The wakeup of millions of ugly, loud but essentially harmless periodical cicadas from a brood that was first observed in Ohio in 1804 has started a few days earlier than expected.

They usually emerge every 17 years when the ground temperature reaches about 65 degrees, according to Gene Kritsky, an entomologist at the College of Mount St. Joseph.

A few have been sighted in Cincinnati suburbs, and by next week there will be millions of them. Experts say they do not carry diseases and do not sting or bite people. They’re just a loud nuisance.

Although they regularly emerge in 17-year cycles, there are different broods and sometimes their territories overlap. The last group to hit Cincinnati was Brood X in 2004.

This group, Brood XIV, inhabits 12 states, mostly in the Ohio Valley but also as far east as Massachusetts, where they were first recorded in 1770.

There are also annual cicadas, which add to the confusion, and the possibility that some broods change their cycles and join other broods.

“Sometimes one brood transitions into another,” said Kritsky, who has been tracking cicadas for 34 years.

He’s devised a Web site (www.msj.edu/cicada) for reporting sightings and to help him and the public track their movement.