2002-02-20 - BARK BEETLES, SLASH, AND FOREST FERTILITY
By Chris Schnepf, Extension Forester,
University of Idaho Cooperative Extension
In recent Extension programs on thinning and insects and disease, I have noted confusion regarding slash,
downed trees, and bark beetles. This is somewhat understandable. On one hand, we encourage people to leave
more tops, branches, and other woody material distributed through the woods (within fire safety limitations) to
enhance forest nutrition, fertility, and understory wildlife habitat. On the other hand, we also encourage people
to remove green slash or winter downed trees that can provide a breeding ground for bark beetles, which may
emerge to attack standing trees.
Many forest owners are unclear about how to reconcile these issues – what types of slash can be left during what
times of year and under what conditions? As a result, they often believe they should remove all slash and
downed trees to guard against bark beetles. That is not necessary.
There are many species of bark beetles; only a few of the ones that kill trees breed in slash or downed trees.
Furthermore, slash or downed trees must be of a specific species, size, and available at a specific time of year to
present a hazard to standing trees. The chart on page 2 may help you sort out these differences, and identify
what is generally safe to leave on the ground and when, regarding the most commonly damaging bark beetle.
Trees dead longer than one year are not bark beetle hazard. You will often find insects in them that are
superficially similar to bark beetle, but they are not usually insects that kill trees. The same goes with large
wood boring insects (commonly found working in dead trees or firewood) that are often mistakenly referred to
as bark beetles. These insects rarely kill trees. In fact, they are beneficial to the forest, to the extent they start
tearing down dead trees, making them less of a fire hazard and releasing their nutrients back into the forest.
They also provide food for a variety of wildlife species.
Wood left in the forest is not necessarily wasted. Large and small organic debris left distributed across the
forest acts much like mulch in a garden, providing nutrients and adding to forest soil structure. It also provides
food and habitat for insects and other organisms that further benefit soil fertility and structure. To quote a
famous worm in a recent children’s book by Gary Larson about a family of worms: I’ll think that I shall never
see, a thing as lovely as a big, rotting tree carcass. Try to practice smarter forestry remove or treat slash only
to the degree that it truly creates an insect or fire hazard or unduly limits access.
TREE SPECIES AND HOW SLASH OR DOWNED TREES MUST BE TREATED TO PREVENT BARK BEETLES.
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Tree Species
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Bark Beetle
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Material needing treatment and method
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Material that may be left for forest nutrition and wildlife
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Ponderosa pine
(Pinus ponderosa) and
Lodgepole pine
(Pinus contorta)
|
Pine engraver
(Ips spp.)
|
Do not leave green pine slash
larger than three inches in
diameter December to June.
Otherwise burn, chip, or dozer
trample slash.
|
Pine stems smaller than
three inches in diameter or
created July to November.
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Douglas fir
(Pseudotsugae
menziesii)
|
Douglas fir beetle
(Dedroctonus
pseudotsugae)
|
Remove or burn green
Douglas-fir slash or downed
trees larger than eight inches in
diameter within one year of
creation (those downed in
May-July should be taken out
before the following April).
|
Douglas fir stems less than
eight inches in diameter or
more than one year old.
|
Grand fir
(Abies grandis)
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Fir engraver
(Scolytus ventralis)
|
Remove or burn green Grand
fir slash or downed trees larger
than three inches in diameter
December to July.
|
Grand fir stems less than
three inches in diameter
created in August to
November.
|
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Original article can be seen here
WSU Cooperative Extension
Cooperating Agencies: Washington State University, U.S. Department of Agriculture, and Spokane County
Cooperative Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, color, gender, national
origin, religion, age, disability, and sexual orientation. Evidence of noncompliance may be reported through your local Cooperative Extension office.
rwalters@spokanecounty.org
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