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Pest Status
Background
Field Trials
Nuclear polyhedrosis
virus
Chapter 63
Orgyia pseudotsugata (McDunnough),
Douglas-fir Tussock Moth (Lepidoptera:
Lymantriidae)
J.C. CUNNINGHAM and R.F. SHEPHERD
The Douglas-fir tussock moth, Orgyia pseudotsugata (McDunn.), occurs in the semiarid
interior of British Columbia and in parts of Washington, Idaho, California, Nevada,
Colorado, Arizona, and New Mexico in the United States. In Canada, Douglas fir,
Pseudotsuga menziesii (Mirb.) Franco, is the preferred host; but Engelmann spruce,
Picea engelmannii Parry, is also attacked and later instar larvae will feed on ponderosa
pine, Pinus ponderosa Laws., when forced to abandon totally defoliated Douglas fir.
Small larvae eat the underside of new needles. The later instar larvae may eat entire
older needles or sever them near the base and leave them entwined in silk webbing.
Heavily infested trees have a reddish-brown appearance. Trees may die after 1 year of
defoliation, but mortality occurs more commonly after 2 or more years of severe
defoliation (Johnson & Ross 1967). Severely damaged trees are also susceptible to
attack by the Douglas-fir beetle, Dendroctonus pseudotsugae Hopkins.
Since 1916, when the first records were kept (Sugden 1957), six outbreaks of Douglasfir
tussock moth occurred in British Columbia. The latest outbreak began in 1971 and
collapsed in 1976; a further outbreak was predicted for 1981 (Sterner & Davidson 1981).
Characteristically, high populations of Douglas-fir tussock moth appear in relatively
small patches of forest of 1-5 ha. The infestation normally extends from these areas in
subsequent years, but more striking is the appearance of new outbreaks often many
kilometres from the original sites. After an average of 5 years (3-6) outbreaks collapse,
the factors responsible being nuclear polyhedrosis virus (NPV), egg parasitoids, and
starvation. However, following several years of severe defoliation, the trees frequently
die before this population collapse occurs. Hence attempts to regulate the population
with biological or chemical agents should be made early in the outbreak cycle.
Two types of nuclear polyhedrosis virus (NPV) have been found in populations of
Douglas-fir tussock moth. In one type the rod-shaped virus particles are embedded
singly in polyhedral inclusion bodies (PIB) and in the other they are embedded in
bundles (Hughes & Addison 1970). These are referred to as single-embedded NPV
(SNPV) and multiple-embedded NPV (MNPV). An SNPV isolated from the whitemarked
tussock moth. O. leucostigma (J.E. Smith), is also pathogenic for Douglas-fir tussock
moth larvae.
Small-scale ground-spray trials in 1962 with field-collected NPV (possibly a mixture
of SNPV and MNPV) gave encouraging results (Morris 1963). The first aerial spray trial
in British Columbia was conducted in 1974 using Douglas-fir tussock moth MNPV
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