Parts of Virginia, West Virginia and North Carolina may soon be awash with the sound of cicada calls, with entomologists predict densities of up to 1.5 million insects per acre.
The reason for this sudden cacophony is the emergence of brood IX (9), which will emerge after spending the last 17 years underground as nymphs. People living in the aforementioned states may recall the spring and summer months of 2003, when the cicadas of this brood were last out in force.
"Communities and farms with large numbers of cicadas emerging at once may have a substantial noise issue," Eric Day, Virginia Cooperative Extension entomologist at Virginia Tech's Department of Entomology in the College of Agriculture and Life Sciences, said in a statement.
"Hopefully, any annoyance at the disturbance is tempered by just how infrequent—and amazing—this event is."
The reason for these intermittent displays is that cicadas can occur annually or periodically, depending on the species. Periodical cicadas—like the Magicicada septendecim, Magicicada cassini and Magicicada septendecula—spend the majority of their lives in the soil in an immature form. After 13 or 17 years, depending on the species, the nymphs transition into adult cicadas, shedding their skins and resurfacing to mate and kickstart the cycle all over again.
According to Virginia Tech, the shift is driven by the year and soil temperature, which enables individual insects to coordinate their emergence and, thus, boost their chances of reproduction. But why the cycle takes place every 13 or 17 years and not every 12 and 16 is "one of the great mysteries of the insect world."
Past research has suggested it may have something to do with avoiding predators. Observations have shown birds can wipe out a whole population of cicadas if it resurfaces at the wrong time, reports Science. This was backed up by research in 2013, when academics writing in The American Naturalist connected cicadas' 13- and 17-year cycles to drops in populations of predatory birds. However, they were unable to explain why bird numbers hit all-time lows at these intervals.
Different broods cover different geographical regions, though they do at times overlap. Brood IX covers southwest Virginia as well as communities in North Carolina and West Virginia. Brood X—due to emerge next year—will hit several states, including Pennsylvania and Washington D.C. It is, according to Magicicada.org, one of the largest broods of 17-year cicadas.
Unlike certain other species, such as the invasive "murder hornet" and Asian gypsy moth, these insects do not pose much of a threat. For most people, these insects will be an annoyance at most. However, vineyard managers and gardeners with young trees and saplings may find they are something of a pest as the egg-laying process can cause parts of a healthy plant to wither and die. For this reason, entomologists advise people not to start growing trees in the year before the cicadas come out.
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