Insects that emerge every 17 years in the eastern United States provide valuable nutrients to forest ecosystems when they die, an ecologist says.

The Brood X cicadas emerge from their burrows on a regular cycle, sing to attract a mate and lay eggs before dying on the forest floor.

Last spring, the insects swarmed forests, raising concerns about their effects on the ecosystem.

Scientists had noticed forests tended to have higher levels of the nutrient nitrogen in their leaves after cicadas emerged and tree growth tended to increase.

The infestation gave researchers a chance to test the impact of cicada carcasses on soil and plants.

Researchers at the University of California, Davis, studied cicada-infested areas in the U.S. in 2002, 2003 and 2004.

In an experiment, Louie Yang measured the levels of nutrients in plots with cicadas compared to control plots without extra insects.

The type of nitrogen used by plants jumped 199 per cent on average in soil littered with cicada carcasses, Yang's team found.

Populations of bacteria and fungi also increased, the researchers said in Friday's issue of the journal Science.

Plants appeared to benefit from the extra nutrients. Bellflower, a plant that commonly grows on the forest floor in cicada territory, produced seeds that were nine per cent larger.

The plants also produced leaves containing 12 per cent more nitrogen than those that grew in cicada-free soil.

The nitrogen from the cicadas came in a unique form, which allowed the researchers to trace it to the insects.