2020 A Peak Year For Gypsy Moths

Gypsy moth infestations peak every 7 to 10 years. 2020 is near one of those peaks. Some communities in southern Ontario are seeing major infestations. Damage to tree foliage by the caterpillars can be extensive. A single caterpillar can consume up to one square meter of foliage per day.


  • invasive – native to Europe
  • this European defoliator feeds on a wide variety of trees
  • the first detection of gypsy moth in Ontario occurred in 1969; however, widespread defoliation did not occur until 1981
  • established populations exist south of a line from Sault Ste. Marie east to North Bay and Mattawa; a separate infestation exists in New Liskeard
  • the Ontario distribution coincides with the range of the insect’s preferred hosts of oak; however, no known populations of gypsy moth exist in northern parts of the range of bur oak north on New Liskeard in the northeast region, and west of Thunder Bay to Lake of the Woods in the northwest region

“Invasive” refers to a species that has moved outside of its native habitat and threatens the new environment, economy or society by disrupting local ecosystems.

Host species

Hosts range from oak (Quercus), birch and aspen (populus) in the north, to various hardwoods such as sugar maple (Acer saccharum) American beech (Fagus grandifolia) and softwoods such as eastern white pine (Pinus strobus) and Colorado blue spruce (Picea pungens) in southern Ontario.

Characteristics and life cycle

  • over-winters in the egg stage often on the bark of trees
  • in spring, eggs hatch and larvae ascend the trees to feed on the new foliage
  • initially, feeding occurs during the day, but as the caterpillars mature feeding occurs mainly at night – often this can delay the detection of infestations
  • mature larvae are 50 mm long, dark coloured, hairy, with a double row of five pairs blue spots, followed by a double row of six pairs red spots, down the back
  • feeding is completed in July
  • male moths are light brown and slender-bodied, while females are white and heavy-bodied
Gypsy moth larvae
Gypsy moth adults laying eggs

Symptoms and damage

  • gypsy moth outbreaks occur every 7 to 10 years
  • larvae chew holes in leaves or devour entire leaves
  • in late July spongy egg masses can be observed on the trunks and branches of infected trees
  • during severe outbreaks, trees and shrubs are completely defoliated over large areas; despite the trees’ ability to produce a new crop of leaves over the summer, the damage causes significant growth loss
  • under story shrubs and plants may also be affected
Gypsy moth defoliation

Municipal Control measures

Insecticides can be applied effectively in urban areas through foliage application in June to protect ornamental and shade trees. Egg masses can also be physically removed and destroyed.

Gypsy moth populations have also collapsed from rapid proliferation of the fungus Entomophaga maimaiga.

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.

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