It’s Science Wednesday! If you live on the east coast of the United States and have been outside recently, you may have noticed a peculiar sight and sound in the air – loads and loads of male cicadas are singing their siren songs through vibrations of their tymbals, a pair of rigid drum-like membranes on their abdomens, to attract mates for a very brief period of time. When synchronized, the calls that the cicadas make can reach 80 – 100 decibels – that’s like hearing a lawnmower or chainsaw! Thankfully the calls are made during daylight hours.
Back in 2004, when Facebook existed only at Harvard University, billions of “Brood X” newly hatched cicada nymphs fell from their homes in twigs and burrowed in the dirt, feeding on fluids from grass roots. As they grew, they tunneled deeper down, finding tree roots to suckle – for 17 years! And now they are emerging – up to 1.4 million cicadas per acre – and molting into adult form. As the male cicadas emerge earlier than the females, they bear the brunt of the early feeding frenzy by predators like birds, spiders, and even cats. But this male sacrifice helps ensure the survival of more females and thus, more offspring. After mating, females lay eggs – about 500 at a time – in fissures in twigs. Despite the casualties during emergence, the sheer number of cicadas in these broods ensures survival, as simultaneous emergence overwhelms the predators.
Though cicadas may remind you of grasshoppers, they’re an entirely different order of insects. They do not swarm, they do not fly well, and they typically don’t travel more than several hundred feet. They pose little threat to plants because they don’t eat plant tissues. They pose no threat to humans. While nearly 3400 species of cicadas exist worldwide, periodical (versus annual) cicadas that emerge en masse like this are unique to the eastern US. There are actually three species of 17-year cicadas—Magicicada septendecim, M. cassinii and M. septendecula—that form mixed-species cohorts called broods whose members arise like clockwork on the same schedule. The broods are identified by Roman numerals, with Brood X (10) being the largest of twelve.
These cicadas are actually quite beneficial from the ecological standpoint for multiple reasons: (1) their emergence tunnels naturally aerate the soil; (2) they provide food, having a positive impact on predator populations; (3) female egg-laying in the fissures of twigs in trees is a natural “pruning” that results in the trees producing more flowers and fruit the following year; and (4) after the cicadas die, their decaying bodies provide nitrogen and other nutrients to the soil.
Climate change over the past 20,000 years, combined with the cicadas’ ability to periodically shift their life cycle to emerge four years early if conditions are favorable (or four years later if conditions are difficult), is thought to have driven the formation of multiple broods across the eastern US. But the 17-year cycle? Scientists still aren’t quite sure why they emerge after that time period. Dr. Chris Simon (University of Connecticut, Department of Ecology and Evolutionary Biology) says that “my colleagues and I hypothesize a molecular clock in periodic cicadas that somehow keeps track of the years. That is what we’re looking for now. We suspect it is tied to yearly cycles of the trees they are feeding upon.”
As for when they start emerging when the correct time has passed, this depends on the ground temperature reaching between 64 – 65 degrees Fahrenheit (18 C).
If you live in the eastern US and want to contribute to citizen science on the emergence of Brood X cicadas, check this out:
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photo: Patricia Chapman Meder