Why is one of the periodical cicadas ( Magicicada septendecula ) a comparatively rare species?

ABSTRACT.1 The same woodland rarely supports two different broods of 17‐year cicadas, and never two that appear in successive years. Theoretically, this is because a build‐up of predators, generated by the leading brood, would eliminate the lagging one, or because nymphs of the lagging brood would suffer a competitive disadvantage, or both. 2 Nevertheless, substantial numbers of 17‐year cicadas of all three species ( Magicicada septendecim, M.cassini and M.septendecula ) did appear in Grant County, Kentucky, in 1974 (Brood XIV, expected) and 1975 (not expected). This is interpreted to mean that many individuals delayed their emergence until the eighteenth year. 3 Dissected eggnests show that the expected build‐up of hymenopterous egg parasitoids failed to occur, although there may have been a build‐up of dipteran parasitoids, which may leave no trace in the eggnest. 4 Excavated young nymphs show that establishment rate was poorer in 1975 than in 1974, which is attributed to nymphal competition. 5 Identification of eggnests from the two years shows that one of the species ( M.septendecula , which is usually the rarest in mixed‐species populations) is much more susceptible to delaying its development by a year than are the other two species. 6 It is predicted that the 1975 emergence will produce no adult progeny, i.e. that there will be no emergence in 1992. 7 The sporadic occurrence of this phenomenon, other factors being equal, would be sufficient to explain the comparative rarity of M. septendecula , or even its local extinction. 8 Historical records from northern Illinois show frequent emergences one year behind schedule. These delayed individuals never produce progeny that survive to the next generation. M.septendecula is absent from northern Illinois and indeed absent from the northern areas of all periodical cicada broods in the northern part of the range.