Periodical Cicada Tunnels

by Carl Strang

On Monday I went to Wood Dale Grove and Salt Creek Park Forest Preserves in DuPage County and looked for periodical cicada tunnels. As mentioned in the previous post, these are dug to the surface by nymphs, often weeks before they emerge to molt into their adult form.

My choice of those two preserves was not random. In the 2007 major emergence there were practically no periodical cicadas in the adjacent cities of Addison and Wood Dale, but residents reported that there were plenty in 2003. I had found no cicadas in either of those preserves when I checked them in 2007. Is it possible that those cicadas had completely shifted their timing to form a separate population? If so, this would be scientifically significant. I started finding cicada tunnels soon after entering the forest at Wood Dale Grove on Monday. I was elated at what I saw when I lifted a piece of old concrete.

Three cicada tunnels were in that one small area.

In 40 minutes of searching I found 10 tunnels. A couple were in chimneys, similar to those some crayfish produce.

Cicada chimney

How did I know these were not crayfish tunnels? Size tells the tale. A periodical cicada tunnel is half an inch in diameter, around the size of my forefinger. Crayfish tunnels are at least as big around as a quarter. Nightcrawler tunnels are much smaller, less than a pencil’s diameter.

Most of the tunnels I saw were sheltered, like those under the concrete, under bark or an overhanging fallen tree stem. Those in unsheltered bare soil had sharp edges, indicating they had been dug in the few days since a very heavy rainstorm passed through. Were it not for that storm, I believe I would have seen many more, as the rain washed soil into the tops of the tunnels.

At Salt Creek Park it was much the same story. In low areas resembling those at Wood Dale Grove I found tunnels at the same rate, one sharp-edged one about every four minutes. I did not find any in the upland woods, which may prove to be significant. Cassin’s 17-year cicadas are associated more with lowland woods, Linnaeus’s 17-year cicadas with uplands.

All of this has me very much looking forward to what I will find in a few weeks in this area. If there is indeed a major emergence in these two preserves, which are fairly close together, what is the extent of the emergence area? Are both species involved, or just the one? Can I document mating and egg laying? And, of course, all of this observing must be done under the social distancing, mask-wearing safety protocols dictated by the covid epidemic. Stay tuned.

Periodical Cicadas’ 2020 Emergence

by Carl Strang

In the previous post I outlined the history of DuPage County’s periodical cicadas. Part of that story remains to be told, and this next month will reveal much.

Newly emerged periodical cicadas

Huge numbers of our two species of periodical cicadas emerge every 17 years. The last major emergence was in the spring of 2007, so the next one will be in 2024. At least as far back as 1969, however, there has been a significant appearance of the cicadas four years early. This has happened every cycle since, in 1986 and in 2003. Will 2020 bring another instance?

This phenomenon first was noted by Henry S. Dybas, a biologist at the Field Museum of Natural History in Chicago. He invited observers to report locations where the cicadas were emerging in 1969. Illinois extension entomologist Phil Nixon followed suit in the next two cycles. These generally were not quantitative records, however, and no one noticed whether the cicadas were reproducing.

Towns where periodical cicadas were reported to Phil Nixon in 2003

As the map shows, most of the locations were in eastern DuPage County, adjacent Cook County, and extending southeast from there to the southern Cook border. I want to repeat this mapping in 2020, with two important modifications: a quantitative aspect, and an effort to document egg-laying. The question that needs to be answered is whether these early emergences are independent of one another. In other words, have they simply represented mistakes by a few individuals each time, emerging after 13 years of development rather than the usual 17? Alternatively, do these cicadas represent a new, splinter population of cicadas whose ancestors emerged four years early at some point, but ever since have reverted to the usual 17-year lifespan and are reproducing each time?

I want to compile site observations with maximum numbers of cicadas observed at each place: counted numbers where they are few, or massed numbers where there are too many to count. Each location also could have one or both species, readily separated by their different-sounding songs. Places with only a few, countable cicadas are worth mapping but have little biological significance, as the insects are munched on by every predator larger than themselves and few if any will survive to mate and lay eggs. The best possibility for an ongoing, separate population will be in places, if there are any, where massed numbers locally overwhelm the predators so that significant reproduction can happen. This is where observations of mating and especially egg laying are important.

Cassin’s 17-year cicadas mating. The area between the red compound eye and the base of the wing is all black in this species. In the slightly larger Linnaeus’s 17-year cicada there is an orange line connecting the eye and the wing.

This female is laying eggs. You can just see the ovipositor that has levered out from her abdomen and is piercing the twig.

The cicadas produce visible rows of slits on the undersides of twigs when laying eggs. This often kills the twig, so that leaves beyond the oviposition site turn brown.

I welcome shared observations to help me in this process. Already in April the nymphs have begun to tunnel to the surface. I suspect this allows them to monitor conditions aboveground so that they can know when best to emerge. Cicadas should begin singing in mid- to late May and continue through much of June, peaking in the first half of June. Location information to report is town plus nearest intersection of streets (or name of park or preserve if relevant). The Cassin’s 17-year cicada song is a rapidly clicking buzz that rises and falls, as with the single individual in this recording:

The Linnaeus’s 17-year cicada song is more of a tone rather than a buzz, starting high and descending, often rendered “pharaoh.” The next recording is of massed songs of Linnaeus’s periodical cicada. I don’t have a good recording of a single cicada singing:

Finally, here is what massed songs of both periodical cicada species together sound like:

Sound recordings and photos will be helpful, especially photos of ovipositing individuals and of twigs with egg damage. Naturally I will be making as many observations as possible myself and will follow up reports of massed cicadas. You can either send me information by e-mail to wildlifer@aol.com, or post your observations on the Insects and Spiders of Illinois Facebook page.

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