Invasive pest threatening Toronto trees swarming in several neighbourhoods

An invasive pest species that is known for its ability to deteriorate trees has been spotted in several neighbourhoods, and here's everything you need to know.

Spongy Moth (Lymantria dispar dispar) is a pest native to Europe that causes extensive damage to trees, especially oak species. The pesky moths can kill trees in one season by feeding on foliage, which can lead to massive loss of leaves and needles and make them more vulnerable to diseases and other insects.

Throughout October, the species has already been spotted dozens of times throughout southern Ontario, with spottings regularly shared on the biodiversity social network, iNaturalist.

Sighting in Wellington County, Ont. Photo: richardlbaxter/iNaturalist.

Mature larvae can be identified by their dark-coloured, hairy bodies that are approximately 50 millimetres long, with a "double row of five pairs of blue spots, followed by a double row of six pairs of red spots, down the back," according to the provincial government. Male moths are usually light brown, while females are white and heavy-bodied.

Over the past few weeks, City staff have observed areas in Toronto where the species is especially prevalent, including Casa Loma, Moore Park, and Baby Point. While current numbers don't require a spray next spring, staff will be injecting 180 City-owned oak trees this fall with a botanical injectable pesticide called TreeAzin, to protect them from the months.

Sighting in Hamilton, Ont. Photo: gnanney251/iNaturalist.

After being treated, the pesticide kills insects that feed on the tree's leaves by preventing them from growing larger. Without preventive measures, the species population can increase drastically, and outbreaks have occurred multiple times in the past in Ontario, the last of which occurred in 2021 and was the largest on record in the province.

To prevent an outbreak, make sure to regularly check your property and trees for egg masses, making sure to inspect closely for fuzzy patches on trunks or branches. If you're dealing with a particularly rough infestation, the City recommends hiring a certified arborist to treat your trees.

Control methods vary drastically depending on the time of year, but trees should be injected from November until heavy frost sweeps in, in order to avoid other potential diseases.