06/06/2011 13:43
Cicadas set to invade state, but not quietly
Brood 19 is coming back.
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COLUMBIA, Mo. – Thirteen years ago they hatched and fell to the ground, only to burrow deep and emerge this spring.
As these creatures rise from the earth and shed their exoskeletons, people across Missouri will begin to hear the deafening sound of their collective love song.
Scientists call them Brood 19, or the Great Southern Brood.
“Some estimates indicate that in heavily wooded areas there may be more than a million cicadas per acre,” said Bruce Barrett, University of Missouri professor of entomology.
These cicadas haven’t been seen since 1998, when a particularly large population of 13- and 17-year cicadas emerged simultaneously in Missouri. This year billions of 13-year cicada offspring will emerge across 15 states, beginning in mid-May and peaking in mid-June.
Brood 19 is composed of three species of 13-year cicadas. Periodical cicadas, which belong to the genus Magicicada, are separated into broods according to species and what year they emerge. They range from 3/4 to 1.5 inches long and are distinguished from the green annual or dog-day cicadas by their black bodies and red eyes and legs.
They spend most of their lives underground, where they suck juices out of plant roots. In the spring of their 13th year, once soil temperatures stabilize around 65 degrees, these cicada nymphs tunnel to the surface and climb the nearest tree, plant, fence post or house. In wet years the nymphs sometimes build 2- to 4-inch turrets of soil for protection and then chew through the tops to emerge. They then molt their exoskeleton, frequently leaving the brown shells attached to trees.
After all that, their adult lifespan only lasts four to six weeks, when their sole goal is to mate and lay eggs. After their eggs hatch, ant-sized nymphs fall to the ground and burrow down to start the process over.
Their unusual life-cycle length and large numbers both evolved to help the species survive predators in a phenomenon called predator satiation.
“Their unique internal biological clock causes most of them to emerge within two or three days of each other in overwhelming numbers,” Barrett said. “All the natural predators in the area can feast on as many as they want without significantly reducing the population.”
While that shrill, shrieking noise may wear on human nerves, it is music to the ears of female cicadas.
“Males will form choruses or singing aggregations in trees and make their courtship calls,” Barrett said. Only males make noise, calling out for their reproductive partners in unique ways.
“If you were to flip over a male cicada, there are two large plates at the beginning of the abdomen. If you were to lift those plates up you’d see a hollowed-out area and a pair of rigid membranes called tymbals that are attached to a muscle. When the muscle contracts, the tymbal bends slightly and makes a click, and when the muscle relaxes it snaps back into place, click, and by rapid contraction you get the click-click-click that makes the cicada’s shrill noise.”
Each species has its own variation of the song. That may help them find proper mates in areas where species overlap.
Those calls begin in the morning and peak in mid-afternoon, reaching a volume of nearly 100 decibels. As temperatures cool into the evening, the cold-blooded insects slow the noise to a stop until the next day.
When females get around to laying eggs, they mean business.
They crawl up branches and use a saw-like ovipositor to cut slits in finger-sized twigs to deposit eggs.
A female deposits 24 to 28 eggs beneath the bark, and can make as many as 20 slits before going to another twig. Each female can lay 400 to 600 eggs, which remain on twigs for six to 10 weeks before hatching.
That process can damage trees and shrubs.
“The process weakens smaller stems, allowing puffs of wind to snap them off,” said Chris Starbuck, an MU Extension state specialist for woody ornamentals. “Then the tips turn brown and often remain attached, dangling downward.”
Starbuck noted that this causes trees to bush out, setting some back in development.
Cutting the tips interrupts a chemical signal that normally limits the growth of buds back along the stems. “When that happens you get multiple branches that normally wouldn’t have developed,” he said. “In the case of certain fruit trees it may actually interfere with crop production because you get vegetative growth that inhibits fruit bud formation.”
These slits in branches also can serve as an entry point for disease.
Starbuck believes the 1998 cicada broods in Missouri increased the incidence of oak wilt, a potentially lethal fungal disease. The fungus is spread by a beetle that’s attracted to the sap that comes from wounds in a tree. “It may very well be that the beetles that spread oak wilt were attracted to wounds caused during the last cicada onslaught,” Starbuck said.
Despite the annoyances to people and risks to plants that cicadas bring, Starbuck said to resist the urge to spray pesticides to control them. Those chemicals can disrupt beneficial insects like bees and allow harmful pests to gain a foothold.
“You might feel good if you’ve created a foot-deep pile of cicadas underneath your tree, but you also might have an explosion of other types of detrimental insects after that,” he said.
Small trees and shrubs can be loosely covered with spun polypropylene row-cover fabric to limit damage from cicadas.
Other pests can find opportunity in a large cicada population.
Itch mites are one pest that may benefit from the cicada invasion. They feed on cicada eggs then fall to the ground, where they can crawl onto humans and cause intensely itchy bites. One study indicated that more than 300,000 mites per tree fell per day from pin oaks in Kansas.
The return of Brood 19 might not be welcome by all, but for many, it offers an opportunity to focus on the natural world. “The outbreak is so overwhelming that it’s almost magical,” Barrett said. “When kids start asking parents, ‘What’s that noise?’ or say, ‘Mommy, look at these shells on the side of the tree,’ parents have to stop, put down their newspaper, come out of the kitchen and talk to their kids about nature.
“It’s something so unique and is really one of the marvels of nature.”
