PRINCETON — The hum of cicadas in the summertime is one most are accustomed to. After all, one type of cicada, the annual cicada, emerges each year. However, the lore and dread surrounding the periodical cicada comes to mind every 17 years.
These periodical cicadas emerge every 17 years. Annual cicadas emerge each summer. These are sometimes known as “dog day cicadas” because of their appearance in the “dog days” of summer. Periodical cicadas emerge in May or late April, according to Dr. Doug Pfeiffer, Professor of Entomology at Virginia Tech.
“But these come out not only earlier, but in massive numbers. So every 17 years, there are huge numbers. That is an evolutionary adaptation to lower the impact of predators,” Pfieffer said. “They come out in such great numbers that predators cannot deal with it. Their most common predators are birds, skunks and opossums. In the first few days of feeding, they just get satiated and they can’t handle eating anymore.”
With their predator’s stomachs full, the periodical cicada is free to reproduce without their predators showing much interest. Their emergence every 17 years is an evolutionary defense to overwhelm their predators with just overwhelming numbers. However, this tactic of overwhelming the population sometimes has adverse effects to the human race, besides the noise and mess they leave behind.
“Cicadas, in general, lay their eggs on leaves and branches,” Pfieffer said. “Normally with the annual cicadas, it is so spread out over time that it is not really noticeable. But these periodical cicadas do all their egg-laying at one time and it can be extremely damaging for food growers and it causes a lot of injury on shade trees too.”
According to Pfieffer, there are three ways that cicadas can cause an effect on trees. The most important is egg-laying in the twigs. The female cicada has a stiff and broad depositor for laying eggs that cut into the twigs. Once they lay eggs in the twig, the rest of the twig dies.
“On the mature tree, it is not really that important, it would get trimmed off and not cause much damage. But on younger trees, this can be devastating because those small twigs are what are supposed to be the main scaffold branches later on,” Pfieffer said. “Those split twigs that were going to be limbs were killed, it upsets that whole growing pattern of the tree so it is very damaging and has long term effects on the proper yield of the tree.”
To avoid this damage, the Entomologists at Virginia Tech recommend that growers not plant fruit-bearing trees for a couple of years before the emergence. It is a difficult process to stop, so it is better to avoid it entirely.
In fact, beyond damaging limbs, the hatchlings of the cicada, nymphs, can do a lot more damage to the overall growth of the tree.
“Another form of damage that we do not really have a good quantity of knowledge on is the tree growth of the nymphs feeding on the roots for 17 years. They are feeding in massive numbers but it is a hidden effect,” Pfieffer said. “The nymphs are out of sight and out of mind and if all the trees are affected the same way, you do not really see anything. The eggs are laid in the twigs and when they hatch, the little first stage nymphs drop down to the ground and they tunnel in and find the roots and start to feed.”
The third type of damage that can be done by cicadas, to trees is a pathogen, in particular, a disease called fire blight.
“The third type of injury is allowing entry of plant pathogens,” Pfieffer said. “There is a disease of apples and pears called fire blight. The bacteria is an invader and a lot of times growers can see fireblight following the egg-laying of periodical cicadas.”
Dr. Pfieffer cited a study done in the Hudson Valley in N.Y. during the emergence of the periodical cicada. In the study, researchers spread sheets underneath apple trees in an attempt to protect the tree’s roots from the burrowing nymphs.
“These researchers put sheets under the trees so that when the nymphs dropped down, they couldn’t get down into the soil, so those research trees were protected from that generation’s feeding,” Pfieffer said. “The trees that were protected took off and they grew a lot more than the other trees, so we know it is having an effect, we just do not really have an understanding of how important that effect is. Some trees have excessive vigor anyway because they are in very fertile soil.”
Dr. Pfieffer acknowledged that a lot more studies would need to be done in order to definitively prove that the sheets helped the trees grow. However, he said that such a research project would be difficult because it is such a long-term development. After all, they could only get results every 17 years.
While more studies need to be done to find out how to protect plants and trees from the periodical emergence, humans have been dealing with cicadas for a few thousand years. For those of us who are not farmers, it is simply an annoyance every 17 years. For farmers, perhaps avoid planting certain fruit-bearing trees in the years before an emergence. For long-term health, try laying a towel or sheet under the tree to protect it from the nymphs of the cicada feeding on it for the next 17 years.
— Contact Emily Rice at firstname.lastname@example.org