Thousands of 17-year cicada sightings were reported this summer across parts of the Northeast, New England and the South.
Some of the expected cicada emergence was delayed because of cold late-spring weather in eastern Pennsylvania, Greg Hoover, a Penn State senior extension associate in entomology, said. Cicadas emerge when soil temperatures reach 64 degrees and there is a light precipitation event, he said.
When the conditions are right, usually in mid- to late May, cicada nymphs emerge from half-inch holes in the soil and climb onto trees or other objects, Hoover said. Within an hour, they shed their skins and become adults.
Periodical cicadas are found only in eastern North America, John Cooley of the University of Connecticut said. There are seven species, three of them with 17-year cycles and the rest on 13-year cycles.
Populations are called broods and are identified by Roman numerals; all eight broods found in Pennsylvania are on a 17-year cycle.
Land development and the loss of wooded/forested area in eastern Pennsylvania diminished the emergence of Brood II, but Brood II is not one of Pennsylvania's larger cicada emergences, Hoover said.
Some damage from the female egg-laying process was reported in northern New Jersey by owners of valuable woody ornamental plants, Hoover said. A female will deposit up to 600 eggs after cutting into the bark of a twig.
"The nymphs prepare an emergence tunnel in the soil days before they emerge to transform into adults. Over the years, I've noted the start of a periodical cicada emergence in Pennsylvania is usually triggered by a rainfall event," Hoover said.
While Brood II was on cycle this year, there are reports of "stragglers," cicadas appearing on the wrong cycle.
"These delays are often attributed to the quality of the food on which periodical cicada nymphs are feeding. Periodical cicada nymphs seem to keep track of annual cycles of their host plants," Hoover said. "Any unusual weather event that affect the quality of their host plants may miscue them and cause an emergence to occur in a different year. The exact cause or prevalence of straggling is not well understood. Straggler records confound attempts to make accurate maps of the periodical cicada broods."
Cicadas showed up pretty much as expected from prior population maps, Cooley said.
"There were a few surprises, including some previously unreported populations in Oklahoma on the same cycle as Brood II," he said.
Two species of periodical cicadas also occurred farther north in the Hudson Valley than expected, Cooley said.
However, the current cicada emergence is over.
"The adult periodical cicadas are long gone. In most areas, the new nymphs have probably hatched, although in the northern parts of the range -- New York and Connecticut -- the hatching is still going on," Cooley said.
The cicadas that folks are hearing now in Pennsylvania are dog-day cicadas that require two to four years to reach maturity, Hoover said. The dog-day cicadas, named for the dog days of summer, usually appear in late July or early August and are larger than the periodical cicadas.
If people missed the 17-year cicadas in their area, there is hope.
"Almost every year in the eastern U.S., a periodical cicada brood emerges. If you weren't in the range of Brood II, odds are you are within the confines of another brood," Cooley said.
The next impressive 17-year cicada emergence in Pennsylvania will occur in western Pennsylvania as Brood VIII in 2019, Hoover said.