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DOUGLAS-FIR
TUSSOCK MOTH
(Orgyia pseudotsugata)
Hosts:
Douglas-fir, grand fir, subalpine fir; rarely pine, western hemlock,
western larch,
Engelmann spruce
Quick
ID:
- Foliage
appears scorched or off-color
- Defoliation
from top down and from outside in
- Thin crowns
and topkill
- Defoliation
from hairy caterpillar up to 1" in length
- Grey cocoons
and webbing present on branches
 Field
Identification
Tree: From a distance, trees appear scorched or off-color,
often with a noticeably thin crown. Tussock moth larvae prefer new
foliage, especially when they are young, hence the defoliation
occurs from the top down and from the tips of the branches inwards.
As the top of the crown has a high percentage of new foliage each
year, it is severely defoliated and often killed. Larvae spin webs
enclosing leader and branch tips. When the population density is
normal, grey cocoons may be found on the underside of old foliage.
When the population density is high, additional cocoons may be found
on branches, on the trunk, or on understory vegetation.
Insect:
Adult insects are charcoal-brown moths. The male has a wingspan
of about 1-1/4" (31 mm), and the female has stubby, vestigial
wings. Egg masses are greyish as they are mixed with black hairs
from the female's abdomen; they are laid on the female's cocoon.
Larvae go through several stages of development. They hatch as tiny,
black-headed caterpillars and finish as larvae about 1" in
length, with four distinct tufts of white hair with red tips in
the middle of the back, two "horns" of hair in front,
and two tufts at the end of the abdomen, one of which may be reddish.
All stages of development are hairy. If you have a defoliator which
has no hair, it's not tussock moth.
May
be confused with: Western spruce budworm, sawflies
Life
cycle: The female, who does not fly, lays eggs on her cocoon
anywhere from August through October. These eggs are mixed with
hairs from her abdomen, held together with a gelatinous substance.
Eggs overwinter in a mass and hatch from late May to late June.
Larvae initially feed on the underside of new needles, then switch
to older needles and next year's buds. Young larvae may be dispersed
to other trees by a "ballooning" effect, where the wind
carries them on a strand of webbing for some distance. Fully developed
larvae spin a cocoon and pupate from mid-August through September,
emerging to mate and die soon thereafter. Only one generation occurs
each year.
Predisposing
factors: The probability of defoliation for a given stand in
Eastern Washington may be related to certain factors. Geographically,
those stands which are lower in elevation, on east-facing slopes,
or on ridgetops are more likely to be defoliated. Tussock moths
are favored by stands of high density, a high percentage of true
fir and Douglas-fir, or many trees with a large crown diameter.
Naturally, the probability of defoliation increases when adjacent
or nearby stands are defoliated, or have a high probability of defoliation.
Water deficiency also predisposes trees to attack. Defoliation by
tussock moths is itself a predisposing factor for bark beetle infestation,
particularly fir engraver and Douglas-fir beetle. Bark beetle outbreaks
often occur shortly after tussock moth outbreaks, once trees have
been weakened.
Impact:
The magnitude of outbreaks in terms of infested acreage is known
to be increasing. Smaller trees suffer more mortality from the effects
of defoliation, and larger trees suffer more mortality from subsequent
bark beetle attack. In a major outbreak in the Blue Mountains, 72%
of the trees heavily defoliated were killed. Growth loss after defoliation
can be significant, with percentage lost varying from 58% to 40%,
depending on severity of defoliation.
Management:
Both direct and indirect methods of control have been used against
the Douglas-fir tussock moth. Direct methods include chemical, viral
and bacteriological sprays, usually applied aerially. Chemical sprays
have been effective in knocking back the population, but do not
address the underlying cause of outbreaks and hence will not prevent
recurrence. They have fallen out of favor as questions have arisen
about their toxicity in the environment.
Attention
has shifted recently to viral and bacterial controls, as these naturally-occurring
agents have historically caused populations to crash. These can
be highly effective but are somewhat more expensive than chemical
sprays. However, they are specific to butterflies and moths and
hence do not disturb other in sects, nor other members of the ecosystem.
Preparations of a bacteria, Bacillus thuringiensis, are available
commercially. Again, like chemical sprays biological agents knock
populations back but do not treat the underlying cause of the outbreak.
The
US Forest Service and others have made the decision to concentrate
on silvicultural measures to control insect pests. Some recommendations
for reducing the hazard of serious defoliation include:
- Thin to
a generate a low-hazard stand. An appropriate stocking level will
reduce stress from intertree competition, particularly water stress.
Stands less than 50 years of age and without a high proportion
of grand fir will be more resistant to defoliation.
- Do not use
equipment that causes soil compaction or erosion. Match equipment
to site to minimize damage on trees that would favor insect and
disease attacks.
- Lop and
scatter slash, pile and burn largest pieces (greater than 6"
in diameter) only if fuel load is unsatisfactory. The idea here
is to encourage nutrients to cycle rather than to remove them
(by removing the slash) or to permit them to be leached (by burning
everything).
- Under even-aged
management systems, schedule harvest cuts to minimize adjacent
stands' exposure to heat and wind and to improve snow retention.
- Favor establishment
of species adapted to drought, such as ponderosa pine in place
of Douglas-fir on dry sites, and ponderosa pine, lodgepole pine,
Douglas-fir and larch on sites capable of supporting true fir
species.
Adapted
from The Douglas-fir Tussock Moth: A Synthesis, USDA Forest Service
Technical Bulletin 1585, M. Brookes, R.W. Stark, and R. Campbell,
eds.
Note:
Use pesticides with care. Apply them only to plants, animals, or
sites listed on label. When mixing and applying pesticides, follow
all label precautions to protect yourself and others around you.
It is a violation of the law to disregard label directions. If pesticides
are spilled on skin or clothing, remove clothing and wash skin thoroughly.
Store pesticides in their original containers and keep them out
of the reach of children, pets and livestock.
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