Periodical Cicada

Background

Weaver's first Geographic article, published in July 1953, focused on the periodical cicada. The title itself captured the insect's remarkable life cycle: "Rip Van Winkle of the Underground: North America's Much Misunderstood Insect, the Periodical Cicada, Emerges After 17 Years in the Earth for a Fling in the Sun." This article marked the beginning of a prolific career. Weaver would go on to write 37 stories for The National Geographic, mostly within the Science beat. His work was widely viewed, as the majority of Geographic subscribers primarily opened the magazine to peruse the pictures. The cicada article was his first contribution to that visual and narrative tradition.

Predator satiation survival strategy

Periodical cicada nymphs emerge in massive numbers, sometimes over 1.5 million per acre, at nearly the same time. This synchronized emergence is an adaptation called predator satiation: despite being easy prey for many animals, so many cicadas appear that predators cannot consume them all, leaving enough to reproduce. One hypothesis suggests that their prime-numbered development times of 13 and 17 years help avoid predators with shorter reproductive cycles.

For instance, a predator with a three-year cycle would not have its reproductive peak align with the cicadas' emergence, reducing predation pressure. Another hypothesis proposes that prime-numbered intervals prevent hybridization between broods, preserving the long-development trait under harsh conditions like Pleistocene glaciations. Mathematical modeling supports this idea. The genetic control of cycle length is thought to involve a single gene locus with 13-year dominance over 17-year, but this remains unconfirmed at the DNA level.

Impact on other populations

Cicada population cycles significantly affect other species. Tree growth declines the year before emergence due to increased root feeding by nymphs. Moles, which eat nymphs, thrive in the pre-emergence year but decline afterward when food becomes scarce. Wild turkey populations benefit from gorging on adult cicadas at the end of their life cycle. Decomposing cicada carcasses provide a nutrient pulse to forest communities. However, cicada broods can also have negative effects: eastern gray squirrel populations suffer because female cicadas damage upcoming mast crops through their egg-laying activity.

Broods

Periodical cicadas are grouped into geographic broods based on emergence year. In 1907, entomologist Charles Lester Marlatt assigned Roman numerals to 30 broods: 17 with a 17-year cycle (I–XVII) and 13 with a 13-year cycle (XVIII–XXX). He noted 17-year broods are generally more northerly. Many of these broods have not been observed; Broods XI and XXI are extinct.

Marlatt attributed disappearances to forest reduction and introduced house sparrows. His numbering scheme is retained for convenience, though only 15 broods survive. Cicadas emerging outside the expected time are called stragglers, often appearing one or four years early or late. Brood XIII (largest emergence by size) and Brood XIX (largest by geographic extent) were expected to emerge together in 2024 for the first time since 1803, but only potentially overlapping in a thin area of central and eastern Illinois. Their next dual emergence will be in 2245.