When is a 2012 Magicicada not a Brood I cicada? When it’s a straggler.
A straggler is a periodical cicada that emerges in a year before or after the year they are supposed to emerge. Typically a straggler will emerge one or four years before, or one year after the year they should have emerged. Stragglers from Broods II (due 2013), Brood V (due 2016) and Brood XIX (backin 2011) are or will emerge this year in limited numbers.
Brood II is set to emerge next year in most of central Virginia (as well as CT, MD, NC, NJ, NY, PA), Brood V will emerge in four years in Virginia and West Virginia (as well as OH, PA), and Brood XIX emerged last year in a few areas of Virginia (as well as AL, AR, GA, IL, IN, KY, MO, MS, NC, OK, SC, TN).
Stragglers present a challenge for people tracking the Brood I emergence because Brood II, Brood V and Brood XIX stragglers will emerge in the same states as Brood I cicadas. Brood II and Brood V overlap Brood I in some places.
Here is a comparison of the I,II & V Broods. The black dots represent where the cicadas have emerged historically.
Here’s a map of Brood XIX in case you are curious: