Okanagan Shuswap Forest District - Forest Health

Choristoneura occidentalis	Orgyia pseudotsugata

Defoliators ...

Most of the major forest defoliators in British Columbia are native pests. Butterflies and moths (Lepidoptera) are considered to be the most destructive of these forest defoliators. The larvae, or caterpillars, feed on the foliage of trees and plants. Forests of any age class may be attacked. Two lepidopterans are particularly important defoliators of coniferous forests: western spruce budworm (Choristoneura occidentalis) and Douglas-fir tussock moth (Orgyia pseudotsugata).

Damage caused by defoliators includes mining of new needles and buds during early larval instars (developmental stages), followed by heavy feeding on both new and old needles by late instar larvae. Late instars are voracious and often wasteful eaters, capable of quickly ravaging the tree of its foliage. Tree tops and branch tips are usually the most severely defoliated. The remainder of the foliage may be destroyed as the larvae migrate down the crown. Sustained severe defoliation can result in tree deformity, decreased growth, top kill, and often, tree mortality. This can translate to significant losses in timber volume.

Symptoms of damage that indicate defoliator attack include discoloration and/or loss of foliage, mined and chewed needles and buds, abundant silk webbing draped over the crown, and the presence of insect pests of certain life stages on the host tree.

Western Spruce Budworm, (Choristoneura occidentalis)

Western spruce budworm is a lepidopteran native to North America. It is the most destructive coniferous forest defoliator in the province. Budworm infestations are usually most severe in uneven-aged, multilevel stands in drier Douglas-fir ecosystems.

Host trees: Despite its name, western spruce budworm attacks primarily Douglas-fir but may also attack amabilis fir and grand fir.

Description and life cycle: Western spruce budworm have a one year life cycle. Moths emerge in late July and early August. They then mate and lay eggs, only to die within two weeks of emergence. Eggs are laid in flattened masses on the underside of needles. The eggs hatch into larvae within 12 days. The larvae do not feed but instead spin silken shelters, called hibernacula, under bark scales and among lichens. The larvae overwinter in these hibernacula. The larvae become active again in late May and mine into vegetative and reproductive buds. As the new shoots elongate and flush, the larvae spin loose webs around the growing tips, creating a feeding shelter. Larvae prefer new foliage, but will feed on old foliage when new growth has been depleted. The larvae develop through six instars and then pupate within the webbed foliage. Adults emerge within two weeks of pupation and the cycle begins again. Spruce budworm is usually encountered during the larval life stages Early larval stages are yellowish green and range from about 0.2 to 0.4 cm long. Later, larvae appear orange-brown, and are 0.5 to 0.7 cm long. Finally, larvae range from "reddish to olive brown…and sometimes have a green tinge on the upper surface" and are between 2.5 and 3.0 cm long (Finck, K.E., 1989). Moths are mottled, with colours including orange or reddish brown, black, and white.

Damage symptoms: Budworm impact is most severe in uneven-aged stands in drier interior Douglas-fir biogeoclimatic zones. Budworm populations favour multilevel Douglas-fir stands with dense understory trees. Mature larvae are sensitive to disturbance and readily drop from their feeding sites. The larvae spiral down from the upper canopy on silk strands to land and feed on intermediate and understory trees. Late instar larvae are voracious eaters and quickly ravage the tree of its foliage. By the end of the feeding period the damaged new growth, webbed together in clumps, turns reddish brown. Fall rains wash away the damaged needles to reveal the full extent of defoliation. Tree tops and branch tips are usually the most severely defoliated. Sustained severe defoliation can result in tree deformity, decreased growth, top kill, and often tree mortality. Defoliated trees are weak and susceptible to further attack by secondary pests, such as bark beetles, thus contributing to additional tree mortality.

Egg masses are laid on the underside of needles in a shingle-like fashion.	Larvae become active again in late May and mine into growing vegetative and reproductive buds.
Western spruce budworm larva develop through six instars, or growth stages, and then pupate within webbed foliage.	Adults emerge from pupae within two weeks to mate, disperse,and lay eggs, thus beginning the cycle again.

Douglas-fir Tussock Moth, Orgyia pseudotsugata

The Douglas-fir tussock moth is a destructive native defoliator of Douglas-fir. Outbreaks of tussock moth occur every ten to twelve years causing significant damage and mortality to Douglas-fir stands in the interior of the province. These outbreaks tend to last up to four years before natural controls such as predators, parasites, pathogens, and starvation lead to population collapse.

Host trees: Primarily Douglas-fir. Occasionally ponderosa pine and western larch may be attacked if adjacent to an infested Douglas-fir.

Description and life cycle: The tussock moth has a one year life cycle. Adults appear from late July to early September. The adult female is stout bodied, wingless and sedentary, usually remaining camouflaged on her cocoon. Males are slender bodied and, approximately, have a 30 mm wingspan. Males emerge before females. Males take to flight in seek of females. Mating occurs on the cocoon, typically, on the same day that the female emerges. Each female lays approximately 200 eggs in a single mass on her empty cocoon. The action of depositing her eggs dislodges hair from her abdomen which mixes with a frothy cement produced during oviposition. The egg masses overwinter. Larvae hatch in late spring and feed voraciously on the current year’s foliage. Because the female does not fly, the primary mode of dispersal is carried out by first and second instars. The larvae produce long silken threads which, along with their light bodies and long hairs, enable them to be carried by the wind. As the larvae mature, they feed on both old and new foliage. In late July the larvae pupate in cocoons on the underside of foliage and emerge two weeks later as adults to begin the cycle again. Larvae are easily identified by three characteristic groups of hairs (tussocks) found on the upper surface of the body. The hairs of the tussocks are yellowish with dark red-brown tips. Larvae also have two sets of dense, long, black hairs at the head, as well one similar group of hairs on the posterior.

Damage symptoms: Young larvae feed only on new foliage while older larvae feed on both new and old foliage. The upper part of the crown and the branch tips are defoliated first. The remainder of the foliage is destroyed as the larvae migrate down the crown. By July, defoliated trees appear scorched. Trees may die after one or more years of severe defoliation. Frequently, the top third of the crown is completely defoliated, which leads to damage in the form of top-kill and branch dieback. Douglas-fir trees that have been weakened by the tussock moth defoliation may also be susceptible to attack by other insect pests, such as the Douglas-fir beetle.

Eggs are laid on empty female cocoons in a mass of approximately 200, covered in hair from the female’s body.	Early instar Douglas-fir tussock moth larva.

Cocoon and late instar larva on the underside of the branch.	Female adult Douglas-fir tussock moths are wingless and sedentary, usually remaining on their cocoons. Cocoons can be found on the underside of foliage and twigs.

Male adult tussock moths are winged and search out flightless females.	Outbreaks of Douglas-fir tussock moth frequently cause significant top-kill and tree mortality over limited, well defined expanses of Douglas-fir stands.