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DEPARTMENT CIRCULAR. 411 10 Unf

Ten, ‘+

Washington, D. C. Santon , 1927

THE RELATION OF INSECTS TO SLASH
DISPOSAL

Prepared by the Division of Forest Insect Investigations, Bureau of Entomology !

CONTENTS
Page Page
Slash a factor in forest protection__________--_- 1 | Pine slash—Continued.
Few species of trees involved______------------ 3 Monterey pine in California________ ify_ fi Q
LNTGHRIES 1a ae Se eee See ee 4 The SOupiernypineS? . 282 ea eee 9
White pine in the Northeast_________-__-- 2 s7oSpruce slush! 2!) )o)1ni24 El Ve eee 10
Red pine and white pine in the Great Lakes Spruce in the Northeast___.-_--_._-._-_- 10
CEs a) 0 ys Oa rll pes aang ee niet nt aa Scag 4 Spruce in the Great Lakes region__________ 10
Western white pine in the northern Rocky Engelmann spruce in the Rocky Mountain
WROD AN TORIOM 2 pine ee 5 TOONS =a he eee
Sugar pine in the Pacific coast region__---_ 6). Douglas fir slasho2 2+ Stes See ee 10
Westenm yellow pine 2.5 =e 3 Gul onclusions.2.- Aas fens ak eee ee ee 11
Jeffrey pine in California__..___-__-.--_--- Se; Hiterature cited. 2_ Jt) TS Ole Sie 12
sod gepole! pine. bax: steese. bes 2. esses 8 |

SLASH A FACTOR IN FOREST PROTECTION

Does slash from cutting operations increase the insect menace to
living timber? Many times this question has been asked by foresters
and timbermen, but there has been little uniformity in the answers.
Considerable confusion has arisen as a result, and a great diversity of
opinion is found concerning the influence of slash upon insect out-
breaks.

Two widely divergent opinions are held by professional forest ento-
mologists in North America. In Canada, Swaine (10, p. 18; 11, p.
762; 12, p. 25; 13, p. 47; 14, p. 231; 15, p. 346) and Hopping (6, 7)
advocate on entomological erounds the necessity of slash disposal as
a phase of forest protection. Workers in this country, Graham (2,
p. 447), for example, almost unanimously take the opposite viewpoint,
that under present economic conditions slash disposal can not be jus-
tified on entomological grounds. Chamberlin (1, p. 23), however,

1 This circular was prepared by entomologists and collaborators of the Division of Forest Insect Investi-
gations. Those primarily responsible for the information contained in the regional statements are as
follows: F.C. Craighead, northeastern and southeastern ferest regions; S. A. Graham, Lake States region;
J.C. Evenden, northern Rocky Mountain region; W. D. Edmonston and F. C. Craighead, southern
Rocky Mountain region; J. E. Patterson, north Pacific region; J. M. Miller, H. E. Burke, and H. L.
Person, California region. F. P. Keen and F. C. Craighead are Jargely responsible for assembling the
paper as a whole. In making reference to this circular it is to be understood that it is a product of joint
authorship, as explained in the foregoing.

? Figures in italic in parentheses refer to ‘‘ Literature cited,’’ p. 12,
31991°—27——1

2 DEPARTMENT CIRCULAR 411, U. S. DEPT. OF AGRICULTURE
j

recommends burning slash of western yellow pine for the control of
bark beetles. In England, Munro (8, p. 27), entomologist of the for-
estry commission, also recommends burning slash for control of bark
beetles. It is fully realized, however, that broad statements applying
indiscriminately to all forested regions are out of the question; the
problem varies with each forest, region, and type. Not only must
the species of tree and the character of the slash be considered but
the various species of insects that may be involved must be taken
into account.

It is recognized that, under special conditions, slash disposal or
seasonal timing of cutting operations is advantageous. Such cases
are referred to in the general text.

Hopkins discusses this question in many of his papers. His
general conclusions are adequately supported by all the detailed
investigations so far conducted by the Division of Forest Insect In-
vestigations. As early as 1899 (3, p. 23) he called attention to the
wasteful methods of lumbering in the Northwest and the attractive
breeding places for forest insects provided by the scarred trunks and
the débris left on the ground. Ten years later, in his monograph,
(4, p. 25-26, 48), he suggests that the cutting of living timber for
commercial purposes may offer favorable conditions for the multiplica-
tion of some of the species, although in some cases it may serve as a
protection to living timber, and suggests burning the slash after it
has attracted beetles away from the living trees. In one of his later
papers on forest entomology (5, p. 8) he broadly summarizes his
opinions as follows:

Since the problem of slash disposal is on the program of this meeting and,
since it is pretty generally considered by foresters and some entomologists that
slash is an insect hazard to the living timber, I want to say that the results of
more than thirty years of observations and some detailed study of the problem
indicate that as a rule, it is not. As inall rules, there are, of course, exceptions
to this one. ‘There are a few cases as related to certain types of forests, time of
year, and cases of sporadic cutting, where the slash is dangerous, not so much
from the msects that breed in the tops, logs, and stumps, as that it serves to
attract the tree-killing insects to the locality and from thus being concentrated
they attack and kill the living timber. Continued logging operations, after they
are once started within a given area, provide continuous breeding places for the
insects and their natural enemies and thus the slash serves as a protection to the
living timber.

The entomologists of the Division of Forest Insect Investigations
feel that a paper setting forth what isnow known about the entomo-
logical effect of slash would help to clear the situation and stimulate
more careful observation and compilation of further and needed data.
In this, as in most scientific problems, the last word can never be said,
and much investigative work is still needed to clear up many phases
of the slash problem. However, considerable work has already been
done, and certain tentative conclusions are presented in this circular.

There are hundreds of species of insects which breed in the slash
of forest trees, but few of these are capable of doing much injury to
living trees. They attack the base, the trunk, the tops, the limbs,
or the twigs of mature standing trees; others emerge from slash and
attack seedlings, saplings, and poles.

In addition to this, slash has a very strong attractive imfluence
which may result in bringing thousands of insects into the general
slash area with a resultant varying damage to living timber.

>

-

THE RELATION OF INSECTS TO SLASH DISPOSAL 3

The direct result of insect attack upon the slash itself is beneficial
because the insects aid in bringing about decomposition. In this
circular the direct influence of insects upon twigs, limbs, and tops
can be ignored and only the role of slash need be considered in
(1) supplying breeding material for insects which emerge and kill
mature standing timber or seedlings, saplings, and poles, and (2)
attracting insects from the surrounding forest and concentrating them
in the vicinity of the slash, where they kill living trees.

In general, the insects with which this discussion is concerned breed
in the same parts of a fallen tree as of a living tree. Insects which
attack the base of standing trees breed in stumps, those attacking
the main trunk breed in cull logs, and those attacking limbs and
twigs breed in the limbs and twigs composing the smaller portions of
slash. There are, however, some important exceptions to this rule,
such as certain species of Ips.

Probably the heaviest losses of all are attributable to the effect of
slash in attracting beetles from the surrounding territory and con-
centrating them in the vicinity, where they are able to do greater
damage to the standing timber than if their attacks had been scat-
tered over a wider area.

This attractive influence has been noted many times. The felling
of trap trees* has often concentrated attacks on adjacent healthy
trees, even in some cases in which the felled trap tree has not been
attacked. The beetle population is usually attracted to fire-scorched
areas, even though the beetles are often unable to breed successfully
in the trees weakened by fire. It is also usually attracted to places
where heavy windfalls have occurred or to trees that have been
broken down by snow.

It is evident that this class of damage can not be minimized through
slash disposal. In fact, burning the slash in the summer, which is
the flight period of the beetles, may even increase the attractive
influence and bring about a still greater concentration.

FEW SPECIES OF TREES INVOLVED

The problem can be further simplified by limiting the discussion
to such trees as are subject to primary damage from slash-breeding
insects. Relatively few trees are thus involved.

All hardwoods can be eliminated from the discussion, as practically
no insects which breed in hardwood slash are of importance as tree
killers. Among the conifers, the slash of fir (Abies), larch (Larix),
redwood (Sequoia), hemlock (Tsuga), cypress (Cupressus, Taxodium),
cedar (Thuja, Libocedrus, Chamaecyparis), and juniper (Juniperus)
either breeds insects of very little significance as tree killers, or the
trees killed are so few or of so little value as to be of small economic
importance.

As a result of these eliminations the problem in the United States,
viewed from an economic standpoint under present forestry practi-
ces, resolves itself into a consideration of slash from pine (Pinus),
spruce (Picea), and Douglas fir (Pseudotsuga). Each of these three
groups will be taken up in sequence.

3 Living trees girdled or felled at the proper time to attract the flying beetles to them and away from
healthy trees.

4 DEPARTMENT CIRCULAR 41], U. S, DEPT. OF AGRICULTURE
PINE SLASH

The pines suffer more than any other group from the attack of
bark beetles. It is not surprising, therefore, that pie slash should
be viewed with the greatest suspicion as a forest menace. Upon
study, however, the danger loses much of its alarming aspect.

WHITE PINE IN THE NORTHEAST

The only really serious bark-beetle enemy of the white pine (Pinus
strobus) which breeds in slash in the Northeast is the Pales weevil (Hy-
lobius pales Herbst). Studies by Peirson (9) indicate not only that
this beetle breeds in stumps and cull logs but that the adult is also
strongly attracted into slash areas for the purpose of feeding. It
kills great quantities of small reproduction by gnawing off the bark
until girdling results, and is frequently the limiting factor in obtain-
ing white-pine regeneration.

No satisfactory method of control has yet been devised. Burn-
ing the bark of cull logs and thoroughly charring the stumps, or
peeling the bark of logs and stumps (both of which are difficult and
expensive processes), may lessen the damage to some extent, but will
have no effect in lessening the attractive influence of the slash, which
is, after all, probably the ereatest source of danger. R. T. Fisher,
in his experimental silviculture on the Harvard forest, has found that
the only practical methods of preventing serious losses are (1) ob-
taining such an abundance of reproduction through reproduction
cuttings that there are still enough seedlings left to produce a stand
after the destruction wrought by the weevil, and (2) clear cutting,
and subsequent planting after an interval of three years.

RED PINE AND WHITE PINE IN THE GREAT LAKES REGION

Graham (2) reports that insect species that normally breed in the
slash of the red pine (Pinus resinosa) and white pine (P. strobus) in
the Great Lakes region have seldom been observed attacking or kill-
ing healthy trees. The insects responsible for the few exceptions to
“abe rule are all found breeding in the stumps and larger parts of the
slash.

The bark beetle Dendroctonus valens Lec. May occasionally kill trees,
and is one of the most common of the species that breed in fresh pine
stumps. It is almost never found breeding in parts of the tree more
than 6 or 8 feet above the ground. All recorded cases of injury
caused by this beetle to a standing tree have been associated with
the attractive influence of logging operations, windfalls, fires, or logs
in storage.

Several species of Ips occasionally kill trees and these, like the
Dendroctonus beetles, usually breed most abundantly in the larger
parts of the slash. The most common offenders of this group are
Ips pint Say, a primary pest of red and white pines, and J. calligraphus
Germar, which breeds in white pine. In the light of present knowl-
edge it ‘is safe to say that in the Lake States only the larger parts
of the trees constituting pine slash have any entomological signifi-
cance so far as living standing trees are concerned. Just how impor-
tant an entomological factor these parts are can not be said, but it is
certain that the stumps provide the most favorable breeding ¢ places.

ad

THE RELATION OF INSECTS TO SLASH DISPOSAL oO

The ordinary method of slash disposal, by burning, destroys only
the small, entomologically innocuous parts of the slash, and leaves the
parts most favorable for insect breeding. Any method of slash dis-
posal that will reduce the number of insects must take care of the
larger material. Even burning slash piled over stumps does not as
a rule sufficiently char the stump to prevent subsequent insect infesta-
tion. Thorough charring can ‘be accomplished only by keeping the
fire close to the sides of the stump throughout the process of burning.
Thorough charring of stumps is too expensive to be practical. Bark-
ing is cheaper, but must be carried down to the mineral soil if satis-
factory results are to be expected.

Unpublished results of experiments im Minnesota indicate that
heavy shade is unfavorable to most potentially injurious insect species.
In one experiment, piling the small parts of the slash over the stumps
very materially reduced the infestation. This experiment was on a
small scale and only about 20 stumps were used, but the results were
very clear cut. Pairs of stumps close together were selected; one
of each pair was covered with the smaller parts of the slash, the
other stump being left uncovered. ‘The uncovered stumps were found
to contain an average of 40 mother tunnels of Dendroctonus valens
per stump, whereas the covered stumps averaged only 4 each.

From the data at hand the writers are forced to conclude that slash
burning in the Lake States is unnecessary from the entomological
point of view. Covering the stumps and larger pieces of the slash
with the smaller pieces promises satisfactory results at a minimum
cost.

WESTERN WHITE PINE IN THE NORTHERN ROCKY MOUNTAIN REGION

The most serious insect enemy of the western white pine (Pinus
monticola) is the bark beetle Dendroctonus monticolae Hopk. This
species breeds successfully in cull logs from logging operations, and
also kills lhving trees. General observations by Evenden indicate
that during continuous logging operations the progeny of such beétles
as breed in the cull logs are absorbed in the new material made avail-
able, so that little damage results to standing trees. Because of the
value of this timber species very close utilization is practiced, and
ordinarily only a very small quantity of slash over 6 inches in diam-
eter is left in the woods. Occasionally, however, where a certain
quantity of such material is left in the woods at the close of a logging
operation, and nofresh material is provided, the insects which emerge
attack and kill standing trees in the vicinity, or seed trees left on the
area occupied by the slash.

As these beetles do not breed in the smaller tops and limbs the
present methods of brush disposal by burning ean have no possible
effect upon their number. And because of the close utilization of
the larger material other measures for preventing injury by insects
do not ordinarily appear to be justified. If, however, for any reason,
any quantity of large slash material is left in the woods and no fresh
material is provided to absorb the emerging broods, then as a pro-
tective measure the bark should be peeled from the infested cull logs
before the beetles emerge; otherwise some damage to standing trees
may result.

Two smaller species of insects (Pityogenes carinulaius Lec. and P.
knechtelr Sw.) breed in the smaller material from the tops. Occa-
sionally they breed in sufficient numbers to attack and kill small

6 DEPARTMENT CIRCULAR 411, U. S. DEPT. OF AGRICULTURE

white pine reproduction. These cases are of such sporadic occur-
rence and of such minor importance that they are negligible.

SUGAR PINE IN THE PACIFIC COAST REGION

The bark beetle Dendroctonus moniicolae Hopk. is also the principal
enemy of the sugar pine (Pinus lambertiana) in the forests of the
Pacific coast. As in white pine, it breeds both in cull logs and in
standing trees. This species of pine is not, however, subject to
heavy epidemics of infestation such as occur in yellow pine, and no
heavy killings have yet been traced to the slash of logging operations.
On the Yawkey tract, near Fort Klamath, Oreg.,in 1922 and 1923,
Patterson, Keen, and Hauge noted a considerable increase in the
numbers of standing trees killed shortly after many trees were blown
down by a high wind. Im general, the considerations applying to
utilization and slash disposal in the case of white pine apply to this
species as well.

WESTERN YELLOW PINE

In California and Oregon the western pine beetle, Dendroctonus
brevicomis Lec., is the principal insect enemy of the western yellow
pine (Pinus ponderosa). Besides attacking living trees this insect
breeds in the trunks of fallen trees which are more than 6 inches
in diameter.

In some cases it has been noted that heavy losses by insects have
followed logging operations, whereas in other cases logging areas have
been singularly exempt from such injury. Keen reports that on the
southern Oregon-northern California project two of the cleanest areas
were adjacent to areas where continuous logging had been in progress
for several years; that one heavily infested area was adjacent to a
logged area where operations had ceased; and that in one unit a
reduction of 60 per cent in infestation, equal to the best obtained by
artificial control work, followed the beginning of logging operations
on ‘adjacent ground.

To determine if possible the reason for this condition, Person
carried out some investigations in southern Oregon and central Cali-
fornia. Eleven logging slash areas, two road slash areas, and one
group of windfalls were studied. By bark counts made on 459 pieces
of slash it was found that the average number of attacking beetles
per square foot was 9.8,and the number of beetles emerging, 16.
Similar studies were made by Patterson in southern Oregon. Five
logging areas, one windfall area, and 24 miles of road slash were
studied; the last named involved the examination of 1,425 pieces of
slash, including the examination of 1 square foot of bark on each of
1,075 trees. Bark counts in this material gave 15.6 beetles per square
foot attacking and 19 beetles per square foot emerging. Bark counts
made on hundreds of standing trees near by showed an average attack
of 23 Dendroctonus beetles per square foot, with 54 beetles emerging.
It is evident that the attack and emergence on slash material is far
below that for standing trees. In all of these studies checks on the
surrounding infestation were Sa before, during, and for several
years after the laying down of the slash.

Usually the first effect of the slash was to increase the infestation
in standing treesin the vicinity. In every case, however, this increase
occurred simultaneously with the attack on the slash, instead of follow-
ing the emergence of the beetles breeding in the slash.

“/

THE RELATION OF INSECTS TO SLASH DISPOSAL 7

Numerous observations made by forest entomologists in the West-
ern States point conclusively to the fact that the western pine beetle
is strongly attracted to the vicinity of slash material, trees scorched
by fire, trap trees, and windfalls. As long ago as 1906, Webb (17, p.
80) pointed out that storm-felled and lightning-struck western yellow
pines became centers of infestation by this insect. If a concentra-
tion of the beetles is brought about through the presence of some
such material, they not only attack this material but go into the
standing living trees in the immediate vicinity.

From these investigations the conclusion is reached that the destruc-
tion of standing trees by insects, consequent upon logging operations
in adjacent territory, is not caused by the breeding up of insects in
the slash, but by the concentration in the slash of beetles from the sur-
rounding territory, and the failure of the slash to absorb them all.
This strong attractive influence can not well be avoided.

Since the western pine beetle breeds only in the slash material
more than 6 inches in diameter, such as cull logs and butts, the
present methods of slash disposal can be of little avail in reducing
the numbers of beetles. Neither can the very strong attractive
influence of the slash be avoided. The best way, therefore, to avoid
injury to neighboring standing trees is to supply continuously a
quantity of logs to absorb the broods of beetles, and, if necessary, to
reduce, through artificial control, the total population of beetles on
the area.

Several species of Ips (J. emarginatus Lec., 1. oregoni Hich., J. con-
fusus Lec., etc.) breed in the smaller pieces of the slash material as
well as in some of the larger pieces. When they become sufficiently

numerous, if there is a shortage of slash, they may kill young trees,

and the tops and limbs of larger trees. This damage is very sporadic,
and usually follows the interruption of logging operations in the sum-
mermonths. Keen has reported three small areas on the North War-
ner division of the Modoc National Forest, in California, where species
of Ips have caused heavy losses of reproduction after the cessation of
logging operations in the summer months. The conditions were com-
plicated by extremely dry weather. Usually a supply of fresh slash,
continuously laid down during the summer months, will absorb all of
these beetles and prevent injury to neighboring living trees.

In general it may be said that, in the light of present information,
no special methods of slash disposal need be recommended to avoid
insect damage to western yellow pine, except in the case of sporadic
cutting or when logging operations cease. In such cases infested
slash material should be destroyed before the broods of insects emerge.
The same general considerations apply to western yellow pine in the
northern Rocky Mountain region.

In the southern Rocky Mountain and Colorado Plateau regions
the Black Hills beetle (Dendroctonus ponderosae Hopk.) is the prin-
cipal insect enemy of the western yellow pine (Pinus ponderosa var.
scopulorum). Edmonston has reported that in Colorado the forest
areas showing least infestation are always those adjacent to places
where logging operations are in progress, particularly where the oper-
ations have been continuous for several years. Several officials of
the Forest Service have reported similar observations. Recent studies
by Craighead, Blackman, and Keen in the Kaibab National Forest
in Arizona indicate that the felled logs and culls act as traps for the

8 DEPARTMENT CIRCULAR 411, U. S. DEPT, OF AGRICULTURE

beetles durmg the fight period. When the material is sawn, practi-
cally all the beetles are destroyed, and in the logs that are left in the
woods during the winter a very high mortality of beetles occurs, so
that the emerging beetles are much fewer than those attacking. At
the same time the cull logs and tops furnish good breeding material
for certain borers (Acanthocinus) and predatory species which are
detrimental to the development of the Dendroctonus broods. As
these enemies increase with this abundance of breeding material they
tend to concentrate on any standing trees which are attacked, and
thus hold the infestation at a minimum. Under these conditions it
is doubtful whether disposal of the larger slash would not in fact be
more injurious than beneficial to neighboring standing timber.

In both the Rocky Mountain and the Pacific regions Dendroctonus
valens Lec. breeds in stumps and butt logs, and to a limited extent
it attacks the base of live trees. This injury is not often severe
enough to warrant the expense of special control methods, such as
piling brush over stumps, prompt removal of logs, and barking of cull
ogs.

In Arizona, New Mexico, and south of the Colorado Plateau, some
outstanding cases of losses to reproduction resulting from insects
breeding in slash have been called to our attention. Craighead
reports that two species of Ips were found breeding in cull logs and
larger tops of western yellow pine on timber sales. The spring
generation of beetles is confined to this felled material, whereas the
summer generations attack and kill pine reproduction in clumps of
from one-tenth to 1 acre. At some places these repeated attacks
have materially thinned out the stands of second growth. Modifi-
cations of logging operations will be necessary to prevent these losses.

JEFFREY PINE IN CALIFORNIA

The Jeffrey pine beetle (Dendroctonus jeffreyi Hopk.), although
often a very destructive insect in living Jeffrey pine (Pinus jeffreyr)
in California, is only weakly attracted to the slash of this tree. This
species and other bark beetles, such as Ips, breed very freely in wind-
falls and large pieces of slash material. On large windfall areas they
may occasionally breed up to the point where they become very
destructive to adjacent full-grown timber, saplings, poles, and the top
of mature trees. Such epidemics are, however, characteristically spo-
radic, and their control is not economically feasible at the present time
except in cases where high timber values are involved. As far as injury
by insects is concerned Jeffrey pine slash is usually of but slight
importance.

LODGEPOLE PINE

The mountain pine beetle (Dendroctonus monticolae Hopk.) isa very
serious enemy of lodgepole pine (Pinus murrayana), often killing
hundreds and even thousands of acres of the standing timber. Here
is another case, and a very striking one, of an insect which is strongly
attracted to the vicinity of logging or clearing operations, yet rarely
attacks the felled trees. Clearings for summer homes or hotels have
often brought about such a slaughter of lodgepole pines in the vicinity
as to mar the beauty of the site. Even control operations are not
immune, and in one instance such a heavy concentration of beetles
followed control cutting as entirely to wipe out the effect of the work.

THE RELATION OF INSECTS TO SLASH DISPOSAL 9

Nevertheless, a felled tree is rarely attacked. Here, again, it is obvi-
ous that slash disposal does not reduce the number of beetles, and the
concentration caused by the slash appears to be unavoidable.
Another lodgepole pine insect (Dendroctonus murrayanae Hopk.) has
habits similar to those of D. valens Lec., discussed under yellow pine.
Evenden has found this species of economic importance in crosstie
operations in Wyoming. Breeding readily in the stumps, it subse-
quently attacks the bases of the larger standing trees not included in
the sales. Repeated attacks of this kind weaken the tree to such an
extent that it succumbs to the attack of other bark beetles on the bole.

MONTEREY PINE IN CALIFORNIA

Several serious outbreaks of Ips on the Monterey pine (Pinus radiata)
near Monterey, Calif., have been accounted for as caused by wood cut-
ting and clearing, where the wood has been left piled in the forest.
Other factors, however, enter into this problem, since similar cutting
in other years has produced no epidemics. The turpentine beetle
(Dendroctonus valens Lec.), which breeds usually in stumps, often
becomes primary in its attack upon standing trees of this species.

Because of the value of the Monterey pine as an ornamental and
shade tree, special precautions in slash disposal may well be taken to
help minimize insect damage to it, which occasionally becomes serious.

THE SOUTHERN PINES

The southern pine beetle (Dendroctonus frontalis Zimm.) and three
species of Ips (J. calligraphus Germ., J. avulsus Eichh., and J. grandi-
collis Eichh.) are the most important pine insects of the South. The
first speciesmentioned rarely attacks anything but living trees, whereas
the three species of Ips are more commonly found in tops, cull logs,
and stumps than in standing pines. All these species are so intimately
associated in the destruction of living trees that it is impossible to
consider them separately. On the other hand, the infrequency with
which the southern pine beetle attacks slash, cull logs, or wind-blown
trees immediately eliminates it as an argument for the disposal of
such material.

Very few reports of dying pine timber-in the South that can be
associated with slash as a factor have been called to the writers’
attention, and extensive observations by several entomologists have
not revealed any striking examples. In fact, experimental cuttings
of timber, conducted throughout the year with the express purpose
of furnishing the breeding material for these insects, have utterly
failed to produce attacks in the surrounding standing trees. Never-
theless, there are many examples which show that these species are
all strongly attracted to areas where cutting is being carried on in the
summer. If sufficient material is on the ground they invariably
attack it, and occasionally small quantities of neighboring living tim-
ber are killed. For example, a freshly sawn pile of lumber, brought
to a home site for building purposes, attracted to that vicinity a suffi-
ey number of these insects to kill a number of trees adjacent to the

e

It is therefore reasonable to conclude that slash serving as breeding
material for these insects is not a menace, but that because of the
attraction which it exerts on them local logging or clearing opera-
tions in the summer should be avoided.

10 DEPARTMENT CIRCULAR 411, U. 8. DEPT, OF AGRICULTURE

SPRUCE SLASH

SPRUCE IN THE NORTHEAST

Several species of bark beetles breed in the stumps, logs, or tops
of spruce (Picea canadensis, P. rubens and P. mariana), and attack
and kill living trees.

The eastern spruce beetle (Dendroctonus piceaperda Hopk.) is a
particularly destructive species which breeds in stumps and cull logs.
It has destroyed large areas of mature spruce forests in the past, and
active epidemics are now in progress in Canada, even in second-
erowth stands. Recent observations by Craighead indicate that it
is most destructive only when accompanied by one or more species of
Ips, which combine with it and attack the upper stems.

Several species of Ips UJ. borealis Sw., I. perturbatus Eichh., and
I. chagnoni Sw.) have in two cases been observed by Craighead breed-
ing in slash, and in the following year attacking and killing near-by
living trees. From these, however, scanty broods developed, and in
the third year the outbreaks completely died out.

Special methods of slash disposal do not appear to be justified
from the entomological standpoint, but cull logs and larger débris
should not be left in the woods (16, p. 85). The increasingly closer
utilization practiced in this region with the development of the
pulp industry will undoubtedly tend to minimize such losses.

SPRUCE IN THE GREAT LAKES REGION

There is no specific evidence to prove that spruce slash in the
Lake States has any entomological significance, although there is
justification for suspecting that the larger pieces may be of some
importance. Most of the spruce (Picea canadensis and P. mariana)
cut in this region finds its way to pulp mills, a fact which makes
for close utilization. A comparatively small quantity of material
suitable for the breeding of potentially injurious insects is left in the
woods after logging operations.

ENGELMANN SPRUCEIN THE ROCKY MOUNTAIN REGION

The Engelmann spruce beetle (Dendroctonus engelmanna Hopk.)
breeds in the culls and stumps of the Engelmann spruce (Picea engel-
manni) and also attacks and kills living trees. The small quantity
of timber of this species which is now being cut makes of small
importance the danger from this source. No opportunity has yet
presented itself for the study of the effect of extensive logging on
this insect.

DOUGLAS FIR SLASH

There are several insects which breed in the large pieces of slash
of Douglas fir (Pseudotsuga taxifolia) and are capable of killing liv-
ing trees, though they rarely do so. The more important of these
are Dendroctonus pseudotsugae Hopk., Scolytus unispinosus Lec., and
Melanophila drummondi Kirby.

In the region along the Pacific coast reports on observations of the
death of Douglas fir from the attack of insects that breed in slash are
so infrequent that they can be disregarded. This fact may be due to

~~:

THE RELATION OF INSECTS TO SLASH DISPOSAL 1

pred

the extensive and continuous character of the logging operations.
Recently, however, reports from the British Columbia Forest Service
indicate heavy killing of Douglas fir adjacent to certain logging
operations on the coast of British Columbia.

In the northern Rocky Mountain region conditions are somewhat

different. The Douglas fir beetle (Dendroctonus pseudotsugae Hopk.)
is apparently much more aggressive in these less humid sections.
In recent years it has been causing considerable damage, and coming
more and more to be considered a serious insect. Evenden believes
that several local outbreaks under his observation are clearly attrib-
utable to slash which has resulted from logging operations, con-
struction of rustic buildings, spruce-budworm defoliation (resulting
in the death of many trees), or windfalls.
_ Two small bark beetles, Scolytus unispinosus Lec., and Pseudosy-
lesinus nebulosus (Lec.,) occasionally multiply in slash, and when no
more material is available they cause damage to reproduction and to
young trees.

The question of the destruction of Douglas fir by imsects needs
much more thorough study and more systematic observations before
any conclusions can be ventured.

CONCLUSIONS

The outstanding conclusions which have been reached to date in
regard to the entomological aspect of slash disposal are given here.

As far asisnow known, pine, spruce, and Douglas fir are the only
species of timber to which insects associated with slash are of any
important economic significance.

Comparatively few species of insects are capable of adapting
themselves to breeding in slash and killing living trees.

The great majority of the important tree-killing insects which
attack slash breed largely in the cull logs and butts. It follows that the
present methods of brush burning can have little influence on their
control.

Where logging operations are continuous, and a constant supply of
slash is provided, the potential danger from insects need cause no
concern.

Slash attracts from the surrounding forests insects which often
concentrate in standing timber in the vicinity of the cutting. No
method of slash disposal will avoid this effect.

In special cases where the value of the surrounding timber is
greatly enhanced, either from an esthetic standpoint as a building
site or for scientific purposes such as experimental thinnings in sample
plots, extreme caution should be exercised in felling green timber.
In such cases the attraction of the felled timber for insects is more
important than its service as a breeding ground, and all such work
should be done in the fall or winter, at least two months before the
eTowing season.

A large number of insects of secondary importance breed in the
twigs, limbs, and trunks of fallen trees, and sometimes become so
numerous as to kill reproduction, pole stands, and occasionally even
mature trees in the vicinity. Such outbreaks are, however, sporadic
in nature and of short duration, and, under present economic con-
ditions, do not warrant special methods of slash disposal.

12

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DEPARTMENT CIRCULAR 411, U. S. DEPT, OF AGRICULTURE

LITERATURE CITED

CHAMBERLIN, W. J

1920. THE WESTERN PINE BARK BEETLE. Oreg. Agr. Expt. Sta. Bul.
172, 30 p., illus.
GraHaM, S. A.
1922. SOME ENTOMOLOGICAL ASPECTS OF THE SLASH DISPOSAL PROBLEM.
Jour. Forestry 20: 437-447.
Hopkins, A. D.
1899. PRELIMINARY REPORT ON THE INSECT ENEMIES OF FORESTS IN
THE NORTHWEST. U.S. Dept. Agr., Div. Ent. Bul. (mn. s.)
PARA Ge oF
1909. PRACTICAL INFORMATION ON THE SCOLYTID BEETLES OF NORTH
AMERICAN FORESTS. I. BARKBEBTLES OF THE GENUS DEN-
pRocToNus. U.S. Dept. Agr., Bur. Ent. Bul. 83, pt. 1, 169 p.,
illus.
1922. BUDWORMINFESTATION VS. PULPWOOD PRODUCTION. Amer. Paper

and Pulp Assoc., Woodlands Sect. Proce. 2: 7-8.

Hoprine, R.

1915. THE ENTOMOLOGICAL ASPECT OF SLASH DISPOSAL. Soc. Amer.
Foresters Proc. 10: 183-185.
1921. THE CONTROL OF BARK-BEETLE OUTBREAKS IN BRITISH COLUMBIA.
Canada Dept. Agr. Ent. Branch Cire. 15, 15 p., illus.
Munro, J. W.
[1920]. SURVEY OF FOREST INSECT CONDITIONS IN THE BRITISH ISLES, 1919.

[Gt. Brit.] Forestry Comn. Bul, 2, 35 p., illus.

Pzrrson, H. B.

1921. THE LIFE HISTORY AND CONTROL OF THE PALES WEEVIL (HYLOBIUS

PALES). Harvard Forest Bul. 3, 33 p., illus.
SwaIne, J. M.

1914. FOREST INSECT CONDITIONS IN BRITISH COLUMBIA. Canada Expt.
Farms, Div. Ent. Bul. 7, 43 p., illus.

1917. SHADE TREE AND FOREST INSECTS IN MANITOBA. Agr. Gaz. Can-
ada 4: 755-763.

1918. CANADIAN BARK-BEETLES. PART II. Canada Dept. Agr. Ent.
Branch Bul. 14, 143 p., illus.

1919. SOME INSECT INJURIES IN WOODLOTS. Quebec Soc. Protect. Plants
Ann. Rpt. (1918/19) 11: 46-48, illus.

1919. THE BALSAM INJURY IN QUEBEC AND ITS CONTROL. Agr. Gaz.
Canada 6: 227-238, illus.

1921. sPRUCE BUDWORM INJURIES IN EASTERN CANADA. Illus. Canad.

Forestry Mag. 17: 345-346, illus.

CRAIGHEAD, F. C., and Bai.ey, I. W.

1924. sTUDIES ON THE SPRUCE BUDWORM [CACOECIA FUMIFERANA CLEM. |
Canada Dept. Agr. Bul. (nm. s.) 37, 91 p., illus.
Wess, J. L.
1906. SOME INSECTS INJURIOUS TO FORESTS. II. THE WESTERN PINE-

DESTROYING BARKBEETLE. U.S. Dept. Agr., Bur. Ent. Bul.

58, pt. 2, p. 17-30, illus.

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