Dendroctonus species of the western United States
From: R.L. Furniss and V.M. Carolin. Western forest insects. Misc. Publ. 1339. Washington D.C.: U.S. Department of Agriculture, Forest Service: November 1977. 346 p.
Members of the genus Dendroctonus (meaning tree killers) (Hopkins 1909, Wood 1963) are by far the most destructive group of bark beetles in the North America. Twelve species occur in the West. Most of them are significant in forest management, hence practicing foresters should recognize and take into account the ones in their areas and forest types. All species breed under the bark of the trunk of living or dying trees or in fresh stumps or logs of various conifers. Some species attack only felled, weak, or dying trees, whereas others attack and kill apparently healthy trees, especially during epidemics.
The adults are stout, cylindrical, dark, reddish-brown to black bark beetles ranging from about 3 to 8 mm in length. The eggs, larvae, and pupae are similar to those of other bark beetles (Thomas 1965). Dendroctonus adults work in pairs, boring through the bark and then extending an egg gallery between the bark and wood. Egg galleries differ by species of beetle. Some wind a tortuous manner, crossing and recrossing the galleries made by other pairs of beetles, while others are straight and parallel to the grain of the wood. Dendroctonus egg galleries are always packed with boring dust, except for the portion where the parent beetles are working. This will distinguish the work of the Dendroctonus beetles from that of most other groups of bark beetles.
Trees attacked by Dendroctonus beetles can at first be detected by reddish boring dust caught in bark flakes or crevices and around the base of the tree, or by pitch tubes that form on the bark at the mouth of the entrance tunnels, but in heavily attacked or decadent trees pitch tubes are often either missing or so small that they can be seen only from a short distance. Later, discoloration of the foliage furnishes a more noticeable evidence of attack. Is is difficult, however, to correlate accurately the discoloration with the status of brood development, as this varies with different tree species, regions, and seasons. Sometimes trees don’t discolor until after the brood has matured and flown away. The most conclusive evidence of attack is the egg and larval galleries on the inner surface of the bark. These form a pattern so characteristic for the work of each species that, when considered with locality and host tree, the identification of the species responsible for the attack is relatively certain.
The roundheaded pine beetle, Dendroctonus adjunctus Blandford (= convexifrons Hopkins) (Chansler 1967, Stevens and Flake 1974, Wood 1963), occurs in Colorado, Utah, Nevada, Arizona, New Mexico, and southward in Mexico. In the Southwest, especially in New Mexico, it is destructive in stands of overstocked, pole-sized ponderosa pines. In southern Nevada, mature and overmature ponderosa pines have been extensively killed by this beetle on high-use areas. Limber pine, Chihuahua pine, and several Mexican pines also are recorded hosts. Outbreaks are sporadic and short-lived. Attacks are made in the basal portion of the bole, often in trees previously attacked by other species of Dendroctonus or by species of Ips. In dense young stands, trees usually are killed in groups of 3 to 15; sometimes up to 100. Recently felled trees also are attacked.
In southern New Mexico, there is one generation annually; attacks are made principally in October and November; eggs and parents overwinter. The foliage of attacked trees fades the following May or June. From the entrance hole, the egg gallery extends horizontally in the cambium region, either left or right, for 25 to 50 mm and then winds longitudinally with the grain an average distance of 300 mm. Galleries of neighboring pairs often cross, but retain an overall logitudinal pattern. Eggs are laid individually in niches on alternate sides of the egg gallery. The larvae mine across the grain in the cambium region until the third instar, then they bore into the outer bark to complete development. The adult is dark brown, averages about 5 mm long, and is similar to D. ponderosae but is more slender.
Penetrating oil spray has been used for direct control on high-use areas. Thinning and control of dwarf mistletoe in dense young stands of commercial forest likely will minimize potential killing by this beetle.
The larger Mexican pine beetle, Dendroctonus approximatus Dietz (= parallelocollis not Chapuis) (Wood 1963), occurs in Colorado, Utah, Arizona, New Mexico, and southward in Mexico. Hosts are Pinus ponderosa, P. engelmannii, P. leiophylla, and several Mexican pines. It is a secondary species, attacking near the base of trees infested by D. ponderosae, D. adjunctus, D. brevicomis, and species of Ips. It also attacks injured trees and the lower side of recently-down trees.
D. approximatus resembles D. adjunctus, but generally is larger and the gallery pattern is different. The adult Mexican pine beetle is dark brown to black and averages about 6 mm long. The individual egg gallery is elongate, winding, and much branched. Overall, the galleries form a crisscross network. Unlike other species of Dendroctonus, the eggs of D. approximatus are laid in niches on the bark side of the gallery rather than in contact with the cambium. The larval mines the pupal cells are entirely in the bark. Flight occurs from June to October, causing the attacks to be correspondingly strung out rather than concentrated. Adults and larvae overwinter. These is one generation annually. Direct control is unnecessary.
The western pine beetle, Dendroctonus brevicomis LeConte (= barberi Hopkins) (Keen 1955, Wood 1963), is periodically destructive to ponderosa pine and Coulter pine. It occurs in California, Oregon, Washington, British Columbia, Idaho, Montana, Nevada, Utah, Colorado, Arizona, and New Mexico. Normally this beetle breeds in overmature trees, in windfalls, in root-rotted trees (Cobb et al. 1974), or in trees weakened by drought, stand stagnation, or fires. Under epidemic conditions it becomes aggressive and kills apparently vigorous trees of all ages having bark sufficiently thick to protect the insect in its development. Trees under 15 cm (6 in) in diameter are seldom attacked, nor does this beetle breed in limbs. During the severe drought in the 1920’s and 1930’s, losses up to 60 to 90 percent of the ponderosa pine forest on extensive areas were recorded. Many stands of prime timber were ruined for commercial harvest. The heaviest losses of mature ponderosa pine have resulted from outbreaks of this insect in California, Oregon, and Washington. It is less important in the northern and southwestern portions of its range.
The western pine beetle is similar to D. frontalis but differs in having uniformly short hairs on the elytral declivity and in part by its distribution. The adult D. brevicomis is dark brown and about 3.0 to 5.0 mm long; other life stages are typical of bark beetles in general.
Flight and attacks start late in spring or early in summer and continue until stopped by cold weather. Parent females produce from one to three broods, causing much overlapping of generations. On the average there are three main periods of attack, hence survey and control crews recognize “spring,” “summer,” and “winter” broods. In the biological sense, there are one to two generations annually in the northern part of the range and from two and one-half to four generations in the southern portion, where activity continues almost without interruption throughout the year.
Initial attacks on a standing tree are made about mid-bole and subsequent attacks fill in above and below. Inconspicuous pitch tubes and red boring dust are indications of success in overcoming a tree. The galleries of individual pairs wind both laterally and longitudinally and are much branched. These galleries cross and recross each other in a mazelike pattern, much like D. frontalis. The larvae feed in the inner bark, working away from the egg gallery for about 12 mm and then turn into the outer bark, where they complete their development. Following successful beetle attacks, blue-stain fungi invade the sapwood. Of these, Ceratocystis minor (Hedg.) Hunt may play a pathogenic role. For detailed information on the biology and control of the western pine beetle, one should consult the comprehensive summary by Miller and Keen (1960).
The western pine beetle frequently attacks a tree jointly with other insects, notably Ips pini, I. paraconfusus, and Melanophila californica in the top; Dendroctonus ponderosae and Ips emarginatus in the midbole; and Dendroctonus adjunctus and D. valens at the base. Sometimes the top-killing associates attack first, thus providing favorable breeding material for epidemic buildup of the western pine beetle.
Woodpeckers, Enoclerus lecontei, and Temnochila chlorodia are natural enemies of the western pine beetle. Many other insects and other small organisms are associated with this beetle in various ways (Stark et al. 1970). Abundance of the western pine beetle is most often determined by climatic influences and the resistance of the host tree. Winter temperatures of -29° C (-20° F) and lower have been found to cause heavy brood mortality. Rapid, vigorous tree growth increases host resistance and discourages epidemics.
During an extended drought in the 1920’s and 1930’s, this beetle was widely epidemic in the Pacific Coast States. Direct control, largely by felling infested trees and burning the bark, was applied to protect commercial stands for later harvest (Miller and Keen 1960). Control measures were repeatedly applied because the beetle bred prolifically in the drought-stricken trees. Epidemic conditions continued prevalent until the 1940’s when rainfall increased and tree vigor improved. Since then, lesser outbreaks have occurred or threatened from time to time. Sanitation-salvage logging is the measure now generally applied to minimize losses caused by the western pine beetle. High-risk trees are removed from the stand and utilized, thus depriving the beetles of favorable breeding material (Keen 1943). The use of pheromones in control is under intensive study but has not been developed to the stage of practical use. In parks and in other area of intensive use, direct control still appears to be the measure of last resort in curbing this beetle.
The southern pine beetle, Dendroctonus frontalis Zimmerman (= arizonicus Hopkins) (Bennett and Ciesla 1971, Dixon and Osgood 1961), is a very destructive enemy of pines in the Southeastern and Gulf States. In Arizona and New Mexico it attacks ponderosa pine, Chihuahua pine, and Apache pine, but is not recorded as a serious pest in those States. The adults are dark brown, average about 3 mm long, and resemble D. brevicomis. They differ in that the hairs on the elytral declivity are sparse and much longer than the width of an interspace; whereas those of D. brevicomis are abundant and much shorter than the width of an interspace. The long winding egg galleries of D. frontalis cross and recross each other, giving a labyrinthlike effect very similar to the gallery pattern of D. brevicomis. A notable difference is that the pupal cells of D. frontalis frequently show on the inner bark surface; those of D. brevicomis do not.
The southern pine beetle has four to seven generations each year in the Southeastern and Gulf States. Outbreaks characteristically develop and subside rapidly. A blue-stain fungus, Ceratocystis minor (Hedgc.) Hunt, occurs symbiotically with D. frontalis, each of these organisms apparently helping the other overcome a tree. In the Southwest, the biology of the southern pine beetle is presumed similar to that in the Southeast and South, but the generations likely are fewer.
The Jeffrey pine beetle, Dendroctonus jeffreyi Hopkins (Smith 1971), is the most destructive bark beetle enemy of Jeffrey pine which is its only host. In the period of 1930-1970, it killed about 55 million fbm annually in California. Normally it breeds in scattered individual mature and overmature trees that are retarded in growth rate. It also attacks lightning-struck trees and recently windthrown trees but does not breed in slash. During epidemics it kills groups of up to 20 to 30 trees regardless of age or vigor.
The adult Jeffrey pine beetle is very similar to that of D. ponderosae, but generally is somewhat larger (Lanier and Wood 1968). The gallery pattern also is similar. There is a slight turn at the bottom of the egg gallery, which then proceeds up the tree in nearly a straight line following the grain of the wood. These galleries are usually 60 to 180 cm long and are packed with boring dust. The eggs are placed in individual niches in alternating groups along the sides of the galleries, and the larvae work out from the egg gallery across the grain of the wood. The pupal cells are formed in the inner bark and are exposed to view when the bark is removed.
As a rule, this beetle attacks the middle and lower bole of trees 30 cm (1 ft) or more in diameter but sometimes attacks trees considerably smaller. The entrance holes usually are in bark crevices. Initial attacks are marked by pitch tubes, later ones by reddish borings. A high percentage of trees killed by D. jeffryi are previously attacked in the top by Melanophila californica and some by Ips pini. Attacks occur principally in June and July but continue into October. Larvae and adults overwinter. One generation per year is normal in the northern part of its range; two may occur in the southern part.
Logging of high-risk trees is a practical measure for minimizing kill by the Jeffrey pine beetle and thus extending the life of old-growth stands. High-risk trees can be recognized by their poor vigor, declining growth rate, dying tops and branches, and short sparse foliage. Lightning-struck, recently windthrown, and currently infested trees should be promptly salvaged and converted to lumber before the attacking beetles emerge. When an outbreak exceeds salvage capacity, it sometimes is necessary to destroy the brood by felling and burning the infested trees, peeling the bark, or spraying with chemicals. Such direct control measures are applied during the fall, winter, and early spring.
The lodgepole pine beetle, Dendroctonus murrayanae Hopkins (Wood 1963), breeds in Pinus contorta and P. banksiana. The western distribution includes Alberta, British Columbia, Idaho, Montana, Wyoming, Colorado, and Utah. It mines in the lower bole and root crown of overmature, injured, and weakened trees and in fresh stumps and windfalls. Characteristically nonagressive, it occasionally kills overmature lodgepole pines left standing after timber harvesting. Normally only a few pairs attack a tree and two or more generations may be required to girdle and kill it. Some trees survive attacks.
The lodgepole pine beetle is very similar to D. punctatus and D. rufipennis, both of which attack spruces. The adult of D. murrayanae is black with reddish-brown elytra and averages about 6 mm long. On living trees, the attacks are marked by large pitch tubes. Upon reaching the cambium, the attacking female constructs an irregularly vertical gallery averaging about 12 cm long. Eggs are laid in groups of 20 to 50 along both sides of the gallery. The larvae feed away from the egg gallery, keeping together in a common excavation or brood chamber between the bark and the wood. Transformation to pupae and adults takes place in the uneaten part of the inner bark or in cocoonlike structures composed of frass in the brood chamber. In Utah one complete generation and a partial second annually appear to be the rule. Farther north, the life cycle presumably takes longer. No direct control of this beetle is recorded and none seem needed.
The mountain pine beetle, Dendroctonus ponderosae Hopkins (= monticolae Hopkins) (Blackman 1931c, Evenden et al. 1943, McCambridge and Trostle 1972, Wood 1963), ranks first in destructiveness among the bark beetles of the West. It ranges throughout the pine forest of British Columbia, Alberta, and the Western States into northern Mexico. Pinus contorta, P. lambertiana, P. monticola, P. ponderosa, and P. albicaulis are its principal hosts. P. aristata, P. balfouriana, P. coulteri, P. edulis, P. flexilis, P. monophylla, and several other pines are recorded hosts. In lodgepole pine, the mountain pine beetle infests mature forests, often decimating them over extensive areas. In ponderosa, western white, and sugar pines, group killing, often on a large scale, occurs both in mature forests and in young overstocked stands. Mostly, this beetle is a primary killer, but at times it occurs as a secondary, for example in association with D. brevicomis.
An outbreak of the mountain pine beetle from 1894 to 1908 in ponderosa pine in the Black Hills of South Dakota first called public attention to the extensive killing by bark beetles in the West. Between 1 and 2 billion fbm of pine were killed in that early outbreak. Since then many other outbreaks of the mountain pine beetle have occurred. Among the most noteworthy were the Kaibab, Ariz., outbreak of 1917 to 1926 that killed 300 million fbm of ponderosa pine and the series of outbreaks from 1925 to 1935 in Idaho and Montana that killed more than 7 billion fbm of lodgepole pine and vast numbers of whitebark pine. Western white pine in Idaho and sugar pine in California have repeatedly suffered heavy losses caused by this beetle. Somewhere in the West it is epidemic almost continually in one or more of its principal hosts.
Trees from 10 to 12.5 cm (4 to 5 in) in diameter up to those of the largest size may be attacked by the mountain pine beetle. Attacks are usually heaviest along the main trunk of a tree from within a meter or so of the ground up to the middle branches but may extend from the root collar very nearly to the top and into the larger limbs. During endemic infestations there is a tendency for the beetles to select the weaker, less vigorous trees for attack, but no such selection is evident during epidemic conditions. Infested trees are recognized first by pitch tubes on their trunk and red boring dust in bark crevices and on the ground at the roots; later, by discoloration of the foliage, as it changes from normal green to light greenish yellow, and then to reddish brown. The wood of successfully attacked trees soon becomes heavily blue-stained by Ceratocystis montia (Rumb.) Hunt.
The adults of Dendroctonus ponderosae are rather stout, black, cylindrical beetles 4 to 7.5 mm long and practically identical to those of D. jeffreyi. They excavate very long, perpendicular egg galleries through the inner living bark, engraving both bark and wood. The galleries may be nearly straight or slightly sinuous, and, sometimes, particularly in sugar pine, decidedly winding, and at the bottom of these galleries there is a short crook, or bend, 25 or 50 mm (1 or 2 in) in length. The perpendicular portion of the gallery ranges in length from 30 to 90 cm (12 to 36 in) and nearly always follows the grain of the wood.
Eggs are deposited singly in niches in groups on alternate sides during the construction of the egg gallery. These hatch in a few days, and the small white larvae excavate short feeding tunnels at right angles to the egg gallery. These feeding tunnels vary in length and are exposed on the inner bark surface. When fully grown, the larvae construct small pupal cells at the ends of the larval mines and in these transform to pupae and then to new adults. These pupal cells are usually exposed when the bark is removed, but in thick-bark trees they may be concealed in the inner bark. The new adults may bore away the intervening bark between pupal cells and congregate beneath the bark, prior to emergence from a common exit, or individual emergence holes may be constructed directly from the pupal cells. Emerging beetles also take advantage of cracks in the bark or holes made by woodpeckers.
The life cycle of the mountain pine beetle varies considerably over its wide and diverse range. One generation per year is the general rule. In portions of California, two and a partial third generation may develop, and in the coldest portions of its range, one generation may require 2 years. Larvae and adults are the overwintering stages.
Several natural factors affect the abundance of the mountain pine beetle, including sub-zero winter temperatures; nematodes; woodpeckers; predacous insects such as Enoclerus sphegeus, Temnochila chlorodia, and Medetera aldrichii; and the insect parasite Coelodies dendroctoni. As stand susceptibility to the beetle increases, the effectiveness of natural control decreases and outbreaks develop. This points to the desirability of harvesting susceptible stand prior to outbreaks or relieving stand stress as in the thinning of dense young ponderosa pine. Control of outbreaks by logging infested timber is good in theory but has proven of limited usefulness because of the large areas to be intensively covered in a brief time and because of the high cost. Direct control measures, such as felling and burning or peeling, and treating with oil or chemical sprays, were used for decades in many parts of the West. These measures now are considered generally uneconomical. This has led to a search for new methods. One such method being tested operationally is to concentrate the beetles during flight by the use of pheromone-baited traps (Pitman 1971).
The Douglas fir beetle, Dendroctonus pseudotsugae Hopkins (Furniss and Orr 1970, Walters 1956, Wood 1963), is the most important bark beetle enemy of Douglas-fir throughout the range of this tree in western North America. It also attacks western larch but produces brood only in down trees. Normally it breeds in felled, injured, or diseased trees. The resulting endemic mortality is large in amount but widely scattered. At times, D. pseudotsugae becomes epidemic and kills apparently healthy trees on extensive areas. In the coastal Douglas fir region from British Columbia to northern California, outbreaks are sporadic and of short duration but are likely to kill large amounts of timber. Usually they develop following extensive windthrow or large fires. In Rocky Mountain forests, outbreaks usually are of longer duration and commonly develop in trees felled by wind, broken by snow, or affected by drought.
Reddish or yellowish boring dust caught in bark crevices or around the base of trees is the usual evidence of attack by the Douglas fir beetle. No pitch tube is formed but resin may exude from the upper attacks. The foliage of attacked trees turns yellow, then sorrel, and finally reddish brown in late summer, in fall, or in early spring, depending upon the region, time of attack, and weather. Viewed from the air these “redtops” provide a means for assessing an outbreak (Greeley et al. 1953).
Adult Douglas fir beetles are dark brown to black with reddish elytra, rather hairy, 4.4 to 7.0 mm long, and similar to the smaller D. simplex and the more robust D. rufpennis, but distinguished from both by host. Douglas fir beetles work in pairs and construct egg galleries which are mostly in the inner bark, though they also slightly etch the sapwood. Typical galleries are perpendicular, usually straight or slightly sinuous, and average about 30 cm long, though they range about 12 cm to more than 90 cm. The first 2.5 to 5.0 cm of gallery angles to the side as with D. ponderosae. The eggs are pearl-white, about 1.2 mm long, and without surface markings. They are laid in masses of 10 to 36 in grooves at intervals along alternate sides of the gallery. The larval mines diverge from the egg groups and are extended through the inner bark close to the wood. They expand as the larvae grow, so the completed work from each group of eggs is somewhat fanshaped. The pupal cells, which are construted at the ends of the larval mines, may be exposed when the bark is removed from the tree, or they may be concealed in it, depending on the thickness of the bark. In these cells the transformation from larvae to pupae and then to new adults takes place. The new adults bore away the intervening bark between pupal cells and congregate, sometimes for rather long periods, beneath the bark. Finally they bore through the bark to the surface, emerge, and fly to make their attack on other trees.
The Douglas fir beetle has one generation annually. Adults and large larvae overwinter, with the adults predominating. Depending upon area and weather, the overwintering adults emerge and attack from April to June. Some of these adults reemerge and attack additional trees, establishing a second brood. Adults from overwintering larvae emerge and attack in July and August.
In the coastal Douglas-fir region, resistance of the host tree evidently keeps the Douglas fir beetle under control most of the time. Even when outbreaks occur, following stand disturbance caused by fire or wind, they abruptly subside because the beetles do not thrive in normal green timber. In interior forests tree resistance is a key factor in control, but trees are subject to greater stress and the effectiveness of natural control varies more than in coastal forests. Among the insects rated as important in natural control of the Douglas fir beetle are Enoclerus sphegeus, Thanasimus undatulus, Temnochila chlorodia, Coeloides brunneri, and Medetera aldrichii.
Direct control of the Douglas fir beetle is of dubious merit and has seldom been attempted. In the coastal Douglas-fir region, where outbreaks are brief but intense, prompt salvage of beetle-killed, wind-thrown, and fire-killed trees before the beetles emerge is the only practical course of action. In this coastal region, slash on clearcut areas does not create a beetle problem, but sometimes does on selectively cut areas. In inland forests, preventive control is obtained by thinning and harvesting. Slash has not caused tree-killing outbreaks of the Douglas fir beetle, but large amounts of cull stems probably should not be left after logging, especially if shaded. Stumps are reported to be significant sources of beetles in British Columbia but have not been involved with tree killing in the Rocky Mountain States. Aggregating pheromones are being developed for use in concentrating attacks in trees to be logged. An antiaggregative pheromone has been shown to prevent infestation of down trees (Furniss et al. 1974).
The Allegheny spruce beetle, Dendroctonus punctatus LeConte (= johanseni Swaine) (Wood 1963), is a transcontinental, little-known species found principally in the Far North. In the West, it attacks Picea glauca and P. sitchensis and is recorded in Alberta, Northwest and Yukon Territories, and Alaska. It attacks the lower bole and kills scattered trees principally in noncommercial stands. The adult is uniformaly brown and averages about 6 mm long. It resembles D. murrayanae and D. rufipennis, but differs in that the front of the head of the adult D. punctatus is not granulate. The female cuts a “C-shaped” gallery and lays clusters of eggs along the outer edge. The larvae feed out en masse, forming a cavity that becomes filled with resinous frasses. The life cycle is presumed to be 2 or more years.
The spruce beetle, Dendroctonus rufipennis (Kirby) (= borealis Hopkins, = engelmanni Hopkins, = obesus (Mannerheim) (Massey and Wygant 1954, Schmid and Beckwith 1975, Wood 1963) occurs throughout the range of Picea in North American. In the West, P. engelmanni, P. glauca, and P. sitchensis are principal hosts. Normally this beetle is present in small numbers in weakened or windthrown trees, large pieces of slash, and fresh stumps. Sporadic outbreaks have killed extensive stands of spruce in Alaska, western Canada, Colorado, Montana, and Utah. For example, the spruce beetle killed an estimated 3.8 billion fbm of spruce in Colorado from 1942 to 1948 (Wygant and Nelson 1949). Such outbreaks commonly develop in windthrown timber. During, epidemics, trees of all ages and diameters, except reproduction, are attacked, preference being shown for trees of larger diameter. Like fire and wind, the spruce beetle is a natural though destructive means for liquidating overmature forests and making way for the new.
The spruce beetle adult is dark brown to black with reddish wing covers, rather hairy, approximately 4 to 7 mm long, and closely resembles D. murrayanae and to some extent D. pseudotsugae. The egg gallery is vertical, 6 to 22 mm long, slightly wider than the beetle, and the basal portion usually is filled with pitchy frass. Eggs are laid in elongate grooves on alternate sides of the gallery. At first the larvae bore out en masse transversely. Later they mind individually and their mines frequently cross each other. The pupal cells are usually constructed in the inner bark, being exposed when the bark is removed, but are sometimes deeper in the bark and quite concealed in thick-bark trees.
The life cycle and habits of the spruce beetle differ widely in various portions of its vast range. Two years are required to complete a generation, from attack to attack, in the main body of Engelmann spruce stands. At high elevations 3 years may be required, and in coastal forests a 1-year life cycle is normal. In Sitka spruce in Alaska this beetle sometimes attacks and raises broods in a tree 2 or more years before killing it. In the Rocky Mountains, the principal flight, attack, and egg laying takes place when hibernated adults emerge after the snow disappears late in June and in July. Some of the parent beetles reemerge and establish another brood. Eggs hatch and larvae develop during the summer. The progeny pass the winter as half- to nearly full-grown larvae and complete development to adults by the following August. The new adults emerge and migrate to the basal trunk and root collar of the host tree from August to October; there they bore beneath the bark and hibernate until the ensuing June and July. Overwintering stages consist primarily of hibernating adults of the previous seasonal attacks and half- to three-fourths-grown larvae of the current seasonal attacks.
Spruce beetle populations are kept at low levels most of the time by a combination of natural control factors. During outbreaks the beetle outruns its natural controls, often for years, until much of the mature forest is killed. Woodpeckers are important predators in the Rocky Mountains. Among the insects, Coeloides dendrtoctoni, a parasite, and Medetera aldrichii, a predator, are most important. The nematodes, Sphaerularia dendroctoni Massey and Contortylenchus reversus Thorne significantly reduce the egg laying capacity of the infested female spruce beetles.
The harvesting of stands of overmature spruce by clearcutting is effective as a preventative measure when comprehensively done. Salvage of windthrown trees and infested standing trees before the beetles emerge from them is important both in prevention and control. Green trees systematically felled as “traps” absorb large numbers of beetles which then can be destroyed by treating or salvaging the infested logs. Infested trees and logs can be sprayed with chemicals or burned to kill the contained brood. While effective on individual trees, direct control measures are costly and the benefits are short-lived during extensive epidemics, hence management practices should be focused upon prevention. The start of an outbreak is difficult to detect, because the foliage does not fade until a year after attack, and it turns pale green only before dropping. There are no pitch tubes. First-year attacks can be detected only by the presence of brown boring dust around the base of trees. Woodpecker work also helps to identify the trees after about October 1, when the larvae are large.
The eastern larch beetle, Dendroctonus simplex LeConte (Hopking 1909, Furniss 1976), infects Larix laricina throughout the range of that tree which includes Alaska and the Western Provinces and Territories. As a rule, D. simplex attacks injured and recently down trees, and those weakened by fire, flooding, and the larch sawfly. However, an extensive infestation of normal-appearing trees in the Kantishna River drainage, Alaska, developed in 1973 and is continuing (1975). Successfully infested trees fade during late summer. The bark of the lower bole often is removed during the winter, presumably by birds feeding on the beetle broods.
The dark reddish-brown adult resembles D. pseudotsugae but is smaller (4.3 mm long) and differs in gallery characteristics, tree host, and geographic distribution. The broods overwinter as callow adults which mature in the spring. They fly and attack in May and June. Egg galleries are rather winding, commonly 30 cm long, and they lightly etch the wood surface. Turn-around niches are constructed at intervals along the gallery, thus distinguishing D. simplex from D. pseudotsugae. Eggs are deposited in niches in small groups (usually one to four per group) alternately on opposite sides of the egg tunnel. Larvae mine laterally in the phloem. By the time they mature, the inner bark usually is completely mined and the original gallery pattern practically obliterated. There is no record of direct control of this beetle in the West.
The red turpentine beetle, Dendroctonus valens LeConte (Eaton and Lara 1967, Smith 1961), occurs in practically all pine forests of the United States and Canada except in the Southeastern and Gulf States. It attacks all species of pine within its range and occasionally spruce and larch. In the West, Pinus ponderosa, P. contorta, P. jeffreyi, P. lambertiana, P. monticola, and P. radiata are preferred hosts.
D. valens normally attacks injured, weakened, or dying trees, and freshly cut logs and stumps. It often attacks leave trees following logging. Fire-scorched trees and trees in campgrounds and around homes in woodlands also are frequently attacked. Ordinarily this beetle is not aggressive and does not become epidemic. Through repeated attacks, it sometimes kills trees but more often weakens them, thus subjecting them to fatal attack by other bark beetles.
The adult red turpentine beetle, averaging about 8 mm long, is the largest in the genus. Its color is distinctly reddish brown. Woodsmen know it as the “barber beetle” because of its ability to clip hairs with its powerful mandibles. Attacks on a tree are characterized by large reddish pitch tubes at the point of entry. On burrowing beneath the bark, the beetles excavate short, irregular, longitudinal to cavelike galleries between the bark and the wood of the lowermost portion of the bole and of the root crown. The eggs are laid in elongate groups packed in frass along the sides of the gallery. The larvae feed through the inner bark in mass formation, producing a cavity ranging from about 1 to 9 dm² (0.1 to 1 sq ft) or more in area. Transformation to pupae and adults takes place in cells of frass in the brood chamber or in short mines along its margin. Attacks occur throughout warm weather but peak by midsummer. Winter is passed as adults and larvae. The number of generations varies from one in 2 years in the coldest portions of its range to two or three per year in the warmest.
In commercial forests, care should be taken to minimize injury to standing trees during logging, improvement operations, and road construction. High-value trees on intensive-use areas also should be protected from injury and soil compaction. Chemical sprays can be used to prevent attacks and to kill beetles that have attacked. Screening the lower bole during the flight period may be practical in protecting especially valuable trees.