Periodical Cicada Update

by Carl Strang

The 2020 periodical cicada emergence in the Chicago region has reached its peak and is winding down. It has been a mad and sometimes confusing scramble to collect data, but an overall complex picture has emerged. First, a wide-angle look:

Collected observations of periodical cicadas in 2020. White dots indicate towns or parks in which at least one or a few cicadas were documented. Yellow dots mark countable numbers, i.e. from one spot you could hear multiple cicadas singing. Orange dots mark small choruses (though sometimes occurring over large areas), in which the cicada songs were blended to the point where individuals no longer could be picked out, but the choruses were not organized. Red dots mark areas with full choruses, formed into periodic waves of song, loud and with both species audible.

These assessments of numbers are conservative. Many of the white dots are taken from iNaturalist submissions based on a photo of one insect. It is possible that in many cases the peak numbers were above that level, some possibly all the way to full wave chorusing. Also, some of the orange dot communities were assessed before the peak time and may well have developed full wave chorusing later. I visited nearly all the red dot communities myself, walking or driving to get some sense of the area involved. Several to many square blocks were typical here, and often I observed mating pairs and females laying eggs or saw lines of eggs in twigs. The wave chorusing lasted up to two weeks in these places.

Cassin’s 17-year cicada laying eggs in Mokena, Will County

Periodical cicada eggs in winged euonymus twig, Woodridge, DuPage County

Areas with wave chorusing generally were imbedded within a matrix of lower level numbers. Overall, the Chicago suburbs saw the emergence of enormous numbers of periodical cicadas. Apart from the specific observations of mating insects, egg-laying females and eggs in twigs, there is reason to believe that this is an on-going, reproducing population separate from the main regional emergence that next will occur in 2024. That reason is that these suburban neighborhoods have relatively few predators. The reason periodical cicadas have been so successful is understood to be that they overwhelm the many species of consumers that eat them. Suburban residential communities cover huge areas and are ecologically depauperate in that they are reduced to mowed lawns and trees, with some shrubs and herbaceous plantings that typically are non-native and therefore support few insects. That results in low numbers and diversity of predators compared to what the remnant forests harbor.

Example of habitat in River Forest, Cook County, a wave chorus area

I simply did not observe predators beyond the rare crow, territorial robins and usual densities of other species typical of these neighborhoods. I suspect that not only has this population split off from the main emergence, it has been building in numbers with each generation (I use the singular here because I am referring mainly to the Cassin’s 17-year cicada. Though Linnaeus’s 17-year cicadas were present in all the red dot areas, their numbers are more difficult to assess, and the only evidence I have of their reproducing is a single submitted photo of a female laying eggs).

Linnaeus’s 17-year cicada laying eggs in southeastern DuPage County. Photo by Leslie Bertram, used with permission.

I do not want to give the impression that predation wasn’t occurring. There were plenty of signs like this. The numbers removed simply were not making a dent in the mass choruses.

Now I want to focus on DuPage County, because I have the background of a detailed study of the cicadas’ history and their 2007 major emergence there.

This is my working map of the periodical cicada emergence of 2007, with superimposed 2020 observations. Pink areas mark the extents of the 2007 emergence areas. Colored dots represent the same levels of 2020 cicada numbers as in the regional map above.

If you find this image bewildering, I welcome you to the club. The emergence pattern was so spotty and localized that I cannot comfortably mark out large uniform areas. Dots are expanded either along my survey driving routes, or to cover the size of areas more expansively explored. To get a sense of the biological significance, try to tune out the white and yellow dots, as those locations had few enough cicadas that predators would have had no trouble wiping them out.

Again the red areas had so many cicadas that I had no trouble finding examples of mating or egg laying. Red areas adjacent to one another could be combined, as the habitat among them was uniform and continuous. This is particularly true of the cluster of red areas in the southeastern quarter of the county where the cities of Downers Grove, Clarendon Hills, Westmont and Hinsdale blend their similar residential neighborhoods. This area in fact continues east across the Tri-State highway corridor into Cook County’s adjacent city of Western Springs.

I am especially interested in the areas with loud, extensive wave chorusing that are outside the 2007 emergence regions. I will be referring to my detailed notes when I return to these locations in 2024.

The big emergences were in residential areas. Forests were a different story. For the most part, forested areas did not rise above the countable numbers of cicadas. I admit to being disappointed by what I observed in Wood Dale Grove and Salt Creek Park Forest Preserves and their connecting and surrounding residential areas. Both species emerged in both preserves, but never got above the countable level at Salt Creek Park. In part of the forest at Wood Dale Grove there was an area of a few acres in which wave chorusing developed, but it was in only part of the forest and lasted at most 3 days before collapsing to countable numbers again. I draw consolation from this result because it supports my suspicion that forests harbor numbers and diversity of predators that contrast with the expanses of residential communities that form the Chicago suburbs. Though the residential areas surrounding Wood Dale Grove and Salt Creek Park resemble those that had large and sustained cicada choruses, it would be easy for predators to spill out of the preserves and consume cicadas emerging in the surrounds. Cicada specialists may be interested to know that I did check Raccoon Grove Forest Preserve in Will County. I heard only two singing Cassin’s males there.

I have one more step this year, and that is to return to many of these places and look for egg damage to trees and shrubs. This will better assess reproductive success than the several but scattered observations mentioned above. Then it will be a wait of four years until the big show, and I wonder if there will be any peculiarities in the spots where there were mass emergences this year.

8 Comments

  1. Julie Larsen said,

    June 23, 2020 at 6:50 am

    Thank you. Very interesting.

  2. June 24, 2020 at 5:13 am

    […] naturalist Carl Strang posted what the 2020 periodical cicada emergence taught him at natureinquiries.wordpress.com/2020/06/23/periodical-cicada-update. . . . Follow the saga of Monty and Rose and their chicks, the piping plovers at […]

  3. June 24, 2020 at 5:17 am

    […] naturalist Carl Strang posted what the 2020 periodical cicada emergence taught him at natureinquiries.wordpress.com/2020/06/23/periodical-cicada-update. . . . Follow the saga of Monty and Rose and their chicks, the piping plovers at […]

  4. June 24, 2020 at 5:17 am

    […] naturalist Carl Strang posted what the 2020 periodical cicada emergence taught him at natureinquiries.wordpress.com/2020/06/23/periodical-cicada-update. . . . Follow the saga of Monty and Rose and their chicks, the piping plovers at Montrose […]

  5. Donna Granback said,

    June 24, 2020 at 8:27 am

    Very interesting! I’m in Downers Grove and immediately knew one of those elongated red marks covered DG! 2007 was a whopper of a year and I’ll admit to thinking about moving away before 2024!! 🤓
    Question—are the cicadas this year smaller than usual? Or at least than the 17-year? I just feel the must be, but no proof of that.

    • natureinquiries said,

      June 25, 2020 at 6:28 am

      These were 17-year cicadas, a splinter group that pops up four years ahead of each major emergence in part of the Chicago area. If you are asking about body size, they are indeed smaller than the cicadas we have later in the summer, but this year’s were the normal size for 17-year cicadas. If you are asking about relative numbers, I suspect that if anything there will be more in 2024, but they will tell us that then.

  6. Susan Kaufman said,

    July 3, 2020 at 7:58 am

    Thank you for this information. I reside in Park Ridge and planted a 5-year old flowering dogwood tree in May. We covered the little tree with netting last month as we were beginning to see and hear many cicadas. This has since died back both in seeing them and hearing them. Do you think they may be done for the season in our area?

    • natureinquiries said,

      July 4, 2020 at 6:28 am

      Yes, if you are no longer hearing the cicadas you can safely uncover your dogwood.


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