This summer, I found myself back in my home state of Iowa, where I had the privilege of witnessing the emergence of the periodical cicadas. In some areas of Iowa, the only thing that can be heard is the buzzing from an amazing number of male cicadas.
Periodical cicadas in the genus Magicicada are unique because they’re extremely long-lived and because they emerge in unison. They begin life when they’re deposited high above ground, in slits of twigs made by the females. Then they drop to the ground, eventually burrowing to the roots of the trees they were deposited on and begin to feed.
Cicadas are, in general, very slow to develop and they owe this in large part to their diets. Cicadas are in the order Hemiptera, a group of insects defined by their unique piercing-sucking mouthparts. Insects with these mouthparts are restricted to liquid diets, and have three general ways of feeding. Some hemipterans — like leafhoppers, mirids, and stink bugs — feed by destroying the plant tissues either mechanically or with enzymes, and then by sucking up the resulting mixture. Others, like aphids, feed on sugar-rich phloem. These types of insects benefit from a diet which is somewhat rich in nutrients.
Cicadas, unlike the other hemipterans mentioned, belong to a relatively small group of large insects which feed on another fluid called xylem. Xylem is one of two main fluids in plants, the other being phloem. Phloem transports sugar from the leaves to the roots, while xylem transports water and minerals from the roots to the leaves.
Xylem, unlike phloem, is nutrient poor, and insects which feed on xylem tend to be very large and develop slowly as a result. The typical cicada develops over a period of two to five years, and many species are staggered so they emerge on an annual basis. Periodical cicadas, on the other hand, take 13-17 years to develop, depending on the species. There are larger species with shorter life cycles, so many researchers believe this lifestyle to be adaptive. Both 13 and 17 are prime numbers, and it’s believed that this sort of extended life cycle would be difficult for predators to adapt to.
Another interesting thing is that they all emerge in unison, and when they emerge they are by far the most common insects in the forest. The reasons for this, like their extended life cycles, are similarly mysterious. Large emergences could allow predators to feed until they’re full, allowing the survivors to roam the habitat unharassed. Another hypothesis suggests that birds may avoid cicadas due to the volume of their calls, which could drown out the bird calls. I’ve always wondered if males drowned out the calls of competing species, crashing populations the next year and giving the Magicicada nymphs more opportunities to become established.
There’s not really any great support for the hypotheses I discussed earlier. Studies are starting to come out, and some suggest that cicada population dynamics interfere with bird population dynamics. However, it’s extremely difficult to work with animals which will only emerge five times in the course of a researcher’s life. Broods comprised of three different species are staggered all over the country, which gives researchers the opportunity to perform some types of experiments by traveling between the broods. However, this biological quirk is still very difficult for researchers to overcome.
The pictures accompanying this article were taken on my travels with friends, and give an idea of the magnitude of this event. Unfortunately, this emergence wasn’t as impressive as the one I saw when I was ten years old, but it was still quite impressive, and even during this emergence, I managed to find something just as noteworthy as the Magicicadas.
Magicicadas, like all other insects, have their own specialized pathogens. Cicadas become infected with a fungus called Massospora, which eats their abdomens from the inside out. In the early stages, the fungus is transmitted through mating attempts, and in the later stages, the cicada’s abdominal segments begin to fall off and the fungus becomes airborne. The fungus goes through two generations during each cycle of periodical cicadas. The first generation produces short-lived spores, and the second generation produces spores which hang around in the dirt for 17 years, waiting for their hosts to emerge.
I’m glad I got to witness this emergence of insects, because it’s something I will only see a few times in my life. Since the last time this emergence happened, I’ve graduated high school, almost gotten married, had a kid, gotten through both undergraduate and graduate school, and entered the workforce. Right now I am 27 years old, and the next time I see this happen, I’ll be 44. While touring this emergence, I had the opportunity to use these insects to introduce the next generation of my family to my love of insects. Unfortunately, there is evidence that some broods are declining and several broods have even become extinct. I hope that the next time these cicadas emerge, the experience will still be shared with the next generation.
Joe Ballenger is an entomologist who specializes in molecular biology and has a thing for wasps. By day, he dreams about having the opportunity to work in pest management research. By night, he loves curling up with a good review paper. Follow him on Twitter at @Stylopidae and at the Biofortified Blog.