Timed to perfection: Cicada's biological clock determines emergence

There are two types of cicadas commonly found in large numbers in Illinois. There are dogday or annual cicadas that emerge every year. Periodical cicadas emerge every 13 years in the southern half of Illinois and every 17 years in the northern half of the state. Cicadas are large bodied and have large compound eyes.

Cicadas are sometimes mistakenly called locusts. In actuality, they are not at all related to locusts, which are a kind of grasshopper. The male cicadas sing during the day to attract females. Dogday cicadas tend to sing more in late afternoon and evening. Each cicada species has its own distinctive sound to avoid attracting the wrong cicada. Typically, periodical cicada emergences consist of three species, that can be distinguished by the male songs as well as by slight differences in their appearance. The nymphs of these cicadas feed on the roots of trees and shrubs.

There are many species of dogday cicadas (also called harvestflies). They are about 1¼ to 1½ inches long and from ½ to 7/16 inch wide. Some species may be as long as two inches. The wings are clear membranes except for the green coloring along the leading edge of each wing. The wings are one and a half inches to two inches long depending on body size. The upper body has a green and brown pattern to green and black pattern while the lower half of the body is mostly whitish.

Periodical cicadas are also edible. Native Americans utilized them in their diet. In 1990, several college students were filmed eating them alive. When asked what they tasted like, the students reported that they tasted like almonds. Drying them in a microwave oven carefully causes them to release an almond smell.

Life Cycle

Dogday cicadas emerge from the soil during the heat of the summer - the dog days of summer. The harvestfly name came about because some crops are being harvested at the time of their emergence. Once they emerge, they mate and lay eggs. The eggs hatch and the nymphs feed on the sap in tree and shrub roots from two to five years. When fully grown, the nymphs emerge from the soil, climb a tree, building, or other upright object, and shed the exoskeleton that protected their body and wings while tunneling up through the soil. By pumping body fluids to the thorax area behind the head, the thorax swells and splits the exoskeleton. This allows the adult to emerge. Once free, fluids are pumped into their wings causing them to unfold and expand. Once the wings are fully extended, the cicadas let them dry before they fly.

Birds and other predators, including humans, feed on dogday cicadas. One of the more noticeable predators is the cicada killer. This is a large wasp that catches the dogday cicada. After catching and stinging the insect to paralyze it, the cicada killer carries it back to its hole and drags it underground to a chamber where it lays its eggs in the paralyzed cicada. When the eggs hatch, the wasp larvae feed on the paralyzed, but still living, cicada.

The damage done to a tree by the dogday cicadas is not significant enough to justify trying to control the cicada. Let nature take its course. As for the cicada killer, leave it alone and it will leave you alone.

Timing

The periodical cicada emergences are composed of three distinct species. Some experts consider the three species that occur in 13 year emergences as different from the three that occur in 17 year emergence, but others do not. Thus, depending on the expert, there are six or three species and each species vary from each other in size, color, and song. These species are further divided into broods, referring to the year and geographical area where they emerge. Some broods are small and cover a limited area. Others are very large and emerge across many states at about the same time. There is a brood emerging somewhere every year. The periodical cicadas found in Illinois tend to be dark brown and black on top with lighter reddish brown patterns at the wing bases. The lower body color is similar to the wing bases. Compound eyes are red with orange major veins in the membranous wings.

The northern Illinois brood, which will emerge in late May 2024, has a reputation for the largest emergence of cicadas known anywhere. This is due to the size of the emergence and the research and subsequent reporting over the years by entomologists Monte Lloyd and Henry Dybas at the Field Museum in Chicago. During the 1956 emergence, they counted an average of 311 nymphal emergence holes per square yard of ground in a forested floodplain near Chicago. This translates to 1½ million cicadas per acre. In upland sites, they recorded 27 emergence holes per square yard, translating to about 133,000 per acre. This number is more typical of emergence numbers but is still a tremendous number of insects. For comparison, a city block contains about 3½ acres. When the cicadas start dying and dropping from the trees later in the spring, there are large numbers on the ground, and the odor from their rotting bodies is noticeable. In 1990, there were reports from people in Chicago having to use snow shovels to clear their sidewalks of the dead cicadas.

• 2020 | Northern Illinois Sub-Brood (part of Marlatt's XIII): 17
• 2021 | Great Eastern Brood (Marlatt's X): 17
• 2024 | Northern Illinois Brood (Marlatt's XIII): 17

Control

The periodical cicada female uses her ovipositor to make a slit in small twigs of trees, but will lay eggs in branches up to one-and-one-half inches in diameter. The female then lays her eggs in these slits. The eggs hatch and the nymphs fall to the ground and tunnel down through the soil to feed on sap in a root until they emerge 13 or 17 years later. Although their feeding on the tree's roots for 13 or 17 years have an impact on the growth of the tree, they do not cause enough damage to justify any control measures. The egg-laying slit made in the twig may cause the twig to dry and break off. This damage, too, is not harmful enough on an established tree to justify trying to control these insects. However, small transplanted trees, particularly fruit trees, commonly have a trunk diameter small enough that egg slits made in the trunk, may result in the tree snapping off.

Insecticide applications kill huge numbers of visiting cicadas, but analysis of egg-slit trunk damage shows little difference between treated and untreated research plots. The only way to protect small trees from serious damage in a heavy emergence area is to protect the trunk with screening or other material. This is expensive in materials and labor. It is much better to delay small-tree planting for a year or install larger stock, preferably those with a trunk diameter of at least 2½ inches.

Some thought about periodical cicadas and their needs can also help. Realize that these insects require a steady supply of sap-supplying tree and shrub roots for 17 years. Housing developments that have had all trees and shrubs removed prior to building will have few cicadas because the nymphs died when the trees were removed. Similarly, housing developments in areas that were originally farm fields or prairie will have few cicadas due to the original lack of trees. The practice of bulldozing all trees off of a housing development site has been common only since the 1960s, so older housing developments tend to have large numbers of cicadas.

The periodical cicada flies only a short distance, apparently less than one-half mile, from where it emerged from the soil; therefore, it expands its territory very slowly considering its long life cycle. We are not used to thinking of insect life cycles longer than one year. Only nine generations of 17-year periodical cicadas ago, Abraham Lincoln was practicing law in Illinois prior to the Civil War. During the intervening time, periodical cicadas have probably spread only about 5 miles. Consider, for a moment, what most of Illinois was like before the 1900s. Large areas of prairie had periodic prairie fires that killed many trees. With prevailing west winds, large prairie fires would have moved east with the wind. Rivers that run roughly north to south, such as the Fox, Des Plaines, Illinois, Rock, and Mississippi Rivers would have functioned as firebreaks. Trees would have survived much longer on the east sides of these rivers, and so would the periodical cicadas. As a result, we see higher populations of periodical cicadas in East Peoria than in Peoria, Rockford east of the Rock River, and similarly along other rivers.

Credits: This information previously appeared on Extension's Cicadas in Illinois website, written by James Schuster and Philip Nixon.