Periodical cicadas have 13 or 17 year life cycles. Since Magicicada are “predator-foolhardy,” relying on mass numbers to satiate predators, most cicadas that emerge in the wrong year ("stragglers") are quickly annihilated by predators such as birds. Yet even with such strong natural selection against life cycle mistakes, a slight amount of plasticity in life cycle length likely remains. Since periodical cicada nymphs appear to keep track of the annual cycles of their host plants, any unusual weather events that affect vegetation may miscue them and cause an off-schedule emergence. Even if no such events occur, in extremely dense populations (e.g., 1 million per acre), if a small percentage of the individuals in the population make mistakes (for example, 1 in 100,000), then a noticeable number of individuals will emerge in an unexpected year (in this example, 10 per acre).
The exact cause, or even the prevalence, of straggling is not well understood. Straggler records have long confounded attempts to make accurate maps of Magicicada broods, which is one of the reasons the Magicicada mapping project exists. Among 17-year cicadas, straggling seems particularly common 1 or 4 years before or after an expected emergence (e.g., cicadas emerging in 13, 16, 18, and 21 years), although stragglers with other life cycle lengths have also been found. Straggling has been detected in all seven Magicicada species. Some straggling emergences are large (such as light choruses in the Chicago area in 1969 and 2003), while others consist of single individuals. There are no known biogeographic patterns of straggling.
We'd like to know more about straggling. If you have found a periodical cicada in an unexpected place in 2013, please submit a record to this site.