Illinois' cicada invasion isn't over yet. Here's why and what happens next

Are you seeing a lot of dead cicadas? You're not alone, but that might not mean the emergence in Illinois is coming to an end.

It could mean cicadas are entering a new phase of the emergence: the mating phase.

Experts had said cicadas would continue emerging from the ground until late May and early June, but once they emerge, it takes several days before they begin their mating calls.

While cicadas have already been spotted in massive amounts in parts of the region, some suburbs are reporting a shift.

Dr. Gene Kritsky, dean of Behavioral and Natural Sciences at Mount St. Joseph University in Cincinnati who has been tracking the emergence across the U.S. and particularly in Illinois, noted at the end of May that while numbers for cicadas emerging from the ground would decline around this time, that won't mean an end to the noise or the sightings. Cicadas will still need to live out their life cycles above ground.

"The adults will still be singing loudly until mid-June, and then dwindle down towards the end of June," Kritsky said.

He noted that while the emergence has slowed, "the singing is still getting louder," which indicates the mating phase is largely underway.

When the bugs initially surface they actually stay silent for a little while. According to Kritsky, it takes five days for a cicada to begin singing after it springs from the ground.

But with only weeks to live after they emerge, the insects are quick to reproduce.

Male cicadas make a loud sound as they rush to attract mates before their life cycle ends.

According to the National Museum of Natural History, adult cicadas only have about three to four weeks to live after they emerge from the ground.

Once they mate, male cicadas finish their life cycle as female cicadas then make slits in tree branches and lay eggs.

Some males may die shortly after reproduction, but most "won't immediately perish upon mating" and will live a short time longer.

"The eggs hatch six to seven weeks later, the nymphs fall to the ground and go into the soil, and the cycle begins again," the Natural History museum said.

While the emergence spanned several weeks across Illinois and the Chicago area, so too will the mating period.

John Cooley, a University of Connecticut entomology professor affectionately known as "Doctor Cicada," said those who see bugs in pairs are about halfway through the emergence.

While some may already be seeing dead cicadas on the ground, others may only be just beginning their mating season.

In any case, residents shouldn't fear dead cicadas, expert said.

Much like their early life, dead cicadas actually bring an environmental benefit.

"The dead adult insects will drop back to the ground and help fertilize the soil. You can even add dead cicadas to your compost pile. It’s a great example of the natural circle of life," according to the Nature Conservancy.

As cicadas begin to die off in larger numbers toward the end of the month, it will take a couple weeks for their bodies to decompose back into the soil.

"They're free fertilizer for the tree," Kritsky said.

The decomposition process could come with a rancid smell, but Kritsky said it will be short-lived and those who can stick it out will reap the benefits.

Depending on the weather, according to Kritsky, the full decomposition process should take a couple weeks -- though the smell will end sooner, he said.

While several suburbs are reporting massive amounts of cicadas, some parts of the region have reported almost none at all, particularly in the northwest suburbs and in Chicago itself, according to a map that tracks cicada sightings in the U.S. These areas likely won't see much of anything now that the emergence phase is likely over.

The historic 2024 emergence involves two broods of cicadas - Brood XIII and Brood XIX - emerging simultaneously. Those two broods of 13-year and 17-year cicadas haven't emerged together in more than 220 years. While much of Illinois saw only one brood emerging, a narrow part of Central Illinois may have experienced both.

For those who experienced large amounts of cicadas in this emergence, there could be another opportunity to witness the historic cicada scene unfold this summer.

That's when the eggs begin to hatch in mid-July.

The hatching takes place between six and 10 weeks after eggs are laid and while it is rare to catch a glimpse of the moment, Kritsky said when conditions are right, the moment could be viewable in the Chicago area.

"If the sun is at the right angle, people have actually seen the nymphs falling to the ground," Kritsky said, noting the sun would need to be behind the tree where the eggs are hatching, "illuminating them as they drop."

Trees in areas that saw large emergences could have as many as 40,000 eggs waiting to hatch, he added.