Coming soon in Illinois' cicada invasion: The smell. When to expect the stench and for how long

Many are wondering when Illinois' massive cicada invasion, which brought trillions of cicadas from two broods in a historic moment to the surface, will come to an end -- but there's something else, something stinky, the area will need to get through first.

The cicada-palooza has brought insect numbers unlike what many have seen before to Illinois and parts of the U.S.

“What you saw was biblical,” said biologist Gene Kritsky, who has been chasing periodical cicadas for 50 years, yet was still amazed by the 3 to 5 million cicadas crowding a small patch of Ryerson Conservation Area north of Chicago. “There are things I've seen this time that I've never seen before.”

It's an only-in-the-United States spectacle, the last of the triple crown of rare forecasted natural wonders.

First, there was April’s solar eclipse, followed by May’s Northern Lights unusually far south. Now the great dual periodical cicada emergence of 2024 — an event of a magnitude not seen since 1803 — has burst from below to join the earlier shows in the sky. It’s lasting weeks longer than the other two fleeting natural rarities, but in many places the cicada invasion is starting to wind down.

That includes Illinois.

The number of cicadas climbing out of the ground has dwindled, but it's not quite over yet. According to Kritsky, the event will likely last through the end of the month.

“In about another two weeks it'll be noticeably over,” Kritsky said. “It's been a blast.”

But before all signs of cicadas are gone, for now, there will be a period of decomposition that could lead to an unpleasant odor for those experiencing the bulk of the emergence.

Currently, the bugs are in their "mating phase," but it won't be long before their lifespan comes to an end.

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.

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.

Some may already be seeing dead cicadas on the ground and while there are plenty of benefits to what comes next, the odor that comes with piles of decomposing insects could be off-putting.

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

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

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

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 largely over.

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 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.

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.