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.

Cicada Emergence Stages

by Carl Strang

The 2020 emergence of periodical cicadas in northeast Illinois is under way now. It has taken a while to get going and is running about two weeks behind the last main emergence in 2007. People have sent or posted reports from 28 towns or preserves so far in Cook, DuPage, Lake and Will Counties. I have begun to visit places where significant numbers of the insects have been reported.

The pattern is reminiscent of what we observed in 2007. A few individuals come out of the ground at first, then a few days later there may be a night that brings out many. It takes them a few days of recovery before the males begin to sing, few or countable numbers of Cassin’s 17-year cicadas in most places. So far only some of the more open residential areas have reached this stage. Forests lag behind because the trees retard the warming of the soil which triggers the cicadas’ emergence.

This Cassin’s 17-year cicada was the first I found in Wood Dale Grove Forest Preserve, the June 4 date well after they had begun appearing in open residential areas.

Where there are good numbers of cicadas, small choruses may develop in which there are too many Cassin’s to count. Such has been the case in 5 communities so far. The ultimate step, which to date I have found in only one spot in southwest Brookfield, has numbers of Cassin’s 17-year cicadas chorusing so loudly that it can be nearly painful to stand under them. Their chorusing produces waves of loud then softer sound on a regular rhythm, and they are joined by the second species, Linnaeus’s 17-year cicada, whose songs add a high-pitched overtone to the Cassins’ buzzing quality. When visiting that place in Brookfield I saw many cicadas on the ground, many nymphal skins, many holes in the soil, and a pair of mating Cassin’s. I saw a crow eating a cicada, but there may be too few predators in these residential neighborhoods to have much impact on this off-year emergence.

Many emergence holes and empty nymphal skins marked the high density area in Brookfield.

Mating pair of Cassin’s 17-year cicadas in Brookfield on June 5

I made the following sound recording in the same area. The surrounding Cassin’s choruses wash out the focal one somewhat, but you may be able to pick out the 6 waves of high volume spread evenly here (5-6 seconds between peaks in this 32-second recording). Also, listen for a few Linnaeus’s “pharaoh” calls along the way. They are not coordinated with the Cassins’ waves.

A visual rendition of the recording

Owls etc. at St. James Farm

by Carl Strang

This has been an eventful spring at St. James Farm Forest Preserve. Thanks to the covid-19 epidemic, volunteers have not been allowed to do restoration work. My response was to take walks through the preserve almost daily in late April and May, gaining a much more detailed understanding of what has been going on there.

Owls, for instance, have provided significant insights. Early in the season I found that great horned owls had commandeered the nest used by Cooper’s hawks the previous two or three years.

My first look at the two nestlings

Though I checked them at a distance, they clearly knew I was there.

Shortly before they branched

The parent, perched nearby, was not concerned.

The youngsters branched (left the nest), and I did not see them beyond a week after that. At the same time, however, I saw an adult great horned owl with a flying youngster at the opposite end of the preserve. Though it is possible that this second nest had been across the road at Blackwell Forest Preserve, the relative number of suitable nest trees leads me to believe there were two great horned owl nests at St. James Farm this year.

The barred owl pair keeps a low profile, and I seldom have heard or seen them. That this is due to the presence of the more powerful great horneds was underlined by my finding on May 13 where an adult barred owl had been killed and plucked. I was left hoping this was a naïve wanderer rather than one of the savvy resident pair, and that hope was intensified when I learned that someone had picked up a young barred owl from one of the interior trails and brought it to the Willowbrook Wildlife Center, the rehab facility for the Forest Preserve District of DuPage County. Some days after that I encountered an adult barred owl with a branched youngster, and Willowbrook deputized me to return their bird to its family.

A leaning black cherry provided a good place for the owlet to climb up.

The youngster quickly climbed more than 10 feet above the ground. His wings were sufficiently developed that he probably could fly.

The owls all were elsewhere by the next morning. Normally I would not share information about owls in this way, but at this point in the season they are so good at staying out of sight that it would be a waste of time for anyone to attempt to find them in St. James Farm’s large forest.

Plenty of noteworthy observations added to my knowledge of the preserve’s flora and fauna. For instance, I found a white river crayfish on the lawn near one of the ponds.

White river crayfish

I got a more detailed list of first flower dates this year, thanks to my frequent visits.

Butterweed pops up in widely scattered open areas of the forest.

This was a cold spring, and first flower dates in May were a median 8 days later than in earlier years of my records. A highlight of the season is the blooming of blackhaw, St. James Farm’s dominant understory shrub.

Though some blackhaws mistakenly were cut last winter by some inexperienced seasonal employees, that was in a limited part of the forest, and even there some were missed, as shown here.

The cut ones will resprout, and the restoration clearing of the forest has been followed by places with bunches of small blackhaws, like this one.

It was an enjoyable season, but now my attention shifts to singing insects. Spring field crickets and green-striped grasshoppers are singing, and periodical cicadas have begun to emerge in some open residential areas.

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