“Ahhhhhhh!”

That’s sometimes the response to the thousands of cicadas that come out of the ground every 17 years.

In summer 2021, Brood X didn’t seem to make it north of South Mountain in the Lehigh Valley region.

The cicadas’ song was strong in the Center Valley area of southern Lehigh County and also in upper Bucks County.

North of South Mountain?

Not so much.

Be that as it may, don’t worry. They’re snappy.

Cicadas do not bite or sting people or pets and are usually at most a mild nuisance because of the noise they produce.

Young trees can sometimes be damaged or killed, but this can usually be avoided by delaying transplanting or covering at-risk trees with netting.

Periodical cicadas are native to eastern North America and are found nowhere else. There are seven species, three of which have 17-year life-cycles and four of which have 13-year life-cycles.

The 17-year cicadas generally have a northern distribution while 13-year cicadas are more southern, although they exhibit considerable overlap in the middle of the United States from North Carolina and Georgia west to Missouri.

Both types may be found in the same forest. Three species of 17-year cicadas occur in Pennsylvania and may emerge at the same time.

Eight different periodical cicada broods exist in Pennsylvania, all of which require 17 years to reach maturity. Several of these broods are very small.

Brood X, known as the “great eastern brood,” is a large brood that emerges across 15 states and has heavy concentrations in eastern Pennsylvania.

Adult periodical cicadas are about one-and-one-half inches long, mostly black in color with reddish-orange eyes and wing veins.

Periodical cicadas may be confused with various species of annual cicadas, which emerge every year. However, they can be distinguished based on size, color and emergence time.

Annual cicadas are larger with greenish wing veins and emerge from July through September instead of late May through early June.

Periodical cicada nymphs live in the soil at depths of two to 24 inches, where they feed on sap from tree roots.

When nymphs determine it is the year to emerge, they burrow to about one inch beneath the soil surface in April.

If the ground is too damp, the mature nymphs build a protective earthen turret, which can help identify where cicadas will emerge. When the soil temperature reaches 64 degrees Fahrenheit, the nymphs exit the ground and crawl one foot or more up tree trunks, weeds or other upright objects. In Pennsylvania this usually occurs in late May or early June, depending on how warm or cold spring temperatures were.

The adult cicadas then shed the nymphal exoskeleton, which is left behind, in one hour or less. At this point, the cicadas are soft, white and unable to fly because the exoskeleton takes a few hours to harden.

Once the exoskeleton is hardened, the adults are capable of flying but are rather clumsy fliers and often slam into objects. This makes them easy prey for various birds, which gorge themselves on the cicadas.

While stragglers may emerge a few days earlier or later, the main emergence of a periodical cicada brood often occurs over one or just a few nights.

Soon after emerging, males begin to sing while females remain silent. About 10 days after emergence, females mate and begin depositing eggs in twigs and branches of trees and woody shrubs.

Using a saw-like ovipositor, a female cicada cuts a small pocket into a twig in which she deposits 24 to 28 eggs. She then moves forward, cuts another pocket and lays more eggs.

The pockets are placed close together in a straight row and sometimes form a continuous slit for two to three inches. Adult periodical cicadas live for three to four weeks above ground. Each female can lay 400 to 600 eggs over a lifetime.

The eggs hatch six to seven weeks after they are laid. The white, ant-like nymphs work their way out of the twig slits, drop to the ground, and enter the soil, where they feed on fluids from plant roots for the next 17 years.

While annual cicada oviposition does not usually damage trees, periodical cicadas emerge in such high numbers that they can collectively cause heavy damage that results in twig and stem dieback.

Large, otherwise healthy trees can withstand this damage without long-term consequences, although they may be aesthetically unpleasing for a time.

However, small trees that have a majority of their branches within the cicadas’ preferred size range can be severely affected and sometimes be killed. This is especially true of small, stressed trees, such as those that have been recently transplanted or are balled and burlapped in preparation for sale or transplanting.

Deciduous trees are preferred hosts, especially oaks, maples, apples and other trees that often have twigs of the appropriate size, although cicadas are not too picky of the species.

Periodical cicadas do not usually deposit eggs in coniferous trees, although it is not unheard of.

A periodical cicada year is a time of feasting for an array of creatures. Grackles, crows and other birds dine voraciously on the adults, while fish will literally gorge themselves on cicadas when they are abundant in trees and shrubs along a stream.

The large number of cicadas present is likely to outstrip the ability of predators in a given area to effectively control the insects.

That’s the whole point of the cicadas mass emerging as a brood, after all.

“Growing Green” is contributed by Lehigh County Extension Office Staff and Master Gardeners. Information: Lehigh County Extension Office, 610-391-9840; Northampton County Extension Office, 610-813-6613.