Stragglers

Sometimes periodical cicadas emerge in places where they are not expected– cicadas that emerge off-cycle are known as “stragglers”.  Stragglers may appear before or after their brood is expected to emerge, and they may appear almost any number of years off-cycle, though increments of 1 and 4 years seem most common.  For example, significant numbers of Brood X stragglers emerged in 2017, four years before the 2021 emergence of the brood.

Many ask whether unexpected emergences of periodical cicadas are related to climate change. There isn’t a simple answer to this question.

Periodical cicadas and long-term climate patterns

Prevailing theories suggest that long-term climate patterns have played a significant role in shaping periodical cicada cycles, species, and broods. For example, the cyclic emergences of periodical cicadas have been tied to glaciation (Yoshimura 1997), “climate shocks” have been invoked to explain the relatively recent formation of 13-year Magicicada neotredecim from a 17-year ancestor (Marshall and Cooley 2000), and the spatiotemporal mosaic of the broods has been attributed to climate variations (Alexander and Moore 1962; Lloyd and Dybas 1966). Moreover, periodical cicadas do not have internal timekeeping “clocks” for their emergences; rather, they have been shown to keep track of and respond to environmental cues (Karban et al. 2000). Taken together, this information suggests that periodical cicadas are sensitive to climate and that they are expected to respond in some way to climate changes or perturbations.

Periodical cicada “stragglers” or unexpected emergences

Periodical cicadas are known for their predictable timing, although it is not uncommon for cicadas to make counting mistakes and emerge in a year when they are otherwise unexpected; after all, given population densities of up to several million per acre, if only a small percentage of cicadas miscount, then noticeable numbers will emerge in “off” years. Cicadas emerging off-cycle are referred to in the scientific literature as “stragglers;” the term has less to do with lateness, since stragglers may appear either earlier or later than expected, than it has to do with the fact that stragglers are typically few in number, separated in time from the main bodies of their broods. Straggler emergences in a given area are generally followed by normal emergences on the expected schedule. Stragglers making errors of 1 or 4 years seem most common, though other increments have also been observed (Lloyd and Dybas 1966; Dybas 1969; Lloyd and White 1976; Simon and Lloyd 1982; Maier 1985; Kritsky 1988; Kritsky and Young 1992; Marshall et al. 2011). Most stragglers are few in number and are quickly annihilated by predators, but there are notable exceptions, including Dybas and Lloyd’s 1969 record (Dybas 1969) of a periodical cicada emergence that included choruses in Chicago suburbs along Salt Creek, four years before the emergence of Brood XIII. The same general area has also had four-year early choruses in subsequent years (e.g., 2003; Cooley et al. 2016).

Selective forces operating on periodical cicada broods

Theories concerning the origins and maintenance of the relatively strict timing of periodical cicada broods rely on the concepts of “predator satiation” and “competitive exclusion” (White and Lloyd 1979; Lloyd and White 1980; Karban 1982a, b; Williams and Simon 1995). Predator satiation is a strategy in which organisms appear suddenly in such great densities that they overwhelm their predators’ ability to consume them; in the case of periodical cicadas, the cicadas also disappear so suddenly and for such extended periods that predator populations are unable to respond numerically (e.g., by adding more predators). For periodical cicadas, competitive exclusion relates to the possibility that cicadas compete for some limiting resource, either as nymphs or adults, and that resource limitations make it difficult or impossible for more than one brood to exist in any given area. Any understanding of off-cycle emergences must account for these modes of selection or suggest better alternatives.

Periodical cicada stragglers and brood formation

Straggling—or some related phenomenon—lies at the heart of the complex spatial and temporal arrangement of periodical cicada broods. Broods give rise to other broods through a process involving off-cycle emergences, which then persist on a new schedule (Alexander and Moore 1962; Lloyd and Dybas 1966). This brood of periodical cicada brood formation does not explicitly address the possibility that brood boundaries may be somewhat dynamic (Kritsky 1987); and that one brood may gradually replace another over time via a process involving successive waves of straggling and the persistence of straggler populations (Lloyd and White 1976; Kritsky 1988; Kritsky and Young 1992). It is a fair—and almost unresolvable—question whether straggling events that occur in the same areas in multiple generations represent new instances of straggling or are the offspring of successfully reproducing stragglers from previous cicada generations. For periodical cicada populations to persist, they must be of sufficient densities to satiate predators; put another way, a “quorum,” not a majority, must adopt a new emergence schedule if it is to persist as a nascent brood. At the same time, if cicadas do compete for resources, then the parent brood and its child brood may not able to coexist stably. An exception may be the case of “shadow brooding,” in which straggler populations are not self-maintaining, but they are continuously replenished by new stragglers before they can be driven extinct (Lloyd and White 1976; Marshall 2001; Cooley et al. 2011). It is possible that such “shadow broods” eventually reach a tipping point where they are able to persist and perhaps even replace their parent broods.

Periodical cicada maps and reporting bias

Periodical cicada maps, such as Marlatt’s (Marlatt 1923), are important sources of information about periodical cicada broods, but they have a number of inaccuracies or omissions related to biases in the way their underlying data were collected and compiled (Marshall 2001). In particular, surveys have revealed the existence of previously unreported disjunct populations (Simon and Lloyd 1982); most recently, internet-based “crowdsourcing” efforts have been key to identifying such populations (Cooley et al. 2015; Cooley 2015).

Climate Change

There is little fact-based doubt that the climate is warming and will continue to do so, with a number of effects, including increased likelihood of severe storms, disruptions of agriculture, and changes to growing season. One of the most informative figures predicting the effects of a warming climate is Fig. 4 in this document, from a publication formerly found at www.climatescience.gov. Periodical cicadas are sensitive to environmental and climatic cues, and the climate is warming. Periodical cicadas may respond to this warming by coming out in advance of their predicted emergence times. There are other possible explanations, but the link between climate change and widespread periodical cicada straggling events is a solid working hypothesis. If indeed unexpected emergences are related to climate change and are not simply a fluke, then other large-scale straggling emergences associated with other 17-year broods are to be expected.

The Future

There is no simple answer to the question of whether climate chance is disrupting periodical cicada cycles. An effective way to resolve questions about unexpected emergences of periodical cicadas is to keep careful records of where cicadas emerge off cycle, noting the densities of their emergences. Then, the same locations should be checked again, and densities during expected brood emergence years should be compared to the densities of unexpected emergences. More long-term, these areas should be checked repeatedly to determine whether stable, off-cycle populations have formed.  Finally, if indeed well-known straggler emergences (such as in 2017) are related to climate change and are not simply a fluke, then other large-scale straggling emergences associated with other broods are to be expected.

References

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