Question:
What is the 2024 periodical cicada "double emergence?"Answer:
Periodical cicadas emerge in 13- or 17-year cycles depending on the brood. These broods emerge in specific geographic regions. Occasionally, two broods will emerge in the same year, and more rarely, there can be some overlap in the geographic region where the two broods emerge. The 2024 double emergence involves two cicada broods (a 13-year cycle brood and a 17-year cycle brood) emerging in a similar geographic region.
Brood 13 (a 17-year cycle brood) will emerge in parts of the upper midwest, and brood 19 (a 13-year brood) will emerge in many areas of the midwest and southeast United States. There's a small number of counties in Illinois that are known to have both of these broods. So, those counties are the most likely to experience the double emergence. All other areas where either brood is known to emerge should only experience one cicada brood.
The last time these two broods emerged together was 221 years ago in 1803. That's when Thomas Jefferson was president of the United States! To see a map of the locations of the known and extant cicada broods, see this map.
