During a periodical cicada emergence, millions upon millions of shrimp-like insects synchronously crawl out of their burrows after 13 or 17 years underground, molt into winged adults, and briefly saturate the local landscape, providing food for a wide range of generalist predators. Despite centuries of scientific research on the bizarre life histories of these eastern North American insects, the ecological impacts of their mass emergences are still poorly understood. 

 

How might this sudden bonanza of palatable, easy-to-catch food alter the functioning of forest food webs? Typically, in what is called a ‘trophic cascade,’ many bird species feed on caterpillars, reducing their numbers and limiting the negative impacts of these herbivores on trees. So, what might happen when the cicadas appear? Would birds temporarily switch over to eating the new arrivals, thereby reducing their consumption of caterpillars, and indirectly resulting in increased damage to trees, or would they mostly ignore this unfamiliar prey? 

 

To find out, our team measured the extent to which birds were feeding on caterpillars before the cicadas showed up. Once a week, throughout the late spring and summer of the year before the emergence, we glued a fresh set of fake caterpillars, made of non-toxic green modeling clay, to understory oak trees at a local field site in central Maryland. The following week we recorded the number of decoys that had been pecked at by birds, which leave tell-tale beak marks in the clay models, providing a simple estimate of the intensity of bird predation on local caterpillars. The next year, during the emergence, we continued to quantify the incidence of bird strikes on our clay caterpillars. We also collected extensive data on which birds were feeding on cicadas, through our own observations and the contributions of community scientists. Additionally, we monitored the densities of caterpillars feeding on the oak trees during the emergence year and for two subsequent years and quantified the extent of herbivore damage to the leaves of those trees for the years before, during, and after the emergence.  

 

Our results were striking! In the year before the emergence, clay caterpillar decoys suffered consistently high rates of attack by birds, which mistook around 30% of them for real caterpillars each week throughout the season. The following year, however, as soon as the cicadas came above ground, the incidence of beak marks on the clay caterpillars dropped dramatically, appearing on less than 10% of the models each week for as long as the cicadas were present, and returning to pre-emergence levels only after the last cicadas disappeared. The year following the emergence, we observed the same high levels of weekly bird strikes that we had seen in the pre-emergence year. 

 

During the emergence, our bird team documented more than 80 species of birds feeding on cicadas, regardless of their regular diet, and ranging in size from tiny gnatcatchers to enormous swans. This avian diet shift dramatically boosted forest caterpillar populations, which experienced a brief summer respite from their normally vigilant predators. Quantitative censuses of oak trees in the spring, before the cicadas emerged, were unchanged from those in subsequent years, whereas the 2021 mid-summer census, conducted near the end of the cicada emergence, saw a doubling of caterpillar densities, as well as an increase in caterpillar size, relative to the next two cicada-free years. Twice as many summer caterpillars, not surprisingly, yielded double the damage to the oak trees, when compared to the flanking non-emergence years.

 

Why are these results important? Our study reveals interconnections among organisms that are ordinarily hidden from our view. We can see, for example, that in the eastern forests of the US, birds regulate populations of free-living caterpillars, benefitting plants by reducing plant damage, and that the presence of cicadas indirectly benefits caterpillars due to the temporary diet shifts of their shared bird predators. Our results also remind ecologists that when studying an influx of resources into a community, it is important to look not just at how these resources directly impact the organisms consuming them, but it is equally important to measure the ecological consequences stemming from the prey that go uneaten during these feeding frenzies. Finally, as avian populations decline worldwide, a cicada emergence can provide a sneak preview of the reduction in plant productivity that we can expect in a world with fewer birds.  

 

In summary, periodical cicadas not only amaze us with their unique and bizarre life cycle – they also have a lot to teach us about ecology!