Bugger! Clouds of aphids return to North Idaho

It’s ash aphid time in North Idaho.

Each autumn, for a couple weeks, the purplish aphids swarm around green ash trees, a non-native ornamental tree imported from the East Coast and used in the Rocky Mountain west to add leafy canopies to urban forests.

The green ash is no longer on the short list of urban forestry departments like it was decades ago, but because the trees are common, so are the bugs.

“There are hundreds or thousands of these trees on the landscape already,” said Idaho Department of Lands entomologist Tom Eckburg. “As long as we have the trees, we’ll have the aphids.”

Specifically, the small, sap-sucking insects are called smokey winged ash aphids and they don’t harm healthy trees, Eckberg said. The aphids follow a seasonal pattern as they move between fir and ash trees.

“They have a complicated lifestyle,” he said.

During spring and summer, the bugs live as wingless insects in the ground, feeding on the roots of firs. They emerge in autumn and swarm to ash trees to lay eggs on the bark. When new bug eggs emerge in spring, they fly to fir hosts — mostly grand fir in town — and the cycle starts again.

People notice the bugs in the fall when the smoky clouds of swarming aphids cover ash-lined streets like a purple haze.

“The masses are searching for a place to lay their eggs,” Coeur d’Alene Urban Forester Katie Kosanke said. “Sunny, mild days trigger them to fly. Typically the worst time of day for them is mid to late afternoon when it is the warmest.”

Autumn infestations are frequently reported from eastern Washington across Idaho’s Panhandle to western Montana, Eckberg said.

The insects don’t bite. They’re mostly just annoying to bicyclers and joggers — particularly when the exercise leads to open mouths.