As cicada emergence begins, just how rare is this event, and what do you need to know

A rare double-emergence of periodical cicadas is getting underway in the eastern United States, and Illinois is right at the confluence of an event that hasn’t occurred in 200 years.

Periodical cicadas, which only exist in the eastern United States, emerge on 13 or 17-year life cycles, and two broods that are both present in Illinois will actually emerge at the same time, according to experts.

Just how rare is this double emergence? What threats do these periodical cicadas pose? And how can you cope with their emergence?

Here's a look at what you should know about the rare and historic moment:

Just how rare is this dual emergence?

According to the University of Illinois, Broods XIII and XIX are both emerging this year in different parts of the state, though there will be overlap in some central portions of Illinois.

While the broods do emerge periodically, as their names imply, they very rarely do so at the same time. According to the university, this marks the first dual-emergence of Broods XIII and XIX in 221 years, with the last occurring during the administration of President Thomas Jefferson.

In fact, it will be another 221 years before such a dual emergence occurs again, with the next one set for 2245, according to experts.

Where will the emergences occur?

For the Chicago area, Brood XIII will be most seen in parts of northern Illinois and Indiana, and possibly even in Wisconsin and Ohio, Dr. Gene Kritsky, dean of Behavioral and Natural Sciences at Mount St. Joseph University in Cincinnati, said.

The Northern Illinois Brood itself is huge, with a reputation for the "largest emergence of cicadas anywhere," according to the University of Illinois.

In 1956, entomologists reported as many as 311 "emergence holes" per square yard in a forested floodplain near Chicago, which experts say translated to 1.5 million cicadas per acre, according to the University of Illinois.

"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," U of I reports. "In 1990, there were reports from people in Chicago having to use snow shovels to clear their sidewalks of the dead cicadas."

Meanwhile, Brood XIX, or the Great Southern Brood cicadas, have a more widespread population, covering parts of Missouri, Illinois, Louisiana, North Carolina, Virginia and Maryland.

"Brood XIX is arguably the largest (by geographic extent) of all periodical cicada broods, with records along the east coast from Maryland to Georgia and in the Midwest from Iowa to Oklahoma," the University of Connecticut reports. "Although 13- year cicadas are generally considered to have a southern distribution, the northernmost known record of this brood is in Chebanse, IL, roughly 75 miles from Chicago’s Loop."

Where in Illinois are the cicadas expected?

Across most of Illinois and the Chicago area at least one of the two broods is likely to emerge, but in a narrow part of the state, both could emerge at the same time, in the same place.

"This is like the year for Illinois," cicada expert Catherine Dana, an affiliate with the Illinois Natural History Survey, told NBC Chicago. "We are going to have cicadas emerging all over the state."

Here's a map of what to expect in Illinois, according to data from the USDA Forest Service.

"Somewhere around Central Illinois, probably like around Springfield, is what some researchers are predicting we may see some overlap of these two ... different broods," Dana said. "It's not going to be a large area. But there will likely actually be some mating happening between these two broods, which is going to be really exciting."

When are cicadas set to emerge in Chicago and Illinois?

The emergence has started earlier than average in Illinois.

"The periodical cicadas have been emerging for the last week and a half," Stephanie Adams, plant pathologist at Morton Arboretum in Lisle, said Tuesday of Brood XIII, which will soon overspread across the entire Chicago area. "We found them both out here in our landscape and also in our East woods."

According to Adams, the emergence of the first cicadas comes about two weeks ahead of the historic average. It will continue to be sporadic, as soil temperature, mulch and turf grass all impact cicadas differently. For example, the soil is warmer near pavement, so cicadas in the those spots are expected to emerge quicker.

Sightings of cicadas, many still in the ground, have already been reported in numerous parts of the city and suburbs.

Cicadas typically emerge as the ground begins to warm in the spring and early summer.

Cicadas have a lifespan of approximately four weeks and typically emerge in mid-to-late May and into June, as the soil temperatures warm in the spring and early summer.

"Research shows that the particular night of the periodical cicadas’ emergence depends on soil temperature," the post read. "Cicada juveniles, or nymphs, emerge after a rainstorm when the soil temperature at 8 inches in depth exceeds approximately 64°F."

How long will the cicada emergence last?

The emergence is set to last through mid-June.

Do cicadas bite?

Generally, cicadas are harmless, Adams said. They don't bite, and they don't have stingers, and they're beneficial to the environment.

What do cicadas eat?

According to Orkin, cicadas have "piercing/sucking mouthparts (similar to a mosquito) they use to obtain their nourishment from below ground roots and young twigs." But what they eat will depend on their age.

Adult cicadas "feed on plant fluids from the young twigs of trees and woody shrubs," Orkin reported.

Which trees will attract the most cicadas to your yard?

Cicadas can cause damage to vulnerable and small trees and shrubs, Adams said.

According to Adams, young plants, between two to three years old, are the most vulnerable, and so are smaller plants, with branches less than two inches in diameter are at risk. They may not be able to recover from damage done by the female cicadas laying eggs, Adams said.

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“That damage is caused when the female starts laying her eggs,” Adams explained. “She has a specialized organ that will cut into the plants and trees and shrubs, and lay her eggs inside the thin bark.”

According to the Arboretum, cicadas are "not particular," meaning they will use a variety of trees and shrubs as hosts.

"However, only very young trees or older trees that are unhealthy or stressed are at any risk," the Arboretum said.

The insects have been known to lay eggs in oak, maple, hickory, apple, birch, dogwood, linden, willow, elm, ginkgo, and pear trees, according to the Arboretum.

"Cicadas also may lay eggs in some shrubs, such as rose, lilac, and forsythia," the Arboretum said. "They tend to avoid conifers such as pine, spruce, and fir."

Arboretum records show that during the emergence 17 years ago, some of the plants most affected by the emergence were maples, cherries, ashes, hawthorns, willows, mountain ashes, oaks, pears, roses, privets, poplars, serviceberries, and beeches.

According to the Arboretum, one way to protect your trees and shrubs would be to use tulle as a wrap and protective barrier. Tulle material is specifically recommended because it is a breathable, and allows sunlight to penetrate the plant.

“You are looking for netting that is no bigger than quarter inch across, any bigger the cicadas can climb through and defeats the purpose of protecting them,” said Spencer Campbell, Plant Clinic Manager.

Homeowners are encouraged to start protecting their young plants as soon as possible, and to keep them wrapped through mid-June when the emergence ends.

Can pets eat cicadas?

As millions of the insects are set to swarm virtually everything outdoors in the region, some pet owners are wondering how their furry friends will react to the cicada emergence.

According to Allen Lawrance, associate curator of entomology at Petty Notebaert Nature Museum, cicadas are going to be viewed as a tasty treat to many animals.

"As far as insects go, they're very large and meaty. They have a lot of nutrients there. That's a big meal just from catching one insect," Lawrance told NBC Chicago.

Lawrence added that squirrels, birds, snakes and dogs may all be drawn to the sight of a cicada.

But are they safe for your dog to eat?

"Yes, they are non-toxic, so they are safe for dogs to eat. However, you may need to be careful and make sure your dog doesn't eat too many too quickly and they get an upset stomach," Lawrance said.