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Brood II: After 17 years, billions of eastern US cicadas rise again

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While the bugs have prompted people to alter wedding dates and forestall planting new, young trees until after the cicadas' festivities end, they play a number of roles in forest ecology, researchers say.

“There still are a lot of mysteries about how periodical cicadas interact in forest ecosystems,” says Dr. Liebhold, but one clear role seems be as a source of nutrients for forest soils.

Once the adults die and fall out of the trees and shrubs they occupied, they decompose. That tends to trap nutrients close to the roots, where the food is less vulnerable to leaching from the soil. This fertilizer feeds the trees and shrubs directly. It also feeds the next generation of cicadas indirectly because the nymphs that burrowed into the ground feed off fluids that move through small roots.

Researchers have found that the nymphs can have a pronounced effect on tree growth. Richard Karban, an ecologist at the University of California at Davis, has looked at tree rings as a measure of nymphs' effect on oak trees and found that the process can reduce tree growth by up to 30 percent.

When the bugs emerge from the soil during their brief appearances on the surface, they act as tiny soil aerators, adds Gene Kritsky, an entomologist at the College of Mt. Saint Joseph in Cincinnati. The tiny tunnels they burrow in the soil to escape can last for months, trapping precious rain when it falls during the summer.

The bugs also act as natural tree pruners, Dr. Kristky adds. The females lay their eggs under the bark of newly forming branches on trees. This can leave these branches vulnerable to high winds.

While such pruning can give a tree the appearance of being badly damaged, it also can spur future growth and higher yields, at least temporarily, in fruit trees.

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