Why do 17-year cicadas come out more often than every 17 years?

Feeding, Mating and Other Habits of 17-year Cicadas

Periodical cicadas, whether they are a 17-year or 13-year species, will live for just three or four weeks aboveground -- just long enough to mate and, in the case of female cicadas, deposit eggs. (Sad as this may seem, this mate-then-die cycle is quite common in the insect world.)

While cicadas don't excel at longevity as adults, they are quite good at getting noticed. Male cicadas attempt to attract females with a loud buzzing that seems to undulate up and down between two distinct notes.

Although each species of periodical cicada has its own song, the response of female cicadas varies little. Exactly one-third of one second after the male's song ceases, a female responds with a click of her wings -- but only if she wishes him to make further advances. The male counters with the same call, hoping to hear another wing click. If he does, he makes two particular sounds in quick succession, waits for a third wing click and then vibrates his forelegs as he gets busy with his new partner [source: Bug Music].

Within about 10 days, the female will deposit some 400 eggs in small twigs and branches of nearby trees and shrubs, choosing from among the 80 different species they prefer to use as a nursery. The females have protruding, needle-like devices at the end of their abdomens, which are used to create an egg cradle in the wood. Female cicadas may repeat the egg-laying process dozens of times [source: Ohio State University].

In about six weeks, white nymphs the size of ants will emerge from the eggs, drop to the ground and work their way into the soil. Like their parents, they'll spend the next 17 or 13 years feeding on fluid found in plant roots.

Although cicadas' root-feeding is considered harmless, damage caused by female cicadas burrowing into trees and shrubs to deposit eggs can be significant. Twigs may break off, which can create production issues for fruit and nut orchards. Some caretakers cover small trees with mesh cloth to prevent access to bark; others apply insecticides before egg-laying occurs or simply opt to wait out the cicadas' limited life span [source: Penn State University]. Not only are these insects generally harmless to plants, they are harmless to people, so no need to panic if one lands on you.

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