by Carl Strang
Last Thursday I searched for the northernmost lyric cicadas, having found them superabundant in Kendall County and absent in the portions of McHenry County I surveyed. This is a woodland species that seems especially common in bottomland forests, so I took advantage of our glacial legacy and followed rivers north and south (rivers developed in low zones between the concentric end moraines), but also stopped at other woodlands along the way.
There clearly is a gradient in density from south to north. In Kendall County, and along the Des Plaines River at the south edge of DuPage County, large numbers of lyric cicadas form loud choruses. In central DuPage County, at locations such as Fullersburg Woods and Mayslake, this is a regular part of the insect fauna, but they are down to countable numbers of individuals.
The clearest indication came as I followed the West Branch of the DuPage River, and continued north beyond it. At Blackwell Forest Preserve, in west central DuPage County, there was a ratio of 11 lyric cicadas to 18 or so Linne’s cicadas (= 0.61; Linne’s has a fairly uniform density through the area). At Elsen’s Hill, a few miles farther north, the ratio was 4:7 (0.57). Several miles farther north, at West Branch Forest Preserve, the ratio was 3:7 (0.43). The farthest north I found this species was at Shoe Factory Woods, in north Cook County, where the ratio was 2:12 (0.17).
By that point, though, cicadas had entered their afternoon lull, and I wasn’t hearing many of any species. Shifting west and driving south along the Fox River, I heard the next lyric cicada at the north edge of St. Charles, a point close in latitude to West Branch Forest Preserve. For now I have a sense of what is happening in the northern edge of this species’ range, but I will continue to monitor them for changes, and to continue seeking that northernmost population in the region.