Forest Insect &

Disease

Leaflet 5

United States

Department of

Agriculture

Forest Service

Douglas -fir

Beetle

Richard F. Schmitz and

Kenneth E. Gibson

The Douglas-fir beetle

(Dendroctonus pseudotsuqae

Hopk.) infests and kills

Douglas-fir throughout most of

its range in western United

States British Columbia and

Mexico. Occasionally western larch trees

are infested when growing among

Douglas-fir under attack. Attacks in standing larch are always unsuccessful, while

those in freshly felled larch are usually

successful and brood emergence is equivalent to that in Douglas-fir.

Douglas-fir beetles normally kill small

groups of trees, but during outbreaks 100

tree groups are not uncommon (fig. 1).

Losses can be devastating during periodic

outbreaks, such as four that occurred in

western Oregon and Washington from

1950 through 1969, during which 7.4 billion board feet of timber were killed. Outbreaks in standing trees range from 2 to 4

years. Those of longest duration coincide

with periods of drought. For example,

thousands of acres of Douglas-fir

supported outbreaks in several western

states following a drought from 1986

through 1988. In 1966, an outbreak in

California killed 800 million board feet of

timber and an outbreak in Idaho killed 109

million board feet of Douglas-fir between

1970 and 1973.

At low or endemic levels, the beetle infests scattered trees, including windfalls

and trees injured by fire scorch, defoliation, or root disease. Where such susceptible trees are abundant, once they have

been infested and killed, beetle populations

can build up rapidly and spread to adjacent green, standing trees. Damage is

greatest in dense stands of mature

Douglas-fir. Various fungi introduced by

the beetles also contribute to mortality of

infested trees.

Figure 1. Group of trees killed by Douglas-fir beetles.
External Evidence of Infestation

Reddish-orange frass, consisting of fragments of phloem (inner bark) expelled

from bark crevices by invading

beetles, is the first sign that a

tree has been attacked (fig.

2). Because wind and rain

remove the frass, and attacks

may be well above eye level,

a tree must be examined carefully to determine if the

beetles are present. Additionally, not all attacks are successful, especially in living

trees. Attacking beetles are

sometimes pitched out by

resin flows. At times, the

most evident sign of attack is

the clear resin exuding from

entrance holes on the stem at

the upper limit of infestation. Pitch flow,

however, is not always present on successfully attacked trees (fig. 3).

Several months after a tree is successfully infested, its foliage becomes discolored (fig. 4). First the needles turn yellow, then sorrel, and finally reddish

brown. Although some trees are discolored as early as August,

Figure 3. Clear resin flow (pitch streamers)

exuding from attacks at the uppermost

infested portions of the tree bole.

Figure 4. Discolored needles on infested

trees first turn yellow, then sorrel, and

finally reddish-brown.

Figure 2. Reddish-orange frass from

entrance hole in bark crevices is the first

indication of beetle attack.
others may remain green until the following June. The

time of year that this discoloration becomes visible varies with locality, date of attack, intensity of infestation,

elevation, and seasonal

weather. Usually needles are

shed and conks of the pouch

fungus, Cryptoporus volvatus

(Pk.) Hubbard, form on the

outer bark during the year

following infestation.

Needles begin to fall from

infested trees the year following attack but

becomes more noticeable the second year

after infestation.

Adult Douglas-fir beetles are stout, cylindrically shaped, and 4 to 6 mm long (fig.

5). The head and thorax are black; the wing

covers reddish brown, becoming darker

with age. The whitish eggs are 1 mm long

(fig. 6). Eggs hatch into white, legless larvae with shiny, lightbrown heads (fig. 7).

Mature larvae transform into pupae before

becoming adults. Pupae are white,

mummy like, with some adult features,

including wings that are folded beneath the

abdomen.

Seasonal History

The Douglas-fir beetle has one generation

per year. Broods remain in the tree and

overwinter mainly as adult beetles, but also

as larvae. Initial adult flights vary from

year to year with location and weather, but

most new attacks take place in mid- to

late-spring.

Figure 6. Eggs are whitish, cylindrical, and

deposited along edge of the egg gallary.

Larvae that have overwintered complete

their development and emerge as adults

in the summer. Also, some adults that

emerge and make early-spring attacks,

re-emerge and make a second attack, usually in late June and July, sometimes continuing into August. Typically, late-season attacks account for less than 20 percent of all attacks in one season.

Figure 5. Adult Douglas-fir beetle.

Description of Life Stages
Figure 7. Douglas-fir beetle larvae.

Figure 8. The female lays groups of eggs,

alternating deposition between opposite

sides of the egg gallery.

Galleries and Characteristics of

Infestation

The distinctive egg galleries are constructed by the female beetles which bore

through the bark and tunnel upward in the

phloem, lightly engraving the sapwood.

These galleries run parallel to the wood

grain, commonly for lengths of 8 to 10

inches (20.3 to 25.4 cm). They are somewhat longer in windthrown trees. Galleries are packed with frass except at the top

where the beetles are actively tunneling.

The females lay groups of eggs alternately

along opposite sides of the galleries as construction progresses (fig. 8). The eggs

hatch in 1 to 3 weeks, depending on seasonal temperatures. The newly hatched larvae mine outward from the egg gallery in

the phloem (fig. 9) . These mines are visible on the inner surface of the phloem and

increase in width as larvae molt and grow

through four stages. During the final stage

of development, larvae construct pupal

cells at the ends of their mines that usually

extend into the inner bark.

Tree stems are infested for varying lengths,

but seldom higher than a top diameter of 6

or 8 inches (15.2 or 20.3 cm). Other bark

beetles, Scolytus monticolae Swain,

Scolytus unispinosus (LeConte), and

Pseudohylesinus nebulosus (LeConte), often occur in the top of the stem. In standing

trees, Douglas-fir beetle egg galleries are

usually more dense and the brood survival

higher in the middle portion of the in-
fested stem. In windthrown trees, egg galleries tend to be uniformly successful, but

are generally more dense on the shaded

underside. Generally, Douglas fir beetle

attacks are more dense, and success rate

higher, in down trees than standing ones.

Figure 9. Characteristic gallery pastern

resulting from larvae mining the inner

bark.

Control

The resistance of live trees to bark

beetle attack is the most important

naturally occurring factor controlling

outbreak development.

Climate and weather also strongly

influence survival of Douglas-fir beetle

populations. Natural enemies include

many parasitic or predacious insects,

nematodes, and mites. Woodpeckers

are not important predators of the

beetle.

While it is feasible to kill developing

beetle broods with chemical insecticides, such a “direct control” strategy

is not likely to be recommended. The

most desirable management approach, therefore, involves prompt detection of blowdown or other stand disturbances, timely removal of threatened or infested trees, and maintenance of a vigorous stand.

The best “control” of a Douglas-fir

beetle outbreak involves pro-active

management to prevent its occurrence.

Although outbreaks typically last but

2-4 years, many standing trees may

be killed before beetle populations

naturally subside; and there is a tendency for their re-occurrence until

stand conditions favoring outbreaks

are changed. That being the case, the

most effective strategies are preventive and involve reducing stand susceptibility to beetle infestations. Likelihood of an infestation developing within

a stand is related to the proportion of

susceptible Douglas-fir and overall

stand density Generally, in
unmanaged stands, attacks are most successful on trees that are mature or

overmature, largest in diameter, and

found in more densely stocked stands. A

very high stand density may increase the

susceptibility of younger and smallerdiameter trees. The higher the proportion

of trees with “susceptible” characteristics,

the higher the susceptibility of the stand

to beetle attack. Correspondingly, silvicultural manipulations which reduce the

proportion of the stand having those characteristics limit beetle-caused damage.

Prompt salvage of blowdown or currently

infested trees will also help forestall epidemics. Because of the beetles’ preference for downed trees; trap trees, used

early in an outbreak sequence, can reduce

beetle-caused mortality in standing trees.

Stands being brought under management should be assessed for presence and

extent of root diseases. A high incidence

of root disease in host trees often results

in high endemic beetle populations. In

such circumstances, stand disturbances

may trigger outbreaks which develop

more quickly and are longer lasting.

Semiochemicals (message-bearing

chemicals), emitted by the beetle and the

host tree upon attack, regulate the behavior of beetles during their mating and

aggregation phase. Certain elements of

beetle-produced pheromones, when combined with specific host monoterpene

odors, serve to guide additional flying

beetles to the

tree and ensure that they are concentrated

in sufficient numbers to overcome resin

defenses of the tree. The attractive elements have been combined into a lure that

is effective in aggregating adult beetles

when placed on suitable host trees. Concentrating attacking populations can

make salvage or sanitation logging more

effective.

Other elements of this pheromone

complex serve to disrupt aggregation and

thereby ensure attack densities are optimum for brood survival. Release of this

antiaggregating component also serves to

divert responding beetles to nearby trees,

resulting in the characteristic group-kill

pattern. The antiaggregative component

has been used experimentally to reduce

the level of attack and survival in

windthrown trees well below optimum

levels. Its usefulness for protecting standing green trees has also been successfully

tested.

Use of synthetic pheromones for beetle

suppression is subject to registration by

the Environmental Protection Agency.

The Agency has determined that the

aggregative pheromone applied to trees

is exempt from registration. Registration

of the antiaggrative component is pending. Field tests to measure the effectiveness of these synthetic pheromones and

determine their most effective deployment are under way. Check with federal

or state forest entomologists for current

recommendations.
Long-term management of Douglas fir

forests offers the best method of preventing or minimizing damage by the

Douglas-fir beetle. Young stands should

be thinned periodically to maintain vigorous growth, reduce moisture stress, and

remove trees

injured by wind, disease, and other

agents. Managed stands which are

mature or overmature should be regenerated. Successful preventive practices

must also include salvage of windthrown

or infested trees before beetle broods

emerge from them.

References

Furniss, Malcolm N. 1979. An annotated bibliography of the Douglas- fir beetle,

Dendroctonus pseudotsuqae Hopk. USDA For. Serv. Gen. Gech. Rep. INT-48, 40 p. Intermt.

For. and Range Exp. Stn., Ogden, Utah.

Furniss, Malcolm M.; McGregor, Mark D.; Foiles, Marvin W.; Partridge, Arthur D. 1979.

Chronology and characteristics of a Douglas- fir beetle outbreak in northern Idaho. USDA For.

Serv. Gen. Tech. Rep. INT-59. 19 p. Intermt. For. and Range Exp. Stn., Ogden, Utah.

Furniss, Malcolm, M.; Livingston, R. Ladd; McGregor, Mark D. 1981. Development of a stand

susceptibility classification for Douglas-fir beetle. In Hedden, R.L., Barras, S.J., Coster, J.E.,

compilers. Proceedings: symposium on Hazard Rating Systems in Forest Pest Management, July

31August 1, 1980, USDA For. Serv., Gen. Tech. Rep. W.O. 27, 169 p.

Furniss, R.L. and V.M. Carolin. 1977. Western forest insects, USDA For. Serv. Misc. Pub. No.

1339, 654 p.
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(USDA) Forest Service is a diverse organization

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discriminated against should contact the Secretary,

U.S. Department of Agriculture, Washington, D.C.

20250, or call 202-720-7327 (voice), or

202-720-1127 (TDD).

Revised June 1996

R1-96-87