Written by: Claire Milsted (May 2024). Claire is a visiting assistant professor of biology at Beloit College, where she teaches botany, genetics and evolution, and conservation biology. She grew up in Chicago and received her PhD in plant biological sciences from University of Minnesota in 2021.

The stateline area is home to many cicadas, insects in the superfamily Cicadoidea (bugguide.net, 2003). We see and hear cicadas every summer, but what makes May and June 2024 special is the emergence of seventeen-year cicadas. These are members of three extremely similar-looking species, Magicicada septendecim, M. cassini, and M. septendecula, which were last seen in 2007 (Johnson, 2024; University of Wisconsin, 2024). The 2007 cicadas mated, laid eggs which then hatched into small juveniles (nymphs) which burrowed into the soil and spent seventeen years slowly growing underground. Because of the recent history of human disturbance and dumping in many parts of Nature At The Confluence, we are curious about whether cicadas will emerge in large numbers all over the site or only in areas with less-disturbed soil.

Cicadas in the genus Magicicada all wait more than ten years to emerge. In addition to the three species we are likely to see this summer, there are four species of thirteen-year cicadas, which are found further south. Thirteen-year cicadas are very similar in biology to seventeen-year cicadas, but their internal “timer” is only set to thirteen years. 

All of these cicadas emerge in multiple broods– basically, the seventeen-year or thirteen-year “timers” start at different times. The cicadas that emerged in 2007 and this year are members of Brood XIII, also known as the Northern Illinois Brood (Johnson, 2024).  Below is a brood map of North America as a whole from the USDA forest service (Liebhold et al., 2013). Note the close boundary between brood XIII and the thirteen-year brood XIX in central Illinois; this part of the country will see a rare double event of thirteen- and seventeen-year cicadas this year.

By waiting thirteen or seventeen years and then emerging en masse, the cicadas manage to overwhelm any threat such as predation. Additionally, thirteen and seventeen are both prime numbers that are not divisible into any factors (Simon et al., 2022). It is thought that this reduces the risk of competition or predation by any other periodically emerging organism that could exist. For example, if there were a sixteen-year cicada species, it might be vulnerable to a predator that was able to wait eight years and feed on the sixteen-year cicadas at every other emergence. However, all insects known to display periodical emergence are herbivores.

Cicadas feed on xylem (plant sap) and are harmless to humans. They can cause minor stress to plants while laying their eggs, but are not major pests. Their digging is thought to aerate the soil and increase fertility, and they provide a great food source for birds, other insect-eating animals, and eventually decomposers.

Male cicadas “sing” (actually using a vibrating membrane on their abdomen) to attract mates. In the coming weeks, you will probably hear their sounds during the daytime, which are often characterized as being like frogs but much louder. If you want to hear a preview, you can check out cicadas.wisc.edu.

Female cicadas seem to be able to recognize members of the same species by their song (Simon et al., 2022). After mating, a female cicada cuts a slit in plant matter such as a twig, lays her eggs, and the seventeen-year cycle begins again. We will see these cicadas again in 2041!

Additional Images

Exuviae (molts) are old exoskeletons the cicadas have shed. Andy Melton, Creative Commons, via University of Illinois

A young nymph from Brood V, seen in Ohio in 2016. Dan Keck, Wikimedia Commons

A Magicicada septendecim adult from the last brood XIII emergence, seen in Illinois in 2007. Joel Mills, Wikimedia Commons

 

Common Questions

Can you tell me any more about the “normal” cicadas that come out every year?

You will see these cicadas later in the summer. They are usually more green in color than the brightly-colored seventeen-year cicadas. One common cicada that comes out every year in the state line area is the Dog-day cicada, Neotibicen canicularis.

I have heard the term “seventeen-year locust.” Is that the same as “seventeen-year cicada”?

Yes! People are using the name “locust” less frequently because it is misleading. Scientists usually reserve the term “locust” for insects in the grasshopper order (bugguide.net, 2003). The locusts mentioned in, for example, the story of Moses, were not cicadas.

How do the periodical cicadas know when to come out? Can they make a mistake?

While they wait 8-12 inches underground, the juveniles are growing very slowly and timing their molting every few years based on a hormonal clock (Simon et al., 2022). Then in their final year, they wait for the soil to reach 64˚F; then they begin the process of tunneling up to the surface (Johnson, 2024). Some cicadas do accidentally come out a few years early or late; these stragglers are unlikely to find mates.

Can I eat the cicadas?

They are not poisonous to humans, but DO NOT EAT THEM IF YOU HAVE A SHELLFISH ALLERGY (AAFA Community Services, 2024). Additionally, be aware that you do not know where they have been before you collected and cooked them; consider risk factors such as nearby pesticide or herbicide application and other possible contaminants before trying them. Bloggers who have tried these insects recommend getting freshly molted cicadas, seasoning them with something spicy, and sauteeing them like shrimp before eating.

How do cicadas pee?

Cicadas have to eliminate a lot of liquid waste due to their diet of xylem, which is mostly water. They urinate a lot relative to their size and this is of interest to scientists (Tamisiea, 2024).

How do they mate?

Females listen for male cicadas of the same species. They do a special wing movement to indicate that they are ready to mate (Simon et al., 2022). If you see two cicadas sitting around abdomen-to-abdomen, they are mating.

I’ve heard that in some metamorphic insect species, the adults can’t eat anything at all–is that true?

Yes, but not for seventeen-year cicadas! They have piercing mouthparts that act as a straw for drinking plant xylem (sap). However, they are more focused on successfully reproducing than on eating, and even if they get plenty of food they will not live very long.

 

Sources:

AAFA Community Services. (2024, April 5). If You Have a Shellfish Allergy, Don’t Eat Cicadas No Matter How Tasty They May Look. Asthma and Allergy Foundation of America. https://community.aafa.org/blog/if-you-have-a-shellfish-allergy-don-t-eat-cicadas-no-matter-how-tasty-they-may-look

bugguide.net. (2003). Genus Magicicada—Periodical Cicadas. Bug Guide- Iowa State University. https://bugguide.net/node/view/6970

Johnson, K. (2024, February 22). The cicadas are coming! Periodical cicadas in Illinois in 2024 | Good Growing | Illinois Extension | UIUC. https://extension.illinois.edu/blogs/good-growing/2024-02-22-cicadas-are-coming-periodical-cicadas-illinois-2024

Liebhold, A. M., Bohne, M. J., & Lilja, R. L. (2013). Active Periodical Cicada Broods of the United States. USDA Forest Service. https://www.fs.usda.gov/foresthealth/docs/CicadaBroodStaticMap.pdf

Simon, C., Cooley, J. R., Karban, R., & Sota, T. (2022). Advances in the Evolution and Ecology of 13- and 17-Year Periodical Cicadas. Annual Review of Entomology, 67(1), 457–482. https://doi.org/10.1146/annurev-ento-072121-061108

Tamisiea, J. (2024). Why Cicadas Power Spray Their Pee. Scientific American. https://www.scientificamerican.com/article/why-cicadas-power-spray-their-pee/

University of Wisconsin. (2024). Wisconsin Periodical Cicadas 2024. Periodical Cicadas in Wisconsin. https://cicadas.wisc.edu/