Will climate change affect periodical cicadas?
Climate represents long-term changes in day-to-day conditions (weather). A number of excellent public datasources (such as https://www.climate.gov/ ) illustrate how Earth’s climate has changed in the past, it is changing now, and it will change in the future.
Periodical cicada species formed millions of years ago (Sota et al. 2013), but they live in landscapes—the eastern United States—that are much younger, due to glacial cycles. Thus, periodical cicadas have clearly persisted through substantial periods of climate change, and indeed, several authors have directly invoked glaciation to explain the evolution of periodical cicadas (e.g., Yoshimura 1997, Marshall and Cooley 2000). Leading hypotheses also posit that brood formation is driven by deviations from well-established climate patterns (Cooley et al. 2018).
All available evidence indicates that periodical cicadas have been strongly affected by past climate change; thus there is no reason to expect they would not be affected by future climate change. Moreover, Magicicada life cycles are cued by the annual cycles of their host plants (Karban et al. 2000) and by soil temperatures in the spring of their emergence years (Heath 1968). To the extent that climate change affects these annual cycles and soil temperatures, it stands to reason that it will affect periodical cicadas.
Questions about climate change and periodical cicadas are usually directed at understanding how anthropogenic, 19th and 20th Century climate change will affect these insects. Richard Primack’s excellent book Walden Warming documents how the climate at Walden Pond has warmed since Thoreau wrote about it, and how warming is reflected in earlier flowering times for plants and changed migration patterns for birds. All available evidence indicates that the climate is warming, and because some parts of the periodical cicada life cycle seem sensitive to temperature, they will be affected by warming climate.
How will warming affect periodical cicadas?
- We predict that warming climates will cause periodical cicada emergences to start earlier in the year, since spring will arrive earlier as the climate warms.
- We predict that any climate-related disruption of the cues periodical cicadas use to pick their year of emergence will lead to an increase in unexpected, oddly-timed emergences, and, in the extreme, a breakdown of periodicity in these insects.
- We predict that if extreme climatic conditions reliably and consistently induce straggler emergences of sufficient density to satiate predators, then permanent life cycle switches could occur (see Marshall and Cooley 2000, Cooley et al. 2001).
Do the data support these predictions?
One the one hand, Magicicada are the most wonderful research animals imaginable, because they are accessible, they are easy to find and collect, and they lend themselves to simple playback experiments. On the other hand, Magicicada are the most frustratingly impractical research organisms in the world, because their long life cycles make longitudinal studies nearly impossible. The periodical cicada mapping project is intended to gather exactly the kinds of data that will allow us to test these hypotheses with confidence… but it will take a while to collect the data, and the project involves multiple generations… of cicadas and researchers alike. Even so, already during the course of this project, notable “straggling” or off-cycle emergences have occurred, such as the unexpected emergence of Brood X cicadas in 2017, although we do not fully understand their significance. Are they harbingers of a breakdown in the periodicity of these remarkable insects? By keeping careful records, we hope to address that possibility…