ABERDEEN PROVING GROUND, Md.- Periodical cicadas are emerging after spending many years underground. Populations of periodical cicadas from Brood X could be as dense as one and a half million cicadas per acre in some areas in the eastern United States. These ancient insects are some of the longest-lived insects on earth and their emergence is unique to North America, but Army bug experts say there is no reason to fear.
“Despite their threatening appearance, periodical cicadas are harmless and they do not bite, sting or transmit any diseases,” says Anne Radavich, chief of the Entomological Sciences Division at the Army Public Health Center.
Scientists are still not sure exactly why these insects
emerge in large numbers every 13 to 17 years, but thoughts are that this is a type of survival strategy.
The 17-year periodical cicadas emerging in 2021 belong to what is called Brood X or Brood ten. A periodical cicada “Brood” is a reference to a geographic localized population of cicadas that will simultaneously emerge as adults in a given year. Brood X is a specific group of 17-year periodical cicadas, known as the “Great eastern brood” and is the largest brood of periodical cicadas in the United States. These cicadas will be emerging in Washington, DC, and 14 states: New York, New Jersey, Pennsylvania, Delaware, Maryland, Ohio, Michigan, Virginia, West Virginia, Indiana, Illinois, Kentucky, Tennessee and Georgia.
Within Brood X there are three different species of 17 year cicadas emerging: Magicicada septendecim, Magicicada septendecula, and Magicicada cassinii. Although all three species look similar, scientists say that the Magicicada cassinii is slightly smaller in size than the other two species.
“The nymphs of periodical cicadas are dark tan in color, have no wings and are about an inch to one and a half inches length,” said Kevin Harkins, an entomologist at the Army Public Health Center. “The adults are about one and a half to two inches in length, have black bodies, red eyes, orange legs, and clear wings with yellowish orange wing veins. The wings of adult periodical cicadas fold over their backs when they are resting, crawling or feeding. The adults can fly but are not very skilled fliers. Both the nymphs and adults have a needlelike mouthpart called a proboscis that they use to pierce plant stems and suck plant juices.”
The nymphs are emerging in large numbers because the soil temperature has reached the magical range of around 64-65 degrees Fahrenheit. Interestingly, the emergence holes in the ground created by the nymphs may help aerate the soil to benefit the tree’s root growth. Once out, the nymphs will crawl up surfaces, most often tree trunks and then molt or shed their skins into adults with wings. As soon as the adults harden their body and wings they will start to make sounds and take flight to find a mate. Adult periodical cicadas will typically live for about four to six weeks.
Scientists say the humming sound they make is the chorus of adult male periodical cicadas calling for female periodical cicadas.
“They make the sound by using specialized structures called tymbals which are found on either side of the male’s body,” said Radavich. “The chorus can be so loud (up to 100 decibels, similar to that of a leaf blower in operation) that the songs may actually confuse or drive away predators that eat them. A fun thing to do with these harmless insects is to watch male periodical cicadas follow your hand when you snap your fingers.”
This simulates the sound of a wing flick from a female periodical cicada which signals that she is ready to mate with the male. After mating with the male, the adult female periodical cicada will lay her eggs in tree branches and then die. The eggs will hatch in six to ten weeks and tiny nymphs will emerge and fall to the ground to burrow into the soil. This is where they will spend the next 16 years feeding on underground tree roots until it is time to emerge again.
Army entomologists say that periodical cicadas don’t have specific predators other than a wasp called the cicada killer, but the cicada killer wasp will not make a dent in the overwhelming numbers of Brood X periodical cicadas.
“Birds, rodents, cats, dogs, and cars may take out a good number of cicadas,” said Harkins. “Both the nymphs and adults make great fishing bait. Pets might eat them, but be warned that they will not be able to digest their hard outer skeleton very well and eating too many periodical cicadas may make them sick or constipated.”
Scientists also say that adult female periodical cicadas may damage young trees when females lay their eggs in softer branches, especially the branch tips.
“This may kill the tip of the branch, and cause it to wilt, called flagging, but should not kill the whole tree,” said Radavich. “Young newly planted trees are particularly vulnerable to damage by cicadas, particularly ash, beech, dogwood, hickory, oak and willow. Pines and firs, along with most flowers and vegetables, are usually not bothered. Both male and female cicadas feed on plant juices with their needlelike mouthparts, but their feeding should not cause damage to trees and shrubs.”
Not a lot can be done to manage periodical cicadas. They will mostly be a nuisance for a few weeks, and numbers could be overwhelming in some localities. Army entomologists say that concerned residents can physically remove specimens from branches or wrap their trees with plastic mesh netting that has a one quarter inch mesh size or smaller.
“When using netting be sure to secure the bottom of the netting around the branch or trunk of the tree,” said Harkins. “This may help prevent the female from laying her eggs in the branches of your tree. Spraying insecticides to kill periodical cicadas will not be effective, is not recommended and may cause harm to beneficial insects that are present.”
Anyone who sees periodical cicada adults, nymphs or shed skins on vegetation or on trees and is interested in reporting the sighting will be able to help researchers map the emergence for future study. Resources such as the Cicada Safari app, available for download for Android and Apple phones, can be used to report these sightings.
The Army Public Health Center focuses on promoting healthy people, communities, animals and workplaces through the prevention of disease, injury and disability of Soldiers, military retirees, their families, veterans, Army civilian employees, and animals through population-based monitoring, investigations, and technical consultations.