Periodical Cicadas’ 2020 Emergence

by Carl Strang

In the previous post I outlined the history of DuPage County’s periodical cicadas. Part of that story remains to be told, and this next month will reveal much.

Newly emerged periodical cicadas

Huge numbers of our two species of periodical cicadas emerge every 17 years. The last major emergence was in the spring of 2007, so the next one will be in 2024. At least as far back as 1969, however, there has been a significant appearance of the cicadas four years early. This has happened every cycle since, in 1986 and in 2003. Will 2020 bring another instance?

This phenomenon first was noted by Henry S. Dybas, a biologist at the Field Museum of Natural History in Chicago. He invited observers to report locations where the cicadas were emerging in 1969. Illinois extension entomologist Phil Nixon followed suit in the next two cycles. These generally were not quantitative records, however, and no one noticed whether the cicadas were reproducing.

Towns where periodical cicadas were reported to Phil Nixon in 2003

As the map shows, most of the locations were in eastern DuPage County, adjacent Cook County, and extending southeast from there to the southern Cook border. I want to repeat this mapping in 2020, with two important modifications: a quantitative aspect, and an effort to document egg-laying. The question that needs to be answered is whether these early emergences are independent of one another. In other words, have they simply represented mistakes by a few individuals each time, emerging after 13 years of development rather than the usual 17? Alternatively, do these cicadas represent a new, splinter population of cicadas whose ancestors emerged four years early at some point, but ever since have reverted to the usual 17-year lifespan and are reproducing each time?

I want to compile site observations with maximum numbers of cicadas observed at each place: counted numbers where they are few, or massed numbers where there are too many to count. Each location also could have one or both species, readily separated by their different-sounding songs. Places with only a few, countable cicadas are worth mapping but have little biological significance, as the insects are munched on by every predator larger than themselves and few if any will survive to mate and lay eggs. The best possibility for an ongoing, separate population will be in places, if there are any, where massed numbers locally overwhelm the predators so that significant reproduction can happen. This is where observations of mating and especially egg laying are important.

Cassin’s 17-year cicadas mating. The area between the red compound eye and the base of the wing is all black in this species. In the slightly larger Linnaeus’s 17-year cicada there is an orange line connecting the eye and the wing.

This female is laying eggs. You can just see the ovipositor that has levered out from her abdomen and is piercing the twig.

The cicadas produce visible rows of slits on the undersides of twigs when laying eggs. This often kills the twig, so that leaves beyond the oviposition site turn brown.

I welcome shared observations to help me in this process. Already in April the nymphs have begun to tunnel to the surface. I suspect this allows them to monitor conditions aboveground so that they can know when best to emerge. Cicadas should begin singing in mid- to late May and continue through much of June, peaking in the first half of June. Location information to report is town plus nearest intersection of streets (or name of park or preserve if relevant). The Cassin’s 17-year cicada song is a rapidly clicking buzz that rises and falls, as with the single individual in this recording:

The Linnaeus’s 17-year cicada song is more of a tone rather than a buzz, starting high and descending, often rendered “pharaoh.” The next recording is of massed songs of Linnaeus’s periodical cicada. I don’t have a good recording of a single cicada singing:

Finally, here is what massed songs of both periodical cicada species together sound like:

Sound recordings and photos will be helpful, especially photos of ovipositing individuals and of twigs with egg damage. Naturally I will be making as many observations as possible myself and will follow up reports of massed cicadas. You can either send me information by e-mail to wildlifer@aol.com, or post your observations on the Insects and Spiders of Illinois Facebook page.

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