by Carl Strang
A year from now is the anticipated main emergence of Brood XIII periodical cicadas in northeast Illinois and adjacent states.
Though the main body of the cicada populations appears at 17-year intervals, each time there are a few individuals that come out a year early (I call them “oops cicadas”). I heard a couple cicada songs on June 6, and would have thought little more about it, but a couple correspondents let me know they had seen surprising numbers emerging in their neighborhoods. I made a list of the places where I had observed significant numbers in 2020 (explained below), and started checking them on June 10.
As expected, some of the sites I visited that first afternoon had no singing cicadas, or at most a few scattered individuals. I was surprised, however, to find some small areas that had large enough numbers to produce wave choruses with both species singing. Levels of periodical cicada numbers singing in an area increase from scattered individuals, to greater numbers that nevertheless are countable, to small unorganized choruses, and conclude with groups that are so large the volume of their (loud!) singing rises and falls in organized synchrony. Most of the singers were Cassin’s 17-year cicadas (Magicicada cassinii), but some of the wave choruses were accompanied by a few of the slightly larger Linnaeus’s 17-year cicadas (Magicicada septendecim). Here is an excerpt from my recorded notes at one of the wave chorus sites:
If you look up in such places you can see cicadas flying in the treetops, while close to the ground you can find the emptied shells left behind when the nymphs come out of the ground and their adult forms break free.
So, what is going on here? In several blog posts in 2020 I shared results indicating that huge numbers of periodical cicadas not only emerged four years ahead of time in some of Chicago’s western suburbs, but that significant numbers of them successfully reproduced. It appears we now have two temporally separated populations of the two species (such 4-year-early emergences also were documented here in 1969, 1986, and 2003). Climate change has been suggested as a possible stimulus (based on the observation that farther south, periodical cicadas emerge at intervals of 13 rather than 17 years). I favor a different hypothesis, proposed by Lloyd and White (1976. Evolution 30:786-801). They suggested that population pressure, experienced by the nymphs as excessive aggressive encounters with one another underground, stimulates a portion of them to come out early. Other more recent results suggest that these insects count years in groups of 4, deciding a year ahead of time when they will mature, so a 4-year-early emergence makes sense. In 2020 it was clear that the big numbers appeared only in residential neighborhoods, not in the forests. Predators are relatively few in residential areas, so that population sizes could become untenably large and stimulate an early escape.
That brings us to this year. It is one year ahead of the main emergence in northeast Illinois. Could it be that populations are so intolerably dense in some spots that some of their members simply can’t wait one more year? The fact that most of the areas with wave choruses in 2020 did not produce such big numbers this year, while a few did, may reflect different stages of population growth taking place in different locations.
In this map of DuPage County, the 2007 areas with large periodical cicada numbers are marked in pink. More brightly colored spots map the 2020 emergence. White dots indicate towns or parks in which at least one or a few cicadas were documented. Yellow dots mark countable numbers, i.e. from one spot you could hear multiple cicadas singing. Orange dots mark small choruses (though sometimes occurring over large areas), in which the cicada songs were blended to the point where individuals no longer could be picked out, but the choruses were not organized. Red dots mark areas with full choruses, formed into periodic waves of song, loud and with both species audible. Many of those dots have elongate shapes because they came out of driving surveys along streets. It is likely that the unmarked areas between some of them were filled with wave choruses.
This month I returned to the areas where there had been wave choruses in 2020. Most of them had few cicadas or none (also true of the areas outside DuPage, in Cook and Will Counties). Four or five had unorganized chorusing. The black dots that I have added to the map indicate this year’s wave chorus locations. None of those seven covered large areas, most being less than a square block.
Next year will be a busy one for me from late May through June. If the Lloyd-White hypothesis is correct here, I would expect all these areas again to have enough cicadas to produce wave chorusing. But then again, they may have more surprises in store.
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