What are expected emergences, and what are not?
Broods are best thought of as all the cicadas emerging in a given year in a given area on a predictable cycle.
Sometimes, cicadas emerge in places where they are not expected to emerge in a given year, but the location has a well-established record of periodical cicada emergences on a different schedule- in other words, cicadas emerge in a brood year, but within the known terrritory of a different brood. These cicadas are best thought of as off-cycle cicadas, or “stragglers.” Usually, the status of these cicadas seems straightforward- they generally occur in small numbers and they do not seem to reach densities that are self-sustaining.
Occasionally, cicadas emerge at low densities along the edge of a brood. These cicadas are ambiguous; they could represent stragglers from a different brood, or they could represent a brood in the process of expanding its range. Because of this ambiguity, it is important that records of periodical cicada emergences include some information about the density of cicadas observed.
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). Published literature (Karban et al. 2000) suggests that periodical cicadas keep track of time less by using a “clock” that keeps track of absolute time than by using a “counter” that keeps track of annual cycles. This information suggests that periodical cicadas may be sensitive to any climate shocks or perturbations that alter or disrupt the cues they normally use for synchronizing their emergences.
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), and stragglers of ±4 years occur in both 13- and 17-year cicadas (Marshall et al. 2017). 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). Significant numbers of Brood X stragglers emerged in 2017, four years before the 2021 emergence of the brood.
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
Many ask whether unexpected emergences of periodical cicadas are related to climate change. There isn’t a simple answer to this question. There is little fact-based doubt that the global 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.
Hybridization
Lloyd, Kritsky, and Simon (1983) proposed a single locus Mendelian model to explain periodical cicada life cycles, with an allele for 17 year cycles dominant to an allele for 13-year cycles**. Sota (2022) proposed a more complex polygenic model in which life cycle length is governed by variations in growth rates and the action of developmental “gates” that allow individuals to move on to the next developmental stage only if they meet or exceed certain size criteria. Both of these models predict that first-generation hybrids between co-emerging 13- and 17- year broods would have life cycles of 13 or 17 years rather than cycles of intermediate length, while subsequent generations may see increasing instances of odd life cycles. Some mathematical models, such as (Narai et al. 2011) assume that hybridization would quickly produce individuals with atypical life cycles. Whether cicadas with atypical life cycles show up quickly after hybridization or take some time to do so, all of these scenarios predict that contact zones between 13-and 17-year cicadas will experience elevated rates of straggling.
**But see Marshall (2001).
What do the data show?
Periodical cicada researchers have noticed an increase in straggler records. Does this increase support any of the hypotheses presented above?
Consider two possibilities:
- Periodical cicada straggling rates are higher now than they were in the past.
- Periodical cicada straggling rates are unchanged from the past, but with the rise of the internet, people are becoming better at identifying periodical cicadas, better at finding periodical cicada researchers, and thus stragglers are more likely to be identified and reported than in the past.
Either of these scenarios could easily explain a perceived increase in straggling, and our data do not allow us to distinguish between these explanations. To obtain baseline information for putting such events in context:
- Keep careful records of off-cycle emergences, including information about densities and species present.
- Revisit these areas in expected emergence years, collecting information about densities and species present.
- Recheck these areas often to determine whether stable off-cycle populations have formed.
More specifically:
- If straggling is a result of hybridization events between 13- and 17-year cicadas, then straggling rates in contact zones between broods of different life cycles would be elevated over rates in all other areas, including locations contact zones between broods of the same life cycles. Moreover, since the brood boundaries are not known to have changed substantially in recorded history, then straggling due to hybridization should remain constant over time.
- If straggling is a response to climate change, then straggling rates would be expected to increase over time independent of location relative to contact between 13- and 17- year cicadas, and large-scale straggling emergences should be increasingly common.