How do cicadas know when 17 years roll around and it is time to appear?

Somehow cicadas know when 17 years has passed, and it is time
 for them to emerge from underground.

Cicadas, locusts, bugs, whatever people call them are coming. Maybe, they are already here. For one brood, 2015 is their year.

Cicadas also are known as locusts, but they are not a true locust, They are however, unusual creatures. Scientists know a lot about them, but mystery still surrounds these strange bugs.

It seems nothing is simple when it comes to cicadas. The ones that make the news are the broods or groups that emerge from underground every 17 years, thus the name “Seventeen year locusts”. But, then there are broods that emerge every 13 years, and then there are some slightly different ones that pop up every year.

But, it is the 17-year bugs named Brood X that is emerging in part of the midwest, including southwestern Indiana and western Kentucky this spring and early summer.

After the cicadas have counted 17 years—"we really don't know how they count the years," a biologist  said. They are ready to emerge, which usually happens in late spring when the soil reaches a temperature of about 64 Fahrenheit (18 Celsius).

People love them, hate them, or just put up with them.

Keith Clay, a biologist at Indiana University, says most people either love of hate the cicadas. He has spent considerable time studying the Brood X variety, which currently is on stage across a good portion of the midwest.

According to Clay, some people so dislike the loud noise made by the thousands, maybe millions of cicadas, they plan vacations elsewhere during the brief life span of the bugs. (Cicadas don’t appear out west.)

However, Clay claims there are those who so look forward to the 17-year awaited emergence, they plan camping trips to be in the middle of the happening.

Purdue University also has been studying these bugs who spend nearly all of their life underground. 

In a news release Purdue said, “As the soil temperature warms, a new love song will fill the air in...After spending the last 17 years underground feeding on tree roots, millions of periodical cicadas - up to 1.5 million per acre - will resurface...to mature, attract mates and lay their eggs.

“In late May, the last immature-stage cicadas will begin to crawl out of the ground and latch onto vertical plants, fence posts or other above-ground structures where they can molt into adults,” said Purdue University entomologist Cliff Sadof.

Adults will mate seven to 10 days later and females will lay eggs into slits cut into the twigs of many tree species. In six weeks, these eggs will hatch into nymphs that will fall to the ground and crawl beneath the soil where they will feed on tree roots for the next 17 years.

Those who are curious can look for periodical cicadas by checking trees that were present 17 years ago and listening for the shrill mating calls that can be heard up to a quarter mile away.

"The cicadas will be most concentrated at forest edges where they will be warmed by sunlight early in the day," Sadof said. "Males need to reach a critical temperature to be able to sing. Early singers out compete their rivals for females.

While cicadas won’t be on the menu at the Junker house, Sadof says there are people who eat them.  "They are edible, but people who are allergic to shellfish shouldn't eat them."

Humans aren't the only ones who can eat cicadas, however. Birds, fish, spiders and snakes eat periodical cicadas, and sometimes dogs and cats will do the same. I’ve also heard of bass fisherman who have had success landing a lunker using them as bait.

To learn more about periodical cicadas, insecticide use, resistant plants and other cicada broods in Indiana, visit http://www.entm.purdue.edu/entomology/ext/targets/e-series/eseriespdf/e-47.pdf .