Not all cicadas appear on the strict schedule. There are also some cicadas, known as stragglers that appear a year later or earlier than the rest of their brood. One-year early or one-year late is the most common time-frame for stragglers, but this isn't always the case. In 2000, for example, many cicadas appeared four years earlier than their brood X schedule. Some observers theorize unseasonably warm weather may cause some stragglers to emerge early; others suggest delayed development may cause some stragglers to arrive late.
Scientists are not exactly sure how cicadas know 17 years or 13 years has passed in the first place. They think it might have something to do with some property in xylem, the fluid they suck out of tree roots for nourishment when they are underground, that reflects the yearly cycle of the tree. Perhaps if the xylem signal is weaker for insects under one type of tree versus another, that will exacerbate the off-schedule appearance. Interestingly, the prime number appearances are thought to help with survival; since the cicadas are not on the same two-year or five-year cycle as their predators, the predators don't become too dependent on them for food (though they do enjoy eating them to a point when they are around) [source: Hayes]. Unfortunately, stragglers who miss the big cicada party more likely to be eaten off as their quantities are smaller.
Even if scientists don't know why stragglers exist, most agree on one thing: Stragglers make it difficult to create accurate emergence maps. Historical accounts of stragglers, because they can range from a single periodical cicada to just a few dozen -- instead of thousands -- are nearly non-existent. To date, scientists and other observers have not been able to pinpoint geographic patterns of stragglers [sources: University of Michigan Museum of Zoology, Magicicada, Cicada Mania].