A Surprising Expansion

June 27, 2024 at 6:25 am (singing insects)
Tags: Cassin's 17-year cicada, Linnaeus's 17-year cicada, Magicicada cassinii, Magicicada septendecim, Massospora cicadina

by Carl Strang

As I mentioned in the previous post, my first goal in surveying the 2024 emergence of periodical cicadas in the Brood XIII area was to determine how much, if at all, the major emergence area in DuPage County, Illinois, expanded relative to the previous emergence in 2007. It was clear that there previously had been a spread from the pre-settlement forested areas of eastern DuPage into the urban forests that had been planted in parks and peoples’ yards. That expansion was connected to the rapid growth of communities around the commuter railways connecting to Chicago.

I did not expect a large expansion in 2024, for two reasons. First, I walked the western edge of the northern main emergence lobe in 2007, mapping the area where cicadas had emerged, as indicated by nymphal skins on the ground and still attached to trees and other vertical surfaces. At the same time, I mapped where cicadas were chorusing. These maps are superimposed on an aerial view of where Glen Ellyn meets Wheaton:

The red line defines the western edge of the main emergence area in 2007. The yellow line indicates where chorusing males had moved beyond their emergence.

As you can see, these singing males had not been too adventurous, going at most a couple city blocks beyond their nymphal homes.

The second reason I did not expect a large expansion was what followed what appeared to be a significant dispersal event. On June 11, 2007, I was driving home from my office at Fullersburg Woods Forest Preserve, where huge numbers of cicadas had emerged. I came out of the forest and saw the sky above the empty area beyond the forest filled with emigrating cicadas. After that, I paid special attention to where cicadas were singing in new places. There were small, countable numbers of males widely scattered beyond the major emergence, but the only new choruses were small ones close to the original emergence area. Though most of the dead cicadas I looked at in these new areas were females, the numbers did not seem large enough to be significant.

So with that background, you can imagine my astonishment as I found that the cicadas had expanded their major emergence area by a huge amount:

Map of DuPage County showing the extents of the 2007 (pink) and 2024 (red) major emergences of periodical cicadas. Red dots mark locations of satellite choruses well separated from the main area and from each other.

The new edge is as much as 5 miles beyond the old one. How to account for this, given my 2007 observations? First, I was wrong in assuming that the dispersal I witnessed was of equal numbers of males and females. In hindsight, I realize that males don’t have much motivation for moving far beyond where they came out of the ground. They are unlikely to attract an unmated female in a place where cicadas had not emerged. Gravid females are another matter. Their offspring have a better chance of survival in a new area, where they don’t have to deal with so much crowding, and where their only specialized consumer, the parasitic fungus Massospora cicadina, has not yet been established.

A second contributing factor relates to my methods. The map outlines the major emergence area, where singing cicadas are so abundant that they cannot be counted. In parts of DuPage County in 2007, as mentioned above, I observed single males and small, countable groups of them outside of the major emergence area. Such small numbers are vulnerable to the many animals that eat them, but perhaps some were able to mate and produce young. Also, it seems likely now that most of the dispersing cicadas were females, which do not sing. If so, the potential for population growth is significant, as each female will lay 400 or more eggs.

The same potential exists this year, as again there have been individuals, small countable groups, and even small, localized choruses, beyond the 2024 major emergence area. Such satellite chorus areas are common in 2024, west and north of the main emergence. The ones noted in 2007 were fewer, and were very close to the major emergence area. One of the frustrations experienced by anyone studying periodical cicadas is the timespan between appearances. When the next major emergence of Brood XIII happens, I will be 90 years old if I remain at all.