UpdatedFri, Apr 16, 2021 at 8:34 am ET Reply Billions of 17-year periodical cicadas will emerge later this spring in parts of 15 Eastern and Midwest states, including Pennsylvania. Researchers want citizen scientists to use the Cicada Safari app and help with data collection. (Scott Olson/Getty Images, File) PENNSYLVANIA — Residents in several Pennsylvania counties have a chance to be a part of an ecological phenomenon that only occurs once every 17 years. An unprecedented citizen science effort gas launched nationwide, as parts of 15 states will see billions of Brood X 17-year cicadas emerge in May or June. The public is invited to help scholars track and better understand this extraordinary natural display.