A low-flying plane buzzing west Bismarck neighborhoods was not attacking the city - it was protecting the area's trees from an insect invasion.

The plane, out early Tuesday morning, was spraying for cankerworm larvae, which eat the leaves of several tree species in the Bismarck area, including elm, box elder, apple, and Canada red cherry.

Jackson Bird, the city forester, said the cankerworms defoliated trees around Dakota Zoo, the wastewater treatment plant and other parts of Bismarck last year. Having their leaves eaten, coupled with the recent years of drought, was a worrisome stressor on the city's trees, Bird said.

The plane sprayed the trees with bacillus thuringiensis, a nontoxic and naturally occurring bacteria, Bird said. He said the spray is safe to humans, pets and other animals.

The spray was applied to treed areas west of Washington Street and from Burleigh Avenue to Pioneer Park, primarily in the Missouri river bottom area.

The forestry department had to wait to spray until the trees had leaves on them and caterpillars hatched, Bird said. He said once those things happened, the department had to wait out the weekend's wind and rain to apply the chemical.

The city notified residents of its intent to spray through media outlets, but some people still didn't know it was going to happen, Bird said.

"We tried to tell folks," he said.

Bismarck Police Lt. Randy Ziegler said police received at least five calls early Tuesday morning concerning the low-flying plane.

"This day and age, if you have planes flying low, it catches people's attention," Ziegler said.

(Reach reporter Jenny Michael at 250-8225 or jenny.michael@;bismarcktribune.com.)