Two notorious pests are waging an all out aerial assault on the patios and decks of Dundas and Ancaster.

Parts of the city are under siege from booming populations of inchworms and gypsy moth caterpillars. It's gotten so bad that they're chewing up foliage and dropping off trees and onto people's property (and heads).

"It's very much on our radar," said Steve Robinson, project manager of forest health with the city. "We're seeing it in pockets throughout Dundas and Ancaster."

"There's a definite increase — and it's synchronized at this point, which we've never seen before. Both pests are rising at the same time."

The inchworm (also known as a cankerworm) is native to the area and part of the local forest ecology. It goes through a natural cycle of boom and bust populations, which are often affected by weather patterns, Robinson said.

The gypsy moth, by contrast, is not native to Ontario, and was introduced in the 1960s. The city is a bit more concerned about that pest, as it doesn't really have a built-in predator here, Robinson said.

Caterpillars can 'completely decimate' tree foliage

The two populations tend to move in natural population cycles — though not one entomologists completely understand.

"This is typical of these cycles that are anywhere between eight and 12 years," Judith Myers, of the University of British Columbia's zoology department, told CBC News.

"It's very, very difficult to find what the actual factor is that causes some outbreaks to be longer than others, some declines to go lower than others."

Caterpillar infestations plague Canadian cities2:15

The city hadn't previously seen both populations grow at the same time, but Robinson theorizes it could be a sort of environmental perfect storm — like last summer's incredibly dry, hot weather, coupled with a mild winter.

Shane Waller, an arborist and nursery sales associate with Harper's Garden Centre in Ancaster, told CBC News he's had lots of customers coming in with the worms in tow, asking him about what is chewing up their trees.

"They can go after an enormous white oak tree," he said. "It completely decimates the foliage of the trees."

Dropping from the sky

So why are the caterpillars dropping out of the trees en masse right now? "They're dropping out of the trees to go pupate in the soil, and then they'll emerge in the fall as moths," Robinson said.

The cankerworm has a one-year life cycle. Once they have their fill of leaves transition to adulthood, the males will emerge as winged moths, but the females will be wingless.

Chris Deathe

Chris Deathe, a senior arborist with Davey Tree, surveys the leaves of a maple tree infested with cankerworm. (Chris Seto/CBC)

The city has sprayed a pesticide called BTK in two public parks in an effort to curb the pests' population growth. Robinson says it's a compound extracted from the soil, and isn't harmful to birds, bees or mammals. They plan to monitor the population growth of the caterpillars, and may spray more, if necessary.

Waller says you can also pick up spray bottles of BTK for home use, but that will only work on smaller trees. For bigger ones, you'd need to call in a professional.

Other options to get rid of the little buggers include pheromone traps, and tree tape, which prevents the female cankerworm from getting too high in the trees.

adam.carter@cbc.ca

With files from Nicole Mortillaro