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.
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.
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).
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.