“Magicicada Brood II will make its 17-year appearance when the ground 8” down is a steady 64°F,” reports Radiolab in this excellent Cicada Tracker DIY project page. And why 17 years underground? From Scientific American:
The curious phenomenon of the cicada’s periodical life cycle is the subject of much debate among scientists, who are limited to no small extent by the infrequency of the insect’s visits to the surface. Most agree, however, that climate shifts — notably the rapid warming following the end of the last ice age — have played a role.
There are seven species of periodical cicadas in North America, four bound to a 13-year cycle, three in a 17-year cycle. All are characterized by black and orange bodies, and males woo their mates with species-specific choruses that can be deafening in large numbers.
The genetic similarity of these seven species suggests a common ancestor in the last 8,000 years. And because emergence seems closely linked to soil temperature and moisture, it is likely that climate has played a role in both regulating their life cycles and cueing their appearance.
Cicadas don’t sting or bite. After a few weeks making noise up in the trees (measured at 94 decibles), eggs will be laid and will hatch. After feeding on sap, these hatchlings will drop down to burrow and live underground, next seen in the year 2030.