Two Cicada Broods That Last Met Under Thomas Jefferson Returned as a Ribbon Across Illinois, Then the Bourbon Brood Followed

The map looked like a nationwide invasion. The math said it was a ribbon. In 2024, Brood XIX and Brood XIII came out of the ground in the same spring for the first time since 1803, when Thomas Jefferson was in his first term. They will not do it again until 2245. Almost nobody, outside a band of counties in central Illinois, was standing where both broods actually touched.

Periodical cicadas are not the green dog-day insects that sing every August. Those are annual cicadas, with overlapping generations, and they show up whether or not a numbered brood is due. Periodical cicadas, genus Magicicada, spend 13 or 17 years underground as nymphs feeding on tree roots, then emerge together, molt, sing, mate, lay eggs, and die in a few weeks. The adults are black, about an inch and a half long, with red eyes and clear wings. They do not sting and they do not bite in any way that matters to people or pets. Birds and mammals treat the emergence as a moving buffet. The bodies that are not eaten rot back into the soil.

John Cooley, who tracks these insects at the University of Connecticut, has the reason the double emergence is so rare pinned to arithmetic. Thirteen and 17 are prime numbers. Any specific 13-year brood and any specific 17-year brood share a year only once every 221 years, because 13 times 17 is 221. “The co-emergence of any two broods of different cycles is rare, because the cycles are both prime numbers,” Cooley said. He also offered the comparison that stuck. “This summer, some will get a chance to witness a phenomenon rarer, and probably louder, than Halley’s comet.”

Seventeen states on a map, one overlap on the ground

Brood XIII is the Northern Illinois Brood, on a 17-year clock. It is concentrated in northern Illinois, eastern Iowa, southern Wisconsin, and a few counties in northwest Indiana. It had last appeared in 2007. Brood XIX is the Great Southern Brood, on a 13-year clock, and it is the periodical brood with the widest geographic spread, through the Southeast and the lower Midwest, including Alabama, Arkansas, Georgia, Illinois, Kentucky, Louisiana, Maryland, Missouri, Mississippi, North Carolina, Oklahoma, South Carolina, Tennessee, and Virginia. It had last appeared in 2011. Together they were reported across parts of roughly 17 states. The Cicada Safari app, built with entomologist Gene Kritsky at Mount St. Joseph University, let people pin photographs to a live map so the range could be checked instead of guessed.

The overlap was not the whole map. The two broods met in a thin slice of central and eastern Illinois, including Macon, Sangamon, Livingston, and Logan counties around Springfield, with scattered reports edging into Indiana. Everywhere else, people experienced one brood at the usual density for that brood. The “trillions” figure describes the combined insects across both ranges, not a single county buried under both clocks at once. Where the ranges did touch, biologists watched for limited interbreeding. That is a scientific question about a few counties, not a science-fiction swarm covering the South and the Midwest in one sheet.

They came up when the soil about eight inches down warmed to roughly 64 degrees Fahrenheit, from late April into May, depending on latitude. Females cut slits in small branches and laid eggs in rows of about 20 to 30. One female can lay hundreds before she dies. The nymphs hatch weeks later, drop, and burrow, which is the start of the next 13 or 17 years. The singing lasted about six weeks and thinned out by late June. Young trees can be damaged by the egg slits. The Environmental Protection Agency’s practical advice was netting or mesh on saplings, not pesticides broadcast across a yard.

The Bourbon Brood, and the year the ground stayed quiet

The dual emergence was not the end of the sequence. Brood XIV, a 17-year brood sometimes called the mother of the 17-year broods, came up on schedule in the spring of 2025. William Bradford described what scholars now tie to this brood in Of Plymouth Plantation, writing of a spring in which the woods filled with large flies out of holes in the ground and a “constant yelling noise” that could “deaf the hearers.” The English colonists, he wrote, had not heard them before. That account is from 1634. The insects Bradford was struggling to name are still on the same clock.

In 2025 the clock ran through Georgia, Kentucky, Indiana, Massachusetts, North Carolina, New Jersey, New York, Ohio, Pennsylvania, Tennessee, Virginia, and West Virginia, with lighter reports into places such as southern Maryland. Kentucky was the center. Jonathan Larson, an extension entomologist at the University of Kentucky, started calling it the Bourbon Brood, a nod to the trail more than to the biology. He told people to expect the first holes when the soil hit the mid-60s, about when irises bloom, and he asked them to treat the insects as a rarity rather than a nuisance. Tennessee and Kentucky logged the heaviest observations. NBC News spent a week explaining that this was one brood, not a repeat of the 1803 pairing. Brood XIV goes back underground until 2042. Brood XIII’s next turn is 2041. Brood XIX’s is 2037.

Then came the quiet year, which is the part forecasts keep having to correct. No major 13-year or 17-year brood was scheduled for 2026. What people heard in July and August were mostly annual cicadas. Periodical stragglers, the few percent that emerge a year early or a year late, or four years off, are normal and easy to misread as a brood. Cicada Safari spent 2026 asking people to photograph those off-cycle insects anyway. Brood II, due in 2030 across parts of the East from Connecticut and New York down through New Jersey, Pennsylvania, Maryland, Virginia, North Carolina, and Georgia, produced scattered early reports. Around Baton Rouge, Brood XXII stragglers showed up a year ahead of schedule, which entomologists treated as a useful preview rather than the main event.

Baton Rouge is the next loud spring

The next full periodical emergence is Brood XXII in the spring of 2027. It is a 13-year brood, last up in 2014, concentrated in Louisiana and Mississippi and in smaller patches of Kentucky and Ohio. The three species in that group are Magicicada tredecim, Magicicada tredecassini, and Magicicada tredecula. The spring after that, 2028, belongs to Brood XXIII, another 13-year group spread across Arkansas, Illinois, Indiana, Kentucky, Louisiana, Missouri, Mississippi, and Tennessee. Two broods on the same cycle do not “double” the way XIX and XIII did. Cooley has been blunt about it. Broods that share a life-cycle length are not scheduled to co-emerge. The prime-number collision is the rare event. Same-cycle neighbors are just neighbors on a calendar.

If you missed 2024, you did not miss a permanent silence. You missed one pairing that will not recur for two centuries, and you may have missed the Bourbon Brood if you were outside its map in 2025. The next chance to stand under a true periodical chorus is smaller and farther south and west, in 2027, unless a straggler happens to pick your tree. The tracking maps live with the University of Connecticut’s cicada project and at Cicada Mania. The University of Kentucky’s naming of the 2025 emergence is at its Bourbon Brood note. More on insects, water, and everything else that does not file a press release is in The AEGIS Alliance’s Environment and Science sections.