impacts on species that use these different strategies, so we'll need to study a greater diversity of species to fully understand the impacts of warming temperatures.”