by Dennis McNair, PhD
First of all, cicadas are not locusts. Locusts are migratory grasshoppers that arise, occasionally, in very large numbers and destroy crops, hence the biblical “plague of locusts.”
In North America, the common dog day cicadas or jar flies are exceptionally large, green bugs that superficially resemble huge flies. The winged adult stage emerges annually, in July or August, especially in southern states, and males produce a loud buzz as a mating call. About 100 species of these annual cicadas exist in North America, of about 2000 cicada species that are found worldwide.
To my knowledge, all mated female cicadas lay their eggs beneath the bark of tender branches of trees and woody bushes, causing little permanent harm. When they hatch, their nymphs drop to the ground, burrow, and feed on fluids from tree roots for one or more years, depending on their species and soil temperatures (1-9 years in US species).
Lately, we’ve all heard a great deal about the emergence of two overlapping broods of the several North American regional broods of periodical cicadas, genus Magicicada occurring in North America. (Brood XIX is a 13-year brood and overlaps with Brood XIII of a 17-year brood. This co-emergence only occurs every 221 years.) Adults of a particular brood emerge almost simultaneously in early summer. Biologists estimate that there are 7 species, each with several broods, in the genus Magicicada in North America. The winged, sexually mature adults in a particular brood all emerge synchronously, so billions of adults appeared in the overlap zone this year. They all emerged within a few weeks in June. (The developmental intervals may vary slightly, depending upon soil temperatures, like other cicadas, but nymphs of periodical cicadas typically remain in the soil for 13 years in the southern part of their range and 17 years for northern broods.)
The evolutionary strategy of synchronized emergence is hypothesized to be that predators will be quickly sated, and only a very small proportion of the mating adults will be lost. Since the typical emergences are spaced so far apart, no predators specialize in eating the sexually mature adult stage. Periodical cicadas are hard to study, because adults are only available for research every 13 or 17 years in a particular locale and the subterranean nymphs are obscure. As a result, a lot of our knowledge about them is speculative.
The males of all cicadas, whether the annual (dog day) ones or periodicals, use specialized thoracic structures, called tymbals, to produce species specific calls that attract females for mating. As you can imagine – or might have witnessed – the combined “singing” of vast numbers of males causes an almost deafening din. The bothersome sound and some minor egg-laying damage to branch tips, caused when the females split bark to insert their eggs, are the only untoward consequences of periodical cicada emergences. They don’t sting or bite and cause only insignificant, temporary damage to plants. The nymphs apparently don’t harm tree roots and have few, if any, parasites. In fact, their burrowing might aerate soil and improve conditions for soil inhabitants.
The subterranean nymphs are so widely dispersed in the soil that the application of insecticides is futile, and so much would have to be applied that their use would be prohibitively expensive. Besides, such pesticides are indiscriminate, so beneficial soil inhabitants would be killed in the process. Egg-laying may cause drooping of branch tips, but no permanent harm is attributable to cicadas – even the hordes of adult Magicicada that appear every 13 or 17 years. So, the only remaining human annoyance is the often too loud noise they produce, and it’s best for us to regard that as just a quickly passing phase of love-sick males calling to their potential mates in the only way Nature has allowed. After a 13- or 17-year wait, can we blame them for being a little loud?!
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