Marlatt’s map of Brood VI (Marlatt 1923) has always been problematic since it suggests that populations of this brood are present across the general range of periodical cicadas. Marlatt’s map shows populations scattered over eastern North America, including extensive areas of emergence in the Philadelphia-New York corridor and in the Midwest. Some Marlatt records, such as those in Wisconsin, are supported by museum specimens (University of Wisconsin-Madison), though as with many older museum collections, specific information about date and locality of collection is lacking. Simon’s (Simon 1988) revised map of Brood VI shows all but the “core” populations in the mountains of North Carolina, South Carolina, and Georgia as doubtful.

In the map below, cicada symbols are verified records in our database as of February 2021. Gold symbols are from Simon (1988); smaller symbols are records with a lower degree of certainty, and crosses represent records that are considered spurious. Blue symbols are from Marlatt (1923); smaller symbols are records with a lower degree of certainty, and question marks represent records that are considered spurious. Symbols are in layered in the order Database, Simon, Marlatt, and symbols in the upper layers may obscure symbols in lower layers. This map may not be reproduced without written permission.

Marshall (2001) noted that many Brood VI records may be the products of biases in earlier mapping efforts; some biases have to do with the ways that records were compiled over generations, while others have to do with varying degrees of effort to search for emergences. A key source of bias is the practice of assigning records to broods on the basis of expectation and timing, a problem compounded by a lack of emergence density data associated with such records. Periodical cicadas have a propensity to emerge off-schedule or “straggle” in increments of four years (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). When broods in close physical proximity (such as II, VI, X, and XIV; Cooley et al. 2009, Cooley et al. 2011, Cooley et al. 2015) are offset by increments of 4 years, the brood membership of low-density populations or populations along the periphery of any one brood can be uncertain. Emerging cicadas could belong to the brood emerging in a particular year, or they could be a “shadow brood” –stragglers from the temporally and spatially related brood; moreover, such populations could be self-sustaining, or they could persist only because they are continually replenished by more stragglers (Lloyd and White 1976, Marshall 2001, Cooley et al. 2011). Shadow brooding seems to be the best explanation for records of “Brood XV” which, if it existed, would emerge four years before Brood II (Lloyd and White 1976, Simon and Lloyd 1982, Cooley et al. 2011). Maier’s (Maier 1985) reports of cicadas in Connecticut emerging on the same schedule as Brood VI may have a related explanation, as they emerged four years after Brood II in the same areas. Because there are so many possibilities for shadow brooding involving Brood VI, many of the records in Marlatt (Marlatt 1923) attributed to Brood VI may actually belong to other broods –they could just be “shadows” of them (Marshall, 2001). Other records, such as those in Wisconsin, are not easily explainable as belonging to other broods, because they are nowhere near likely sources of stragglers (e.g., II, X, or XIV). Thus, the temporal and spatial context of Brood VI makes it especially challenging to map.

Brood VI is one of the few broods of periodical cicadas not yet mapped using GPS technology. Marshall and Cooley (unpublished data) mapped the main body of the brood in 2000, marking paper maps and later transcribing them:

Detailed, georeferenced maps are the key to understanding the evolutionary relationships of periodical cicada broods (Cooley et al. 2009). 2017 provides an opportunity to revisit the map of Brood VI using newer technologies (Cooley 2015). Mapping –and being ready to investigate disjunct populations, newly discovered for Broods I (Cooley 2015), II (Cooley et al. 2015), and most recently, V –is made more challenging by the possibility that Brood VI’s boundaries may be unusually dynamic (Kritsky 1987); interactions between neighboring broods offset by increments of four years may create the conditions in which one brood may replace another over time (Lloyd and White 1976, Kritsky 1988, Kritsky and Young 1992).

In order to resolve some of these outstanding questions concerning Brood VI, we will be mapping the emergence, taking special care to note population densities (Marshall 2001, Cooley et al. 2011, Cooley et al. 2013) and to investigate areas where Brood II is in close proximity to other broods.

Literature

Cooley, J. R. 2015. The distribution of periodical cicada (Magicicada) Brood I in 2012, with previously unreported disjunct populations (Hemiptera: Cicadidae). The American Entomologist 61:52-57.

Cooley, J. R., G. Kritsky, M. D. Edwards, J. D. Zyla, D. C. Marshall, K. B. R. Hill, G. J. Bunker, M. L. Neckermann, and C. Simon. 2011. Periodical cicadas (Magicicada spp.): The distribution of Broods XIV in 2008 and “XV” in 2009. The American Entomologist 57:144-151.

Cooley, J. R., G. Kritsky, J. D. Zyla, M. J. Edwards, C. Simon, D. C. Marshall, K. B. R. Hill, and R. Krauss. 2009. The distribution of periodical cicada Brood X. The American Entomologist 55:106-112.

Cooley, J. R., D. C. Marshall, A. F. Richards, R. D. Alexander, M. D. Irwin, J. R. Coelho, and C. Simon. 2013. The distribution of periodical cicada Brood III in 1997, with special emphasis on Illinois (Hemiptera: Magicicada spp.). The American Entomologist 59:9-14.

Cooley, J. R., C. Simon, C. Maier, D. C. Marshall, J. Yoshimura, S. M. Chiswell, M. D. Edwards, C. W. Holliday, R. Grantham, J. D. Zyla, R. L. Sanders, M. L. Neckermann, and G. J. Bunker. 2015. The distribution of periodical cicada (Magicicada) Brood II in 2013: Disjunct emergences suggest complex origins. The American Entomologist 61:245-251.

Dybas, H. S. 1969. The 17-year cicada: A four year mistake? Bulletin of the Field Museum of Natural History 40:10-12.

Kritsky, G. 1987. An historical analysis of periodical cicadas in Indiana (Homoptera: Cicadidae). Proceedings of the Indiana Academy of Sciences 97:295-322.

Kritsky, G. 1988. The 1987 emergence of the Periodical Cicada (Homoptera: Cicadidae: Magicicada spp.: Brood X) in Ohio. Ohio Journal Of Science 88:168-170.

Kritsky, G., and F. N. Young. 1992. Observations on periodical cicadas (Brood XIV) in Indiana in 1991 (Homoptera: Cicadidae). Proceedings of the Indiana Academy of Sciences 101:59-61.

Lloyd, M., and H. S. Dybas. 1966. The periodical cicada problem. II. Evolution. Evolution 20:466-505.

Lloyd, M., and J. A. White. 1976. Sympatry of periodical cicada broods and the hypothetical four-year acceleration. Evolution 30:786-801.

Maier, C. 1985. Brood VI of 17-year periodical cicadas, Magicicada spp. (Hemiptera: Homoptera: Cicadidae): New evidence from Connecticut (USA), the hypothetical 4-year deceleration, and the status of the brood. Journal of the New York Entomological Society 93:1019-1026.

Marlatt, C. L. 1923. The Periodical Cicada. United States Department of Agriculture, Bureau of Entomology Bulletin 71:1-183.

Marshall, D. C. 2001. Periodical cicada (Homoptera: Cicadidae) life-cycle variations, the historical emergence record, and the geographic stability of brood distributions. Annals Of The Entomological Society Of America 94:386-399.

Marshall, D. C., J. R. Cooley, and K. B. R. Hill. 2011. Developmental plasticity in Magicicada: Thirteen year cicadas emerging in seventeen and twenty-one years (Hemiptera: Cicadidae). Annals Of The Entomological Society Of America 104:443-450.

Simon, C. 1988. Evolution of 13- and 17-year periodical cicadas. Bulletin of the Entomological Society of America 34:163-176.

Simon, C., and M. Lloyd. 1982. Disjunct synchronic population of 17-year periodical cicadas: Relicts or evidence of polyphyly? Journal of the New York Entomological Society 90:275-301.