Aerial spraying program hopes to stem Dundas Valley gypsy moth outbreak

The Hamilton Conservation Authority will spray five areas of the Dundas Valley with a biological insecticide this spring to stave off a gypsy moth infestation that is threatening oak, sugar maple, beech and white pine trees.

Mike Stone, manager of watershed planning services, said a contractor will use a single-engine helicopter to target 113 hectares of forest where ground surveys found concentrations of egg masses likely to cause severe defoliation.

“We’re also concerned that it will lead to tree mortality,” he told authority directors, who approved the $30,000 spraying and a further $10,000 for followup monitoring.

The authority last sprayed the valley in two of the five areas in 2008, using the same bacterium, known as Btk. This year’s effort is expected to proceed between mid-May and early June as weather permits.

McDonell said walking and plot surveys in February found valley moth-egg concentrations of up to nearly 12 times the 5,000 per hectare that can cause severe defoliation.

The infestation is far higher than in 2017, when staff used sticky tree bands and egg scraping to try to keep the caterpillars from devouring leaves. McDonell said trees defoliated last spring managed to rebound, but it took a toll.

“It’s very stressful for the tree to lose its leaves and then have to pull those resources back up again and push the leaves out,” she said. “They need the leaves in order to make food to store for the winter just to survive.”

The five areas to be sprayed include:

• south of Powerline Road and east of Paddy Greene Road in Ancaster;

• south of Lynden Avenue and Little John Road in Dundas;

• a south-central portion from the Hermitage to the end of Old Lions Club Road in Ancaster;

• a central portion from the Merrick Orchard to near the former Maplewood Hall site in Ancaster;

• west of Sanctuary Park in Dundas. Surveys at Borer’s Falls, Tiffany Falls and Iroquoia Heights also found egg masses in spots that could cause severe defoliation, but Stone said they won’t be sprayed because the impact on the overall forest canopy isn’t expected to be as broad. He noted the city is planning a far bigger spraying program that will cover more than 2,000 hectares in Ancaster, Dundas, west Hamilton and Flamborough.

“It’s a bit of a balancing, if you will, in terms of our interest in trying to protect forest health and make sure trees that are very special are protected, but at the same time limiting our use of an insecticide,” Stone said.

“I think we’re spraying to the level that we feel is required.”