Thursday, August 06, 2009

On the hunt for prairie cicadas

On the hunt for prairie cicadas

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Spend part of an August day at one of the rare, high-quality remnants of original tallgrass prairie in Illinois and you’ll be treated to the reverberating sounds of male prairie cicadas (Tibicen dorsata) singing. Or buzzing. Or droning. The terms used to describe the sounds they produce tend to reflect the status accorded to insects by the person doing the describing.

In any case that sound tells female cicadas love is in the air. It tells people who hear it that they’re in a special place, since in central Illinois prairie cicadas occur only where native plant communities remain relatively undisturbed.

I had the opportunity to get up close with prairie cicadas recently when I accompanied Rob Stanton to Loda Prairie Cemetery Nature Preserve in Iroquois County to assist him in his study of their ecology. [Photo: Rob Stanton scans for prairie cicadas.] Stanton is a graduate student in biology who hopes to promote the conservation of prairie cicadas by developing a better understanding of their needs for food and habitat. Our objective on that day was to capture and mark as many prairie cicadas as we could as part of a study to estimate the total population at the preserve.

At first we had difficulty locating our quarry, since the bare flower stalks on which prairie cicadas normally perch and feed have been slow to develop at Loda this summer. But we devised a capture method of walking through the low growth to flush them and then watching where they landed so we could net them there. During my shift we managed to catch and mark 15 cicadas.

The bodies of prairie cicadas measure between an inch and a half and two inches long. Their wings are clear—the texture of cellophane—and divided by dark veins that give them the appearance of leaded glass. Held together at an angle, the wings extend past the end of the cicada’s body to exaggerate its size.

Viewed from above, a prairie cicada’s body resembles an intricately decorated shield, with a dark background marked by symmetrical designs in various shades of tan and white. [The orange paint mark on the right wing of this cicada indicates that it has already been caught.] This shield is topped by two knob-like compound eyes and a set of small, fine antennae. On the underside, male prairie cicadas are distinguished from females by a pair of rigid plates at the base of the abdomen. These plates are hinged, and when open they uncover the audio-speaker-like membranes called tymbals that male cicadas employ to produce sound.

Like other annual cicadas, including the familiar dog day cicadas of cities and suburbs, prairie cicadas spend the greater part of their lives—anywhere from two to four years—underground, feeding on sap from the roots of plants. (Prairie cicadas are referred to as an annual cicada because their generations are staggered, so that some individuals reach adulthood each year.) When they reach maturity, they emerge from the ground to spend a short time in the sun. During that interval they mate and the females lay eggs in the stem of a plant. The nymphs that later hatch from those eggs drop to the ground and burrow down to find a root to feed on, completing the cycle.

During the conversation that took place as we sought to catch prairie cicadas, Rob Stanton met many of my inquiries with the same response, which was “That’s a good question.” The problem wasn’t that he hadn’t done his homework, but that scientists still know little about prairie cicadas beyond the basics of their life history.

Whether we are able to conserve prairie cicadas where they occur now, or reintroduce them where prairie is restored, will depend on developing a more detailed understanding of these charismatic insects.

I was motivated to seek out prairie cicadas by an article that ran recently in The Illinois Steward Magazine, which is published through the University of Illinois.

The article, written by Joseph Spencer, an insect behaviorist with the Illinois Natural History Survey, relates the excitement caused by a photograph he took last summer (left) when he and Rob Stanton went to the Loda Cemetery Preserve for an initial look at the cicadas there. By chance Spencer tripped the shutter just as a female cicada expelled excess fluid from her digestive system.

For more on why entomologists would be so interested in such an event, see “A Taste of Loda Prairie” in the Spring 2009 issue of The Illinois Steward. It is available at bookstores and libraries or, better still, by subscription: Call (217) 244-2851 or visit