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

7 Comments

  1. Marie O’Connor said,

    July 22, 2020 at 6:30 am

    I always enjoy reading your research, Carl. Thanks for your outstanding efforts!

  2. James Govednik said,

    July 25, 2020 at 8:45 am

    Late to this, but we had large numbers of chorusing cicadas on the Wheaton-Glen Ellyn border, south of Roosevelt. On the shorter trees, the branches seemed to almost quiver from the level of activity. I have a photo of downed “flags’ from a maple at the intersection of Darwin and Pembroke, if you’re interested. Love this blog!

    • natureinquiries said,

      July 26, 2020 at 6:29 am

      Thanks, James. That was not an area I checked, but I will note your observation for comparison in 4 years.

  3. August 2, 2020 at 8:05 am

    […] 33: High count of flags (wilting or dying branches because of cicada egg laying) on a tree (in Brookfield) surveyed by retired naturalist Carl Strang; one of many interesting notes in his recap of the 2020 periodical cicadas around Chicago. […]

  4. August 2, 2020 at 8:07 am

    […] 33: High count of flags (wilting or dying branches because of cicada egg laying) on a tree (in Brookfield) surveyed by retired naturalist Carl Strang; one of many interesting notes in his recap of the 2020 periodical cicadas around Chicago. […]

  5. August 2, 2020 at 8:28 am

    […] 33: Excessive rely of flags (wilting or dying branches due to cicada egg laying) on a tree (in Brookfield) surveyed by retired naturalist Carl Strang; one in all many attention-grabbing notes in his recap of the 2020 periodical cicadas around Chicago. […]

  6. August 2, 2020 at 8:35 am

    […] 33: High count of flags (wilting or dying branches because of cicada egg laying) on a tree (in Brookfield) surveyed by retired naturalist Carl Strang; one of many interesting notes in his recap of the 2020 periodical cicadas around Chicago. […]


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