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Pest Status
Background
Chapter 62
Orgyia leucostigma (J.E. Smith),
Whitemarked Tussock Moth (Lepidoptera:
Lymantriidae)
D.G. EMBREE, D.E. ELGEE and G.F. ESTABROOKS
The whitemarked tussock moth, Orgyia leucostigma (J.E. Smith), is a noxious insect in
eastern Canada, particularly in Nova Scotia. It is primarily a forest pest, capable of
killing entire stands of mature balsam fir, Abies balsamea (L.) Mill., in a year and of
defoliating extensive areas of hardwoods. It is particularly devastating to balsam fir
Christmas trees which it deforms, defoliates, or kills. The white marked tussock moth is
also a pest in urban areas where it feeds on a wide variety of hardwood shade trees,
crawls over houses and lawns, and spins unsightly cocoons on trees and shrubs. Some
people are allergic to the larval setae. Moreover, O. leucostigma is a pest of blueberries
and is the target of an extensive spray programme to protect the commercial crop.
Egg masses are laid on empty female cocoons and covered with froth. On hatching,
larvae feed gregariously on the froth. Once this is consumed, larvae spin downwards on
silken threads and are carried great distances by the wind. On occasion, during severe
outbreaks, swarms of airborne larvae appear as a grey mist and the insects can cover the
surface of small lakes and ponds when they land.
The larvae feed on a wide variety of hardwoods and conifers such as tamarack, Larix
laricina (Du Roi) K. Koch, but their primary host is balsam fir. Larvae feed in the shade
on the underside of leaves and twigs. This feeding habit causes a unique form of damage
to balsam fir Christmas trees because larvae also feed on the thin bark of developing
leaders and upper lateral branches causing them to warp downwards. The result is a
pronounced crook in the stems of surviving trees, visible for years afterwards. In heavy
infestations, trees are festooned with silk.
Pupation usually occurs on the underside of twigs and branches or on the stems of host
trees, but it can occur almost anywhere, the yellowish white cocoons often being spun
on fences, houses, or logs. Females remain on the surface of their cocoons when they
emerge, mate with winged males, and lay from 100 to over 500 eggs.
Outbreaks are controlled ultimately by a nuclear polyhedrosis virus (NPV). These outbreaks
of O. leucostigma, at least in Nova Scotia, appear to originate in balsam fir
stands at high elevations and occur at intervals of 6-10 years between outbreaks. The
infestation spreads by wind-borne larvae and eventually large areas of balsam fir and
hardwoods are defoliated. Such outbreaks become widespread and last for 5-6 years,
then collapse because of the gradual spread of the virus disease. The initial infestation
collapses in about 3 years, subsequent infestations in 2-3 years, and in the final stages of
the outbreak infestations appear and collapse in the same year.
Over 25 species of parasitoids attack the whitemarked tussock moth but the only
control they have over populations is to kill survivors following the collapse of outbreaks
caused by NPV. As a result, whitemarked tussock moth populations are reduced suddenly
to extremely low levels, which delays the build-up of subsequent outbreaks.
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