Gypsy Moth

The European Gypsy Moth (Lymantria dispar dispar) is an invasive forest pest that was introduced accidentally in the United States in 1869. Since then, the Gypsy Moth has expanded its range over much of the eastern United States and Canada. It was first detected in Ontario in 1969 and has quickly spread across southern Ontario over the next decade. It is now well established throughout southern Ontario and as far north as Sault Ste. Marie.

How we are controlling Gypsy Moth

Gypsy Moth surveying has been underway since the winter of 2016 with the completion of egg mass surveys. An increase in egg masses suggests a coming season of increased pest activity.

The monitoring initiated in late 2017 identified approximately 2,000 hectares that have exceeded an action threshold of 2,500 egg masses per hectare, and require treatment.

In the spring of 2018, the City of Hamilton will be conducting an aerial spray program to control Gypsy Moth with the use of a bio-pesticide: Foray 48B Biological Insecticide Aqueous Suspension, containing active ingredient Bacillus thuringiensis, subsp. Kurstaki strain ABTS-351, (PCP# 24977) under the Pest control products Act (Canada). As indicated in the map, the locations for the application of Btk via helicopter include Ancaster, Dundas, portions of Flamborough, West Hamilton, and the Hamilton Mountain.

When will the City be spraying?

The spray will occur in the month of May. Exact dates and times for the spray are weather dependent. The City will share up to date details on this page and via @CityofHamilton on Twitter. 

For further information, call 905-546-2489. If you live in the vicinty but the number is long distance, you can call 1-877-446-2424 toll free.

Marking trees

The City of Hamilton has marked trees that are monitored as part of the Gypsy Moth aerial spray program. These trees have pink ribbons, a pink spray painted dot, and some smaller white paint dots. Please do not disturb these trees. The data being collected is very important for determining when the aerial spray program will be most successful at targeting Gypsy Moths.

Tree marked for the Gyspy Moth Aerial Spray Program

Gypsy Moth Treatment Area

To find out if you live in the treatment area, enter an address or intersection into the search box. If your search result appears in the pink highlighted boundary, you are located in the aerial spray zone.


Once the spray program has been completed this year, the Forestry Section will be conducting follow up surveys of the spray area, as well as areas outside of the spray area to verify that the gypsy moth population has decreased below threshold levels. If it has been determined that action thresholds have been reached in other areas, the Forestry Section will address it the following year.

Gypsy moth larvae or caterpillars will feed on tree leaves. If the larvae population is high, they can defoliate whole trees and forests in a short amount of time. Gypsy moth prefer oak trees, but will feed on a variety of hardwood tree species. Under normal circumstances, defoliation caused by gypsy moth won’t kill a tree. However, trees can decline to the point of death in some cases when defoliation is coupled with dry hot summers, or impacted by other forest pests like Spring or Fall Cankerworm.

Bacillus thuringiensis ‘kurstaki’ (Btk) is a soil-borne bacteria that is applied to the leaves of affected trees while caterpillars are in their early instar stage (immature). Once ingested, the bacterium disrupts the caterpillars’ digestive system with cessation of eating within 24-48 hours. Within days, caterpillars that have ingested Btk will succumb to its effects. Btk has very low residual qualities in the natural environment. Sunlight and fungi deteriorate the bio-pesticide within 1-4 days. Because Btk requires an alkaline environment in the gut in order to be effective, it does not have any detrimental effects to humans, birds, or bees. Btk will affect other caterpillar species (known as non-target species). Due to its low residual and the narrow spray window due to larval development, the non-target impact is expected to be low. Conservative and measured application will be undertaken with the goal of reducing the population below the 2,500 egg masses per hectare. This program is not intended to eradicate the pest entirely.

Close up of Gypsy Moth Larva

Figure 1: Late instar gypsy moth larva

Close up of 3 Gypsy Moths

Figure 2: Female gypsy moths laying eggs

Gypsy moths spend the winter as partially developed larvae in eggs. The eggs hatch in the spring and the young larvae begin feeding by cutting small holes in the surface of leaves. As the larvae develop, they feed on the edge of leaves. Larvae have five pairs of blue and six pairs of red spots along their backs. Feeding is normally completed by early to mid-July.

Pupation occurs in a cocoon which can be found on a variety of surfaces including trees, rocks, houses, boats, trailers, fences, picnic tables, and firewood. In 13 to 17 days, the moths emerge. Both sexes have wings, but only the males can fly. The male moth is dark brown to beige, medium-sized, and is a very erratic flyer. The female is mostly white and has a wingspan between 60 to 70 mm. The female lays eggs in masses of 100 to 1000 on tree trunks, branches, houses, fences, etc. as well as under rocks and forest floor debris. The eggs are covered with fine brown hairs from the female’s abdomen. The egg masses will remain all winter and caterpillars will hatch in the spring, from late April to mid-May.

Homeowners are encouraged to monitor their hardwood trees like oaks, maples, beech and walnut for the presence of Gypsy Moth larvae. How you can help save trees:

  • Install Sticky bands to monitor and control caterpillar populations
  • Install Pheromone Traps in trees to lure and trap male moths to prevent them from mating with female moths
  • Contact an Arborist if you think a tree on your property is being heavily defoliated by Gypsy Moth
  • Contact us at 905-546-2489 if you think a tree on city property is being heavily defoliated by Gypsy Moth
  • Water your tree and trees near your home in the public right-of-way once every 3 to 5 days during periods of high heat or drought

Consult your local Nursery or Hardware store for Sticky Bands and Pheromone Traps.