May 27, 2021
Gypsy Moth Spring 2021

​The Ontario Ministry of Natural Resources and Forestry "observed record-breaking areas of defoliation in southern Ontario since the moth’s introduction over 50 years ago," reports Jennifer Leweelyn, ​ OMAFRA Nursery and Landscape Specialist on her website ONnurserycrops. "Nearly 600,000 hectares of trees were defoliated by this pest last year, beating the original record of 350,000 hectares in 1991. OMNRF is predicting moderate to severe defoliation in Gypsy moth hot spots in 2021."


Watch Jennifer Lewellyn's "Gypsy Moth in the Landscape" video:




Gypsy Moth Impacts

Gypsy moths are most destructive in their larval stage – as caterpillars – stripping away foliage from a broad variety of trees. Repeated defoliation stresses trees and can lead to mortality, especially in urban or drought-stricken areas, and can weaken tree regeneration due to impacted seed production and root sprouting. An estimated 595 million hectares of North America are considered climatically suitable for the establishment of gypsy moth populations (Gray, 2004). The efforts to control and manage the pathways of gypsy moths is crucial to mitigate the possible economic, ecological, and social impacts on potential areas of spread.

Economic impacts:

An infestation can defoliate a broad range of tree species, impacting the aesthetics of a forest. In urban areas, this can impact property values and impose costs for tree removal and replacement. In addition, tree defoliation can negatively affect tourism.

The gypsy moth was estimated to have caused the loss of roughly $120 million in residential property value per annum in the US from 1998 to 2007, and $298 million in US federal expenditure targeted at gypsy moth for the same ten-year period (Aukema, 2011). The amount for the US federal expenditure included suppression, research, and ‘slow the spread’ programs.

Ecological impacts:

Invasions disrupt the ecological services provided by trees, impact wildlife habitat and reduce food production for dependent species. The dieback of twigs and branches due to defoliation causes canopy thinning, thereby weakening trees. Invasions are capable of affecting forest tree composition. It is a concern that gypsy moth might affect the oak regeneration success in eastern North America, given that oak species are one of the preferred.

Social impacts: 

Direct skin contact with gypsy moth caterpillar hairs could result in a rash and/or skin irritation. On a recreational level, defoliated trees and thinned canopies degrade the aesthetic value of trees. 

*Impacts sourced from the Invasive Species Centre

How do I know if I have a problem?

The gypsy moth hibernates in egg masses that are covered with tan or buff-coloured hairs, and may be found on tree trunks or bark, outdoor furniture, or the sides of buildings. The egg masses are about the size of a loonie, and may contain from 100 to 1,000 eggs. You can tell how bad the infestation is by the size of the egg mass. When populations are on the decline, egg masses tend to be smaller, about the size of a dime. Larger egg masses are a sign of stable or growing populations. The eggs hatch into caterpillars when tree buds begin to open. This stage, lasting up to seven weeks, is when the insect feeds, so it is important to control gypsy moth infestations early in the growing season.

How can I get rid of gypsy moths?

Egg masses Gypsy moths spread easily, as the young larvae can be carried by wind currents for a distance of up to one kilometre. More commonly, however, they hitch a ride (mainly egg masses) on objects like vehicles, tents, trailers, and lawn chairs to infest new areas. Vacationers, especially campers, should be aware of this and should check their equipment before moving on. It is important to be thorough when looking for egg masses as they can be difficult to locate. Common hiding places include: the underside of branches tree trunks fences firewood outdoor furniture swing sets, boats trailers under the eaves of buildings When an egg mass is found, it should be scraped off with a knife and dropped into a bucket filled with hot water and household bleach or ammonia. Remove picnic tables, swing sets, and lawn furniture from around the bases of trees, because these objects provide the insects with protection from the heat of the sun.


Caterpillars and pupae can be handpicked and crushed. The long hairs of the caterpillar can cause skin irritation or allergic reactions in some people. To be safe, wear gloves when handling them. Caterpillars can be successfully trapped. To make a trap, wrap a 45-cm (roughly 17-inch) wide strip of burlap around the tree trunk at chest height. Tie a string around the centre of the burlap and fold the upper portion down to form a skirt, with the string acting as a belt. The caterpillars will crawl under the burlap to escape the sun and become trapped. Later in the day, lift the burlap. Pick off the caterpillars and dispose of them.

Biological control
  • Bacillus thuringiensis kurstaki (Btk) is a selective biological insecticide that controls caterpillars. Bacillus thuringiensis kurstaki crystals release a toxic protein when dissolved in the alkaline digestive system of the insect. The caterpillar stops feeding soon after, and dies within five days. Other insects, mammals, birds, and fish are not affected by Bacillus thuringiensis kurstaki.
  • The small wasp (Encyrtidae family), introduced in eastern North America in 1909 as a parasite of the gypsy moth egg, is now commonly found throughout the area and has become an important biological control of the gypsy moth.
*Information sourced from the Government of Canada

Learn the difference between Gypsy Moth and Box Tree Moth