No Shadow of a Shadow

by Carl Strang

I didn’t find any periodical cicadas out in Addison or Wood Dale this year. That might seem like a strange statement, given that our local main emergence last happened in 2007, and the next is due in 2024.

A 2007 photo of representatives of DuPage County’s two species of 17-year periodical cicadas: Linnaeus’s on the left, Cassin’s on the right.

A 2007 photo of representatives of DuPage County’s two species of 17-year periodical cicadas: Linnaeus’s on the left, Cassin’s on the right.

I had reason to think I might find a few of these amazing critters here this year (they are peaking in Ohio in 2016, by the way). For several generations, now, starting in 1969, significant numbers of the cicadas have emerged 4 years early in the western suburbs of Chicago. This phenomenon, called a shadow brood, since has been found in a few other locations in eastern North America. It generally is thought to be a one-time deal, but the repetitive nature of this local shadow brood has me thinking there has been reproduction each time. Furthermore, the cicadas in the adjacent cities of Addison and Wood Dale appear entirely to have switched to the shadow timing. Residents reported them to be abundant in 2003. I found hardly any there in 2007.

The next shadow brood emergence therefore should happen in 2020. That assumes that there was indeed reproduction in 2003, or at least that local conditions again will result in some cicadas emerging at age 13 rather than 17. Those numbers are significant, as southern broods of related cicada species always are 13-year cicadas. Something caused a switch in some of our cicadas, in 1969 at least, bumping them onto the 13-year track. If they have been reproducing, then the subsequent shadow broods have resumed the 17-year life span. If you have followed this convoluted story, then you can guess why I thought I might find a few periodical cicadas this year. If the shadow brood indeed is all that exists now in Addison and Wood Dale, and something were to cause a few of them to make the 13-year jump now, 2016 is when they would have emerged. Perhaps a few did, but if so I did not hear any singing, nor did I see any shed nymphal exoskeletons, in this year’s tour of the two cities.

I will repeat my route each year, as I have done starting in 2014. A few cicadas out of the millions emerge a year or two early. I will be very surprised if there are any next year, but the anticipation will build as I look to a possible major emergence in Addison and Wood Dale in 2020.


1 Comment

  1. June 8, 2016 at 10:23 am

    What an interesting and significant observation! John Cooley will absolutely wan to know about this.

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