Butterfly survival reduced by warmer and longer autumns caused by climate change

Pupae of the green-veined white butterfly use more energy if autumn is long and warm, which leaves them too weak to emerge as butterflies in spring – and the results might apply to other butterfly species too

By Carissa Wong

Longer and warmer autumns resulting from climate change may reduce the number of butterflies that emerge the following spring.

Many butterfly species are declining due to rising global temperatures, but most research has focused on how changes to spring affect these insects.

“Our study shows that fall [also] has really strong effects on mortality that only appear the following spring,” says Matthew Nielsen at the University of Oulu in Finland.

Shortening daylight hours in the autumn causes the pupae – or chrysalises – of some butterfly species to enter a dormant state that enables them to avoid harsh winter conditions. At the end of winter, these pupae exit the dormant state and continue to develop before emerging as butterflies.

Nielsen and his colleagues collected 459 dormant pupae of the green-veined white butterfly (Pieris napi) – which is found in Europe and Asia – before exposing groups of them to different autumnal conditions. The team transferred the pupae to chambers kept at either 15, 20 or 25°C for periods of between one and 16 weeks. After the simulated autumn, the researchers transferred the pupae into dark chambers at 2°C for 24 weeks to mimic winter.

When they measured the rate of carbon dioxide produced by pupae during the simulated autumn period, the team found that pupae kept in warmer conditions for longer had a higher metabolic rate and therefore used more energy, compared with pupae kept in cooler conditions for shorter periods.

By measuring the weight of the pupae at the start and end of the experiment, they discovered that the pupae exposed to longer and warmer simulated autumns also lost more weight than those that had undergone shorter and cooler simulated autumns.

“These pupae are just using more energy when it’s warmer, which causes them to lose more weight. Exposing them to warm conditions for longer also causes more weight to be lost,” says Nielsen.

The researchers found that this weight loss was linked with a higher mortality rate among pupae at the end of winter, when they continue to develop before emerging as butterflies. For the longest simulated autumns, survival rates were around 60 per cent for pupae at 15 and 20°C, but roughly 10 per cent for pupae at 25°C. The mortality rate was probably due to dwindling energy reserves.

“Emerging as a butterfly is a stressful process that requires a lot of energy, so while the pupae made it through winter, they couldn’t then emerge as adults,” says Nielsen.

The researchers expect these findings to apply to other butterfly species and different kinds of insects. Nevertheless, further work is needed to establish how well the results translate to the real world, says Nielsen.

Journal reference: Functional Ecology, DOI: 10.1111/1365-2435.14037

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