July 15, 2009
Francesco Pacelli
Nursery technical analyst

Insects and diseases can threaten plant health. As soon as you notice any abnormality in your plant’s appearance, begin a careful examination of the problem. By identifying the specific symptoms of damage and understanding the causes, you may diagnose the problem and select appropriate treatment. Horticulture Review will begin publishing articles on pests and diseases to assist growers understand pest and disease

Lymantria dispar (gypsy moth)

Gypsy moth is native to Europe and southern Asia, being first introduced to North America in the Boston area around 1869. It has since spread over much of eastern North America. In Canada, it was first found in Québec in 1924. The first Ontario record is 1969.


Gypsy moth egg masses are typically laid on branches and tree trunks, but egg masses may be found in any sheltered location. The egg is the over-wintering stage. As the female lays them, she covers them with hair-like setae from her abdomen. Many individuals find these hairs irritating, which may offer the eggs protection. Egg masses contain from a couple of hundred to about 1,200 eggs.


The hatching of gypsy moth eggs coincides with budding of hardwood trees. Larvae (caterpillars) emerge from egg masses from early spring through May. Newly hatched larvae are typically hairy and about 2-3 mm long; mature larvae are up to 60 mm long, with two rows of large spots along the back, usually arranged in five pairs of blue and six pairs of red from head to rear. Many long hairs cover the body. The young larvae crawl up the tree and begin feeding on the expanding foliage. Young larvae chew small, round holes in leaves. Older larvae feed from the leaf edges, consuming the entire leaves except for the larger veins and the middle rib. They feed mainly during the night and rest during the day, congregating in large masses on the underside of the major limbs or on the lower main stem.


The pupa is dark reddish brown, usually with some yellowish hairs attached. The length of females varies from 15 to 35 mm. Males are often smaller, measuring 15 to 20 mm. They are found in protected places such as bark fissures or crevices, loose moss and on foliage or hanging from branches.


The brown male gypsy moth emerges first, flying in rapid zigzag patterns searching for females. The males are active at night and even daytime. When heavy, black-and-white egg-laden females emerge, they emit a chemical pheromone that attracts the males. After mating, the female lays eggs in July and August, close to the spot where she pupated. Then, both adult gypsy moths die.


A gypsy moth has one generation per year, over-wintering in the egg stage. Hatching depends on warming weather, commonly occuring from mid to late-April, occasionally to late May. The newly-hatched, small and hairy larvae move up host plants to newly emerged foliage where they begin feeding. Feeding continues for six to eight weeks, varying with weather, host conditions and location. There are normally five male and six female larval instars. Larvae in the first three instars feed mainly at night; those in later instars feed day and night and consume the largest quantity of foliage. Feeding is usually completed by late June or early July and most larvae move to protected locations to pupate. Moths start emerging in July, peaking about mid-month and extending into August in eastern North America. After mating, adult females indiscriminatly lay egg masses from late July to September.


There are over 300 known host plants for the gypsy moth. In North America, the list of preferred hosts includes oak, linden, cherry, white birch, maple, alder, willow, elm, hazelnut, hemlock, pine, poplars, hawthorn, walnut, plum, peach, pear, apple, apricot and
trembling aspen.

Natural control

Larvae have several predators that help decrease the population. Among the predators are deer mice, Tachinid flies and Braconid wasps. There is no evidence that releasing or enhancing gypsy moth predators or parasites will reduce moth populations.

Biological and microbial control

Microbial and biological pesticides contain living organisms that must be consumed by the pest. Microbial include bacteria, viruses and other natural organisms; biological include man-made synthetics of naturally occurring organisms. These pesticides should be applied before the larvae reach the third stage of instars of development. Mature larvae become more resistant to microbial pesticides and therefore, more difficult to kill. The most famous biological and microbial pesticide to control gypsy moth and other  pests is Bacillus thuringiensis var. Kurstaky (BTK). With Bt, the insect becomes paralyzed, stops feeding and dies of starvation or disease.

Chemical control

The most commonly used chemical pesticides currently registered against the gypsy moth, contain spinosad carbaryl, diflubenzuron. Malathion, methoxychlor, phosmet, trichlorfon, and synthetic pyrethroids (permethrin). Diflubenzuron represents a new class of pesticides, called insect growth regulators. It kills gypsy moth larvae by interfering with the normal molting process. Diflubenzuron has no effect on adult insects. Aquatic crustaceans and other immature insects that go through a series of molting stages are often sensitive to
this pesticide.
Active ingredient Representative trade names
Diflubemzuron          Dimilin
Spinosad     Success 480 SC
Malathion        Malathion
Carbaryl Sevin XRL Plus

To reach Francesco Pacelli, contact Landscape Ontario at 1-888-211-5606, ext. 2377, or e-mail fpacelli@landscapeontario.com.