There has been discussion in the news and in social media this week of two broods of periodical cicada this year. Is this true, and how does that work? The answer to the first question is a qualified yes, but first I’ll give some background.
Cicadas are sucking insects that feed on xylem sap of various tree species. Annual cicadas emerge every year (though taking more than one year to develop feeding on tree roots). Periodical cicadas have a synchronized development, feeding on roots for 17 years (usually). A few broods have a 13-year cycle, mainly in more southern areas. When adults emerge, they do so in staggering numbers. They make remarkable “chorusing centers”, where males make a shrill singing sound, attracting both males and females. After mating, females use their sharp ovipositors to insert eggs into twigs of about pencil diameter. This is the source of economic injury – the area beyond the oviposition scar dies, creating a brown flagging of twigs. This is extremely damaging in a young orchard or vineyard.
There are about 12 17-year broods, and 3 13-year broods. These have different, but often overlapping geographical ranges. Some of these are widely spread, others sharply limited. The following map from the Forest Service, shows the distribution of periodical cicada broods, color coded. Because each county is assigned to only one brood, the degree of overlap of broods is sometimes vague, but there is some idea given of the range. Brood X is the most important to fruit producers in the mid-Atlantic region, but its range is somewhat obscured by coloration for other broods.
The first person known to have figured out the 17-year cycle of periodical cicada was Benjamin Banneker, a free African American living in Maryland. After observing outbreaks on his land in 1749, 1766 and 1783, he predicted an outbreak in 1800 (McGreevy 2021). That reference refers to Banneker working with Brood X of periodical cicada. He would not have known that term, because the recognition of the different geographical broods in different years, and the codifying these with Roman numerals was made by Charles Marlatt (Marlatt 1907), chief entomologist at USDA, and who also began some of the first regulations protecting the US from invasive pests.
In 2024, we can expect emergence of Brood XIX in Virginia (light blue in Fig. 1). But while Broods IX and X are the major broods affecting Virginia’s fruit production, the range of Brood XIX is limited here. It is nicknamed the Great Southern Brood because of the broad swath through the Deep South. But Virginia it is limited mainly to Southside (Hanover, Charles City, Prince George, Brunswick, Halifax and Wise Counties). In our Commercial Spray Bulletin for Commercial Tree Fruit Growers (VCE 2023), we have a map of expected emergence years of adult periodical cicadas, by county, for Virginia and West Virginia (see map). That map uses circles to indicate 13-year broods; all others are years for 17-year emergence.
Now for the explanation of the “qualified yes” regarding the double emergence of cicadas – emergence of more than one brood at a time. This can only happen rarely, when 17 year and 13 year cycles eventually meet in time. This year, 2024, we expect emergence of the 17-year XIII and the 13-year XIX. However, Brood XIII is nicknamed the Northern Illinois Brood for a reason (see Fig. 1). It will occur nowhere near Virginia! So this year, Virginians in Southside (and perhaps extreme southwestern Virginia) can expect to see periodical cicadas. Ecologically, this dual occurrence poses an interesting possibility – the chance for genetic exchange between broods that see each other only every few hundred years!
References
Marlatt, C. L. 1907. The periodical cicada. U. S. Dept. Agric. Bur. Entomol. Bull. 71: 1-181.
McGreevy, N. 2021. Meet Benjamin Banneker, the Black scientist who documented Brood X cicadas in the late 1700s, Smithsonian Magazine. (https://www.smithsonianmag.com/smart-news/meet-benjamin-banneker-black-scientist-who-documented-brood-x-cicadas-late-1700s-180977676/)
VCE. 2023. 2023 Spray Bulletin for Commercial Tree Fruit Growers. Va. Coop. Ext. Publ. 456-419. 180 p.
