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WESTERN
SPRUCE BUDWORM
(Choristoneura occidentalis)
Hosts:
Grand fir, subalpine fir, Engelmann spruce, Douglas-fir, western
larch
Quick ID:
- Foliage
with scorched or blighted appearance at tips
- Needles
bound together with webbing at branch tips
- Defoliation
caused by 1" or smaller caterpillars
- Caterpillars
brown with green markings, white spots on sides, not hairy
- Topkill
or death following defoliation
 Field
Identification
Tree:
Affected trees will have foliage with a scorched or blighted (as
by frost) appearance at the tips of branches. Early in the summer,
when budworms are feeding on new foliage, needles will be bound
together at the tips with webbing. Defoliation is most obvious in
the upper crown and at branch tips. Tree decline, topkill, and death
may follow severe defoliation.
Insect:
Adult insects are orangish-brown triangular moths, a little less
than an inch across. Egg masses on needles are pale green. Larvae
go through several development stages, beginning as tiny green caterpillars
with brown heads and finishing as 1" long, olive-brown caterpillars
with pronounced ivory "warts" on either side. They are
not hairy; if you have a defoliator that is hairy
it's probably Douglas-fir tussock moth. Pupae are golden-brown.
May be confused
with: Frost damage, sawflies, Douglas-fir tussock moth.
Life cycle:
Adults lay eggs in a shingle-like fashion on the underside of needles
in midsummer. Eggs hatch in about 10 days. The larvae molt once,
then hibernate for the winter on rough bark surfaces in the canopy
and down the bole. In the spring, larvae re-emerge and begin to
tunnel into older foliage, or float away on silken threads. After
about two weeks, they move on to expanding buds and finish with
the new foliage. Larvae spin webbing among the needles of expanding
shoots, and feed until disturbed or the shoot is completely destroyed.
When feeding on new foliage early in the summer, budworms construct
a shelter by binding the needles together at the tips with webbing.
They retreat to this shelter when disturbed. In midsummer, they
pupate either in their feeding webs or in other webs in the foliage.
Adults emerge from the pupae and fly to seek a mate.
At night, females emit pheromones to attract males; after mating,
females begin to lay eggs a day later.
Dispersal redistributes
budworms within the crown and between trees and stands. It may occur
at any point in the life cycle, but most commonly it is the younger
larvae that are carried by the wind to other trees. This can be
significant as the tallest trees are preferred by the egg-laying
adults, and larvae are blown to shorter, intermediate or overtopped
trees. Adults are also dispersed by wind currents and over a long
range by frontal systems.
Predisposing
factors: Pure stands of firs and spruce are more susceptible
to infestation and receive more damage than mixed stands that include
other species the budworms don't eat, like pine, hemlock and western
redcedar. Physiological stress in host trees influences susceptiblity
to attack and concurrent damage to a high degree. Agents of stress
can take many forms: drought, overstocking, disease, insect attack,
poor site conditions, fire, harsh frost events, etc. Nonvigorous
trees are more susceptible to attack and receive more damage than
vigorously growing trees. Dominant and codominant trees are more
susceptible to infestation than are intermediate and overtopped
trees, but overtopped and intermediate trees receive more damage
when attacked. Stands managed under uneven-aged structure are more
susceptible and suffer more damage than stands that are managed
for even-aged structure. Budworms are also favored in stands that
are overmature, as trees in those stands are usually declining in
vigor. However, most tree mortality occurs in regeneration, sapling
and pole-sized stands. Warm, dry spring weather and warm, dry sites
in general favor budworms.
Impact:
As western spruce budworm is a defoliating insect, it doesn't necessarily
kill trees. However, severe defoliation will kill a conifer in one
year, and repeated infestation year after year will severely weaken
the tree, causing loss in growth and predisposing it to attack by
bark beetles. The USFS estimates that the western spruce budworm
affected 234,430 acres in Washington in 1990, and 1,027,671 acres
in 1991.
Management:
Direct control of spruce budworm has been attempted on many occasions
during this century, and still is ongoing in places. Chemical spraying
operations, although occasionally highly successful in reducing
budworm populations, have fallen out of favor as questions about
undesirable impacts on the ecosystem have been raised Fine-tuning
of insecticides against the budworm are ongoing. One successful
agent that has been discovered is a naturally-occurring
bacteria, Bacillus thuringiensis, or B.t. It is specific to moths
and butterflies and quite effective in knocking the population back
without having any adverse effects on the environment.
Up until this
century, true fir stands were limited to cooler, wetter areas that
were less subject to fire. After the advent of fire suppression,
true firs took over drier sites from pine. While this was not "abnormal"
succession, it has lead to conditions that favor the spruce budworm.
Whether or not we are entering a long-term period of global warming
and climatic change, the Pacific Coast has always been subject to
periodic droughts that can last for several years. Insufficient
water reduces the tree's ability to defend itself and at the same
time increases the nutritional quality of the foliage to spruce
budworms. When all interrelated habitat characteristics, including
diversity in crown-class structure, are analyzed collectively, budworm
habitat has improved as the forest has changed from pine to fir.
To lessen the
damage, agents of stress and other predisposing factors should be
removed. It is not possible to control abiotic events such as drought,
but management activities can lessen the severity of drought and
other agents of stress:
On warm, dry
sites, stand managers should choose species well-adapted for them
such as pine and larch. That may involve planting these species
and thinning or eliminating true firs. On good fir sites, encourage
species diversity through thinning and planting. Western larch is
more resistant to budworm than true fir, spruce or Douglas-fir,
and western redcedar, western hemlock, and all pine species are
not considered to be more than occasional hosts. Climax budworm
host species (true firs and spruce) should not make up more than
1/3 of the stand. Use silvicultural treatments to maintain stands
at a proper stocking level and increase stand vigor. At age 10,
300 to 600 seedlings/acre should be appropriate, depending on site
conditions and seedling distribution. Thinning from above will not
reduce stand susceptibility to spruce budworm. Manage even-aged
patches rather than uneven-aged stand structure. Clearcutting, seed
tree cuts or shelterwood cuts
may be used, depending on the site. If using a seed tree or shelterwood
cut, remove the overstory within 10 years. Creating a mosaic of
varying stand conditions over a large area may help. Cut intermediate
and overtopped host species in a middle-aged stand. Harvest on time
so the stand does not become overmature. Subject to other management
constraints, a large harvest area is preferable to a small one as
insects, both larvae and adults, are found with decreasing frequency
as one moves from the edge to the center of a cut. Harvest scheduling
should concentrate on cutting the most susceptible stands first
and regenerating them to thrifty, fast-growing, shade-intolerant
species.
Adapted from
Brookes et al., Western spruce budworm, USDA Forest Service Cooperative
State Research Service, Technical bulletin No. 1694.
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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