Cicadas, Itch Mites and Welts, Oh My!

Published: April 1, 2011

It won't be long until we can hear the distinctive mating call of the periodical cicada in eastern Missouri. Periodical cicada nymphs have been underground in the soil at a 2 to 18 inch-depth feeding on plant roots since 1998, when both "13-year" and "17-year" broods emerged in the same year. When soil temperature reaches 67°F at a 4-inch soil depth (usually in May), nymphs emerge from the soil and climb up weeds, vines, shrubs, tree trunks, fence posts, buildings, or other vertical structures where they molt to adults. During this molt, they shed their exoskeletons, frequently leaving them attached to tree trunks and limbs. In some cases, nymphs also build 2 to 4 inch turrets of soil when air temperature is warm and soil moisture is high. For a 4 to 6 week period, the male periodical cicadas vibrate membranes on the sides of their abdomen, producing a courtship "song" to attract females for mating from sunup to sundown. As daytime temperatures increase, the mating call can become deafening, especially in wooded areas with a high cicada population.

The plant damage begins when females oviposit in small-diameter (usually less than quarter-inch-diameter) branches of trees and shrubs. A female slits the twig, depositing 24 to 28 eggs beneath the bark. Moving forward, she can make as many as 5 to 20 slits before going to another twig. Each female can lay 400 to 600 eggs, which remain on twigs for 6 to 10 weeks before hatching. Newly hatched nymphs then fall to the ground and burrow into the soil to feed on roots for 13 years.

Slits in the bark from ovipositing females cause the shoots to snap and dangle from the branch tips, where they wilt, and die. These wounds can become points of entry for other insects. Damaged shoot tips are especially detrimental in young plants. In the following growing season, damaged shoots tips are much like heading cuts made when pruning. Because the shoot tip was removed, three to five new terminal shoots will be produced, causing "bushier" growth around the perimeter of shrubs. This proliferation of new growth shades lower interior branches and can cause dead areas in the plant. On fruiting shrubs and trees, such as blueberry, currants, gooseberry, peach, plum, apricot, and cherry, a portion of fruiting wood may be lost. For one or two-year-old apple or pear trees, fruiting is delayed as new vegetative growth develops behind the injured shoot tips. Spring flowering may also be reduced on ornamental trees, shrubs, and vines. Recently planted ornamental trees and shrubs may be disfigured by heavy cicada ovipostion damage, requiring remedial pruning to restore them to good form.

Small plants can be covered with a layer of spun row cover fabric to protect them from damage. When covering plants, secure the material around the base of the trunk to exclude the nymphs before emergence and remove it in late June. For uncovered plants, remove the damaged shoot tips immediately to reduce the nymphs that will establish on the roots. Chemical control in home landscapes is generally not recommended. While periodical cicada damage can make ornamentals temporarily unattractive with dead twig tips, mature trees or shrubs rarely suffer sufficient damage to warrant control. Applications of Sevin (carbaryl) repeated at 7 day intervals can kill pollinators and in some instances, increase spider mites on blooming plants. Sevin used within 30 days after full bloom will thin apples. For commercial nurseries and orchards, labeled insecticides can provide limited control of cicadas, as new ones infest the plants after spraying.

Periodical cicadas are often called locusts. However, the term "locust" can also refer to migratory grasshoppers that plague crops. They are also different than the dogday cicada, which appear during long, hot days of July and August. These cicadas have 2- to 5-year life cycles, but appear annually because of overlapping broods. They are also larger than the periodical cicada. Dogday cicadas are generally found on mature ornamental trees and usually do not cause much damage, as they are controlled by predatory birds.

While those in eastern Missouri will enjoy the chorus of the 13-year cicadas this year, the northern part of the state will hear the mating calls of the 17-year cicadas in 2014. Yet another brood of 17-year cicadas will emerge in west-central Missouri in 2015.

In addition to the plant damage they cause, periodical cicadas may also promote an outbreak of itch mites (Pyemotes herfsi). These small mites (0.2 mm in length) are believed to feed on cicada eggs. As the population of mites increases, they fall from trees and can bite humans and pets, causing itchy welts. These bites are not dangerous, but can be intensely itchy. One study indicated that more than 300,000 mites per tree fell per day from pin oaks in Kansas. People who spent considerable time outdoors in urban areas were most affected by the itch mites in early August. However, those who rarely went outside were also bitten, indicating that the mites can enter homes through screen doors and windows.