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The swarm of aphids that invaded the Toronto area has lingered longer than expected because they're an Asian species that recently infested Ontario's soybean fields, experts say.

"My prediction that they'd be gone by now might have been a bit hasty," said Chris Darling, curator of entomology at the Royal Ontario Museum. "They're not so bad anymore, but they're obviously still here."

Clouds of bugs descended on the Blue Jays game at SkyDome last night. Players rubbed their eyes while fans in the lower bowl swatted the air with blue promotional bandannas.

Home-plate umpire Tim Welke tried to stop the influx by ordering the roof closed in the third inning, around 8:30 p.m. The game wasn't delayed.

Dr. Darling said he'd assumed the aphids were a native species living on local trees.

But closer inspection revealed they're Chinese soybean aphids, Aphis glycines, which arrived in Canada this year.

They're carried on the wind to the Toronto area from soybean fields in southern Ontario, Dr. Darling said. But they can't survive in the city because they eat only soybean plants.

"They're doomed," Dr. Darling said. "But there are two million acres of soybeans in Ontario. More of them will blow in, if the wind is right."

Dr. Darling predicts the bugs will disappear in a few days or weeks, when they've completed their migratory stage.

The first clue to the bugs' origin came from Eric Richter, a sales representative for Sygenta Seed in London, Ont., where soybean plants have been infested with the leaf-munching bugs for the past two weeks.

Mr. Richter, a self-described "nature nut," said he realized the bugs were migrating after media reports that the bugs had descended on Toronto, and contacted Mr. Darling to offer an answer to the mystery.

"I was listening to the report, and I'm just howling and thinking, 'It's got to be the same aphid,' " Mr. Richter said.

"Now people in the city know what some of the farmers are dealing with out here."

The aphids, which sprout wings and ride the wind in search of additional food, began taking to the skies just last weekend, leaving in their wake damaged soybean crops and, likely, heavy losses for farmers.

"It's almost like the locust plague in Africa," Mr. Richter said. "In some fields, if you tried to put on another aphid it would fall off, there's that many."

By Mr. Richter's math, an area the size of a football field can hold about 600,000 soybean plants, and each infested plant has thousands of aphids on it, making for billions of bugs in a relatively small area.

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