A female cicada laying eggs on a small tree branch. John Lill/Courtesy of George Washington University hide caption
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John Lill/Courtesy of George Washington University
Gone are the noisy choruses, bug-gut-strewn sidewalks, and ice-cream shop cicada specials.
But Brood X cicadas are still with us — billions of tiny eggs lying ready to hatch in the treetops above. You can easily see the evidence all over the D.C. area: dead leaves at the ends of tree branches where the female insects have cut incisions to lay eggs.
"Driving around town, it's really obvious," says John Lill, a biology professor at George Washington University. "I'm sure people have seen it and maybe not known what it is."
The phenomenon is known as "flagging." It's especially prominent in oak and beech trees, many of which now have brown clusters of dead leaves dotting their canopies. In most trees it's completely harmless.
Clusters of dead leaves in this large oak tree indicate cicada eggs. John Lill/Courtesy of George Washington University hide caption
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John Lill/Courtesy of George Washington University
The presence of flagging is a good indicator of areas where Brood X periodical cicadas are likely to come out the next time they emerge, 17 years from now. But Lill says the absence of flagging does not mean a tree isn't hosting cicada eggs.
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"It just means that the plant is more tolerant of the wound or is able to tolerate the damage a little bit better," Lill says.
Female cicadas use a sharp, tubular organ called an ovipositor to slice open tree branches — they prefer small branches that are about as thick as a pencil. In each slice, the cicada will lay a nest of 20 to 30 eggs, then move a little ways up the branch and lay another nest. One female can lay as many as 500 eggs.
Cicada eggs take 6 to 8 weeks to develop, so Lill says, they will likely begin hatching in late July or early August. At that point tiny nymphs will drop from the treetops, burrow into the ground below in search of roots to feed on, while awaiting their next emergence in 2038.
Cicada eggs are tiny, white, and oblong. Though small, they are easily visible to the naked eye — the size of a few grains of sand.
A cicada nest. John Lill /Courtesy of George Washington University hide caption
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John Lill /Courtesy of George Washington University
Lill says cicadas are generalists, in terms of which tree species they lay their eggs in. In fact, he says he's seen cicadas trying to lay eggs in non-tree species, plants like blackberry brambles.
"There were so many of them and they were just trying lay all their eggs before they expired," Lill says.
While flagging damage is purely cosmetic on most trees, it can cause damage in small trees or vines, says Doug Pfeiffer, an entomology professor at Virginia Tech.
"If you were a fruit grower, orchardist or vineyard manager, you could be having a problem with that flagging in your fruit trees or grape vines," says Pfeiffer.
In small saplings, the pencil-diameter branches targeted by cicadas are vital parts of the tree.
"Instead of just being way out on the periphery of the canopy, those are supposed to be the weight bearing limbs of the tree when the tree grows up. So that affects the future growth of the tree," Pfeiffer says.
"I haven't seen an actual economic analysis done by an economist, but I know it can be a significant economic impact for individual growers," Pfeiffer says.
There are 15 different broods of periodical cicadas spread across the eastern U.S., emerging in cycles of either 13 or 17 years. They live most of their lives as nymphs underground, feeding on tree roots. In the spring of their 13th or 17th year, the nymphs emerge, shed their exoskeletons, spread their wings and mate in a noisy frenzy before laying eggs and dying.
In the D.C. area, reports of the Brood X emergence varied widely this year: some areas, particularly the suburbs, were overrun by cicadas, while other areas saw few if any of the insects. But Lill says it appears to have been a successful emergence.
In many cases, the patchiness of the emergence has to do with females being choosy about which habitat to lay eggs in. Lill says they appear to prefer areas with full sun exposure, especially forest edges.
"The idea is that when their offspring hatch later this summer and drop to the ground, those trees, they're banking on those trees being vigorously growing trees for the next 17 years for their offspring," Lill says. "These trees are likely to grow well because they're not light-limited."
Lill says many suburbs are ideal for cicadas because, while there are many trees, there are also lots of gaps in the tree canopy that let in sunlight. Urban parks can also be great for cicadas. But because they like the sun, you'll find more cicadas — and more flagging — in sunny edge areas, rather than dense forested parkland. So, for example, more flagging can be seen along the roadway of Rock Creek Parkway and Beach Drive than in the heavily shaded heart of Rock Creek Park.
"Essentially we humans have created the ideal habitat for these periodical cicadas," Lill says. "By fragmenting some of these forests, we've created tons of edge habitat, which is the preferred habitat for the periodical cicadas."
There is a way to help cicadas out: if you see dead branches that have fallen from trees — caused by cicada-induced flagging — don't throw out the branches. They likely contain cicadas nests, Lill says (you can look for the half-inch-long incisions). Instead, move the dead branches to a spot where they'll stay moist.
"If you relocate them to a shady spot near some woody vegetation, they should be able to hatch and dig in and start the next generation," Lill adds.
"If you want to be a friend of cicadas, that's one way to do it."
This story is from DCist.com, the local news website of WAMU.
