Cicadas are coming

Wednesday, March 23, 2011

Insects that have the longest developmental period of any in North America are likely to be seen in the county this spring.

Brood XIX of the 13-year cicadas had a spectacular emergence in Middle Tennessee in 1998, so this is their year to emerge again, probably in May, when the temperature of the soil four inches below the surface reaches 67 degrees.

Early colonists were familiar with the Biblical story of locust plagues in Egypt, so when they first saw cicadas, they called them "locusts" and thought a "plague of locusts" was punishing them. The name "locust" correctly refers to certain species of grasshoppers.

The large numbers of cicada adults that emerge often arouse fear that they will destroy crops, but cicadas do not feed on foliage. They may feed on twig sap, but females making slits in the bark of trees to deposit their eggs do the real damage.

Adult cicadas, 1 to 1-1/2 inches long, emerge in large numbers, and, four or five days later, the males start "singing," with a high-pitched shrill call to attract females. After mating, the females begin laying eggs. The female cicada has a knife-like ovipositor that she uses to slit the twigs of woody plants. Apple, pear, dogwood, oak and hickory are favorite hosts, and a twig the diameter of a pencil is most often chosen. In each slit, 24-28 eggs are laid, and the process is repeated until a total of 400-600 eggs have been deposited.

Cicada eggs hatch in six to seven weeks and the ant-like white nymphs drop to the ground and work their way into the soil until a suitable root is found. Nymphs grow slowly, feeding by sucking sap from tree roots. This has no noticeable effect on the trees.

Then, after 13 (or 17) years, usually after sunset, the nymphs burrow upwards and leave the soil. They seek upright structures, like posts and trees, on which to molt, and the adult emerges several hours later. At first, adults are soft and white, but they quickly become harder and darker and are able to fly.

According to advice from the University of Tennessee Extension Service, if you have young fruit trees, delay pruning until after cicada emergence so that damaged branches can be removed. Small valuable shrubs, trees and ornamentals may be covered with cheesecloth or spun row cover for protection from cicadas. Insecticide spray applications to trees are not effective for preventing egg-laying damage by female cicadas - they can still lay eggs before being killed by the insecticide residue.

There are two races of periodical cicadas, the 13-year and the 17-year. Both occur in Tennessee, though Marshall County is only on the map for one of them, Brood XIX that is coming this year. Scientists have identified 12 different broods of 17-year cicadas, and three 13-year broods.