What are gypsy moths?

The gypsy moth, Lymantria dispar, is a moth in the family Lymantriidae of Eurasian origin. Gypsy moth is a pest of trees in many regions of the world but is not native to North America. Two strains of gypsy moth have been recovered in North America: the European strain, of which the female is flightless; and the Asian strain, of which the female is capable of flight. Originally ranging from Europe to Asia, it was introduced to North America in the late 1860s by a French naturalist attempting to cross the European gypsy moth with North American silkworms. His intent was to establish a silk industry on this continent. Some of the insects escaped and have since established themselves in Northeastern United States and Eastern Canada.

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What does the gypsy moth look like?

The gypsy moth overwinters in egg masses attached to the bark of trees. The egg masses, usually about the size of a one dollar coin, are buff to tan in colour and may contain from 100 to 1,000 eggs. The eggs hatch into caterpillars (larval stage) when tree buds begin to open. This stage, lasting up to seven weeks, is when the insect feeds.  The caterpillar is easily recognizable in the latter part of this stage: charcoal grey with a double row of five blue and six red dots on its back. The adult female is approximately 30 mm long, white, with zigzag markings on its wings. The female is incapable of flight and dies about one day after laying its eggs. The male is a brownish colour, much smaller, and survives about one week, mating with several different females.

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What does gypsy moth damage look like?

Gypsy moths are a concern because the larvae feed voraciously on the leaves of both deciduous and coniferous trees. During the larval stage, a gypsy moth caterpillar can eat an average of one square metre of foliage. Leaves play a major role in food production for the tree, converting light into food by photosynthesis. Reducing the number of leaves available to capture sunlight results in a loss in food production. Deciduous trees can sometimes produce a second crop of leaves, but after repeated defoliations trees may become so weakened that they are susceptible to secondary infestations. Evergreens may die after only one defoliation.

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How can I control gypsy moths?

Physical Control

It is important to be thorough when looking for egg masses as they may be difficult to locate. Common hiding places are the underside of branches, tree trunks, fences, firewood, outdoor furniture, swing sets, boats, trailers and under the eaves of buildings. When an egg mass is observed it should be scraped off with a knife and dropped into a bucket filled with hot water and household bleach or ammonia. Larvae and pupae can be handpicked and crushed. Some persons are sensitive to the long hairs of the larvae. As a precaution, gloves should be worn when handling the insect.

Biological Control

Bacillus thuringiensis var. kurstaki (Btk) is a selective biological insecticide which controls lepidopterous larvae (caterpillars). Btk 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 Btk.

Chemical Control

Dormant oil, applied to the tree in late winter, will smother the eggs before they hatch. Traditional insecticides containing carbaryl, pyrethrin or permethrin are registered in Canada to control gypsy moth larvae. These are contact insecticides which are most effectively applied at night, when the caterpillars are feeding.

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