17 years later, billions of Brood V cicadas make a noisy emergence

Many parts of Ohio and West Virginia will soon see (and hear) swarms of the insects, as will patches of neighboring states.

Insect enthusiasts in Ohio and West Virginia, and several states bordering the two, are in for an exciting spring this year, because a cicada outbreak of biblical proportions is about to ensue.

Several species of the insects – Magicicada septendecim, M. cassini, and M. septendecula – that emerge from underground every 17 years are starting to come to the surface.

By May or June, billions of them will swarm Ohio and West Virginia, plus patches of southwestern Pennsylvania, western Maryland and northwestern Virginia, to mate and lay eggs. This process will last four to six weeks to the accompaniment of a deafening soundtrack of mating calls. It ends in a massive die-off of the red-eyed, 1.5-inch long insects, whose corpses cover the ground in a blanket of brown.

Though some people may find the event unpleasant, or even repulsive, these mostly harmless insects – they don’t sting or bite – are among nature’s most intriguing creatures.

The 17-year cicadas coming up now are one of several different broods of periodic cicadas – this year’s are part of Brood V – each one made up of different species with 13- or 17-year life cycles. Since development of the broods is staggered, and each one emerges in different parts of the country on different years, a cicada outbreak happens almost every year, according to the US Department of Agriculture.

The creatures spend almost their entire lives alone underground as nymphs, feeding on the sugary juices of plant roots. Their adult lives last for only several weeks, starting after cicadas emerge from the ground, climb up trees, and shed their nymphal skins.

Their brief adulthoods last just long enough for them to mate – accompanied by the notoriously loud mating calls sung made by males from morning through night – and for the females to lay hundreds of eggs inside slits they carve into the branches of trees.  

Once the insects mate and the females together lay billions of eggs, the adult cicadas die. Their eggs hatch six weeks later, and a new generation of nymphs emerges. The young cicadas crawl to the ends of their branches and drop to the ground to burrow and to disappear for 17 years, thereby starting a new cycle.

As The Christian Science Monitor has reported, some biologists predict that cicadas' long lifespan – one of the longest in the insect world – may be an evolutionary trick to help the insects evade predators that were not able extend their own lives to keep up.

They also hypothesize that the 13- and 17-year life cycles of periodic cicadas is significant.

As the Monitor reported in 2013:

Both 13 and 17 are prime numbers, that is, they are numbers that can be divided only by themselves and the number 1. This mathematical trick, say biologists, could be another strategy to avoid being eaten.

'The philosophy is that if cicadas have 12-year cycles, all the predators with two, three, four, and six-year cycles will eat them,' Mario Markus, a physicist at the Max-Planck Institute for Molecular Physiology, who led a 2001 study investigating the relationship between prime numbers and cicada cycles, told Nature magazine. 'If the cicadas mutate to 13-year cycles, they will survive.'