Flagging in the Heat

by Carl Strang

On Saturday I completed my survey of 2020 periodical cicada reproduction in the Chicago suburbs. I focused on 18 suburban cities where I had observed wave chorusing, mapping out walking routes where I counted the flags that indicate where female cicadas deposited eggs in tree and shrub twigs. Altogether I walked 33.4 miles on hot afternoons, but the main physical challenge was the stiff neck I developed from constantly looking up into the trees. I will need to consult with periodical cicada specialists to be sure, but it seems to me that the numbers support a persistent, parallel splinter regional population that will continue to appear 4 years ahead of each major emergence.

Here I will share two sets of data. First, the highest numbers of flags in single trees.

Map of Chicago suburbs I surveyed for periodical cicada reproduction. Numbers indicate the highest count of flags in a single tree. Flags are withered twig ends that result from female cicadas inserting their eggs and cutting off the twig’s vascular system.

The high count of 33 was in Brookfield. I was walking residential streets, so my counts are transects rather than area surveys. The next map gives average numbers of flags per 100 meters. I used that number as the divisor because I often observed 1-2 wave choruses per block while driving earlier in the season, and 100m represents a middle-of-the-road block length.

Average counts of flags per 100m in Chicago suburb survey routes.

The highest counts of 10.5 and 9.1 are in LaGrange and Western Springs, respectively. These counts are conservative: I usually could not see all sides of a tree, canopies blocked part of the view, and to an unknown but certain degree flags had fallen to the ground and were removed by homeowners (this takes away some eggs, but there are others in the twig above the break).

Flags fallen to the ground at Madison Meadow Park, Lombard. In a residential yard these likely would have been removed before I could count them.

This fallen flag carried some eggs, leaving others still on the tree. Sugar maples were a popular choice by female periodical cicadas.

In addition to these transects in the cities, there were isolated parks and sites worth considering. At Pioneer Park Forest Preserve in Naperville, a single isolated non-wave chorus tree produced 7 flags spread between two trees. A similar case at Cook County’s Bemis Woods produced 18 flags. A park in Downers Grove had 37 flags. Other forest preserves had enough flags to suggest that their local off-year populations will continue: 22 at Lyman Woods, 13 at Greene Valley, and though I was disappointed by the size and timespan of the wave chorus at Wood Dale Grove, there were well over 30 flags there.

Tree with flags at Pioneer Park

Periodical Cicada Species and Regional Distribution

by Carl Strang

 

In earlier posts I outlined the major results of my study of periodical cicadas in DuPage County, Illinois. Human history has played a huge role in determining both where the cicadas occur and where they are absent. In this installment (I anticipate one more for the future) I want to pass on my observations from the 2007 emergence in some areas outside the county, and to account for the three species of 17-year cicadas.

 

The periodical cicadas I had observed in the off-year emergence of 2003, and the few I found in 2006 (Oops! cicadas I call them), all belonged to one species, Cassin’s periodical cicada (Magicicada cassini). One other species had been observed abundantly in northeast Illinois according to published studies, so I expected to find Linnaeus’ periodical cicada (M. septendecim) in 2007. A third species, the little periodical cicada (M. septendecula), is regarded in the literature as a more southern species, but it is less known than the others and I hoped to find it locally, too.

 

M. septendecim left, cassini right. Can you find differences?

M. septendecim left, cassini right. Can you find differences?

 

 

What I described in earlier posts applies mainly to the Cassin’s species. Linnaeus’ periodical cicadas occurred in all the areas of abundant emergence, but as a rule were less common than Cassin’s, and intriguingly their outermost song choruses consistently were about 100 meters back from the edge of the emergence area defined by Cassin’s.

 

I did not find the little periodical cicada at all. Not only was it absent from DuPage, but I failed to find it in what seemed like the most promising place in the state. The literature points to Livingston County as the one location from which the little periodical cicada is known in Illinois. That is mainly an agricultural county, but it does have a significant site, Humiston Woods Nature Center, which is forested and has a large area dominated by shagbark hickory, supposedly one of that cicada’s preferred nymphal food trees. On two trips to Humiston Woods I found Cassin’s and Linnaeus’, but no little periodical cicadas. Their songs are distinctive enough that I doubt I could have missed them if they were there.

 

Quiet feeding group

Quiet feeding group

 

As for other counties, there were abundant reports of cicada concentrations in the portion of Cook County east of DuPage, as well as the Palos area to the southeast. I found only a very few scattered individuals in parts of Cook north of DuPage County. Along the Fox River to the west, in Kane County, I found none, but did hear a report of at least one location away from the river where they were observed. I drove south along the Fox River one Sunday morning, spot checking likely forested areas, but did not find periodical cicadas until I was within a couple miles of the Fox’s confluence with the Illinois River, near Starved Rock State Park. That was surprising. Also surprising was the limited distribution of cicadas in the parts of north Will County (south of DuPage) that I checked. I found plenty along the Des Plaines River just downstream from DuPage County, at Keepataw Forest Preserve, but none at Veterans Woods just a short distance farther, nor at Isle la Cache, nor at Hammel Woods on the DuPage River. They were at McKinley Woods on the Des Plaines, and at the southernmost part of the DuPage River. Though I did not investigate further, I suppose that agricultural history has played a role in this patchy distribution, as it did in my county.

 

In the final installment of this series I will touch on high points of the periodical cicadas’ behavior. They provided one more big surprise…

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