Ips Species of the Western United States

From: Furniss, R. L. and Carolin, V.M., Western forest insects. Misc. Publ. 1339. Washington, DC: U.S. Department of Agriculture, Forest Service: November 1977. 383 p.

Among the western bark beetles, Ips is second in destructiveness only to Dendroctonus. Twenty-five species are currently recognized in the West. Predominantly, they attack pines and spruces. In this manual they are discussed in 11 groups basically as proposed by Hopping (1963b), and subsequently modified by Lanier (1970a, 1970b, 1972, and in correspondence). The adults are reddish-brown to black, often shiny, cylindrical bark beetles ranging from about 3.0 to 6.5 mm long. A distinguishing feature of the genus is the pronounced concavity at the rear end (declivity) of the elytra of the adult, which is margined on each side with three to six toothlike spines. The number and character of these spines are major means for identifying the species. For that purpose, the spines are numbered from top to bottom. The kind of host, geographic location, and gallery pattern are other practical aids in identification.

Hosts and distribution of western species of Ips

The first evidence of attack by Ips beetles is yellow or reddish boring dust in bark crevices or little piles of such dust around the entrance holes or on the ground beneath. Pitch tubes are seldom formed, and the boring dust is usually dry and free from pitch. Rapidly in summer and more slowly in fall and winter, the color of the foliage changes progressively from green to yellow, sorrel, and reddish brown. Upon removal of the infested bark, the tunnels of the beetles will be found grooving the inner bark surface and, where the phloem is thin, lightly to deeply grooving the sapwood. The egg galleries differ from those of the Dendroctonus beetles in that, instead of being tightly packed with boring dust, they are open runways in which the adult beetles are free to travel the entire length. A second difference is their polygamous social habit of constructing a central nuptial chamber from which fork or radiate several egg galleries.

Attacks are made by these bark beetles with the coming of warm weather in the spring. An adult male bores through the bark and constructs a small cell or nuptial chamber in the inner bark. Several females then join in the work and each constructs an egg gallery in which eggs are laid in niches along the sides. The larvae, upon hatching, feed in the inner bark and work away from the egg galleries, leaving gradually widening, excrement-packed tunnels behind them. When their feeding is completed, oval pupal cells are formed in which the transformations from larvae to pupae and then to adults take place. The period from the time of attack to the emergence of the new brood is ordinarily about 1 ½ to 2 months. From two to five generations of these beetles may develop during the summer, depending on the altitude, latitude, and species, there being considerable overlapping of generations. The winter is usually spent in the adult stage, although occasionally eggs, larvae, and pupae are found. Some species congregate in large groups under the bark of standing trees and feed on the inner bark. Others emerge and hibernate under the bark of old stumps, among the bark scales, or in crevices and litter at the base of old brood trees.

Ips beetles have a number of predaceous and parasitic enemies, but apparently these do not affect the numbers of the beetles so much as does the lack of suitable host material. Given a quantity of freshly cut slash or windfalls, a large beetle population is almost certain to be produced, but it will not long survive after the supply of this material is exhausted.

Since outbreaks in standing, healthy trees are sporadic and of short duration, the application of direct control measures seldom contributes much to reducing the damage. Efforts should be directed toward preventing outbreaks of destructive species by eliminating situations favorable to the development of excessive progeny. Thus, slash should be piled and burned before the Ips beetles emerge, or should be scattered in the open where the sun will dry it out and make it unsuitable as a breeding medium. These precautions are most needed with slash created during the spring and early summer, particularly in times of below normal precipitation. Prompt salvage of windthrown and storm-damaged trees will help lessen the likelihood of Ips outbreaks. At times direct control by felling and burning infested trees or by treating with chemicals may be needed on intensive use areas, as around homes and on recreation areas.

Species in Group 0 (Hopping 1963a, Wood 1966) are intermediate between Ips and Orthotomicus and have been recorded in both genera. The adults, about 3.0 to 3.5 mm long, are the smallest western Ips. The elytral declivity is nearly vertical and is armed with three slender spines on each side. Pines are the only hosts.

Ips latidens (LeConte) (= guildi Blackman) attacks weakened or dying pines, usually in the tops and limbs of mature trees and in the bole of pole-sized trees. Under favorable conditions it has demonstrated its ability to kill trees, particularly those weakened by dwarf mistletoe or drought, and in some instances, apparently healthy trees of small diameter. During the severe drought of the 1930’s, I. latidens was one of several bark beetles that bred in the top of “high risk” ponderosa pines in Oregon. During epidemics of mountain pine beetle in lodgepole pine, it sometimes develops in such numbers as to attack and kill many small trees. Its typical work consists of from two to five rather short, sometimes curved, egg galleries radiating from the central nuptial chamber.

Ips spinifer (Eichoff) (= sabinianae (Hopping)) closely resembles I. latidens but is appreciably more aggressive. It attacks the bole and larger branches of its principal host, Pinus sabiniana, frequently killing large trees, especially those damaged by fire or weakened by drought.

Species in Group I (Hopping 1963c) are about 3.5 to 4 mm long. The elytral declivity is nearly vertical and is armed with three spines as in Group 0. The egg galleries often form on overall “S” or “E” pattern and are unique among Ips in that two to five, usually four, eggs are laid in each egg niche. Spruces and pines are hosts.

The Sitka spruce ips, Ips concinnus (Mannerheim), commonly attacks the bole of injured, dying, and down Sitka spruce throughout its range, causing no appreciable economic damage. It closely resembles I. mexicanus, but differs in host.

The Monterey pine ips, Ips mexicanus (Hopkins) (= radiatae Hopkins) (Struble 1961), attacks the bole of living, injured, dying, and recently down pines. Usually it is a secondary species and is associated with other bark beetles in its attack. In California it sometimes is a significant pest of Monterey pine, especially in plantations. Outbreaks are most likely where weakened trees are growing near accumulations of fresh slash which are favorite breeding grounds for the beetle. Inland, I mexicanus commonly attacks lodgepole pine but does not become a pest. Up to three generations per year occur in coastal California. One generation may be the rule in the upper portions of its range. In areas where I. mexicanus is a problem, prompt disposal of slash larger than 80 mm in diameter is recommended.

Group II (Hopping 1963d) contains two species. Ranging from about 4.5 to 6.5 mm long, they are the largest western Ips. They usually have four spines on each side of the declivity, although the fourth one is quite small or sometimes missing. These species are unique in having the third spine emarginate at the apex. Only pines are attacked.

The emarginate ips, I. emarginatus (LeConte), is most frequently found associated with the mountain pine beetle in its attacks on the bole of ponderosa, lodgepole, and western white pines, and with the Jeffery pine beetle in Jeffrey pine, but occasionally kills trees by itself. The adults are dark brown, shiny, cylindrical bark beetles. Their work is characterized by the long, straight, nearly parallel egg galleries from 0.6 to 1.2 m (2 to 4 ft) long, which run up and down the tree to connect at different points. Owing to the similarity in length and width of the egg galleries, their work is often confused with that of the mountain pine beetles, with which they are so often associated. However, the presence of a nuptial chamber and the absence of packed boring dust distinguished the Ips galleries. In the northern part of its range this species has two completed generations a year, but in the southern part there is a number of summer generations with considerable overlapping of broods. This species has been included in control projects directed against Dendroctonus, but on its own has never required control.

Ips knausi Swaine usually is a secondary species associated with tree-killing Dendroctonus. In it habits, the character of work, and appearance it closely resembles its near relative, I. emarginatus, and may be considered the southern Rocky Mountain form of this beetle.

The two species in Group III (Hopping 1963d, Lanier 1970a) are the only four-spined Ips with the sutures of the antennal club strongly and acutely angled at the middle. They are structurally very similar, hence difficult to distinguish. Pines are the principal but not exclusive hosts.

Ips integer (Eichhoff) (Lanier 1970a) occurs widely in the West, but is most common in the Rocky Mountain Region. It breeds in the large portion of the bole of weakened and felled trees, principally pondeosa pine. It often attacks in combination with other bark beetles, such as Ips pini, and seldom if ever is primary. The adults are 4.5 to 6.2 mm long and have a prominent vertical ridge on the front of the head. This species constructs three or four straight longitudinal egg galleries that fork from the common entrance or nuptial chamber. The egg niches are so thickly and evenly spaced along the sides of the egg galleries as to give these a sawtoothed appearance - a distinctive feature of this species’ work.

Ips plastographus (LeConte) (Lanier 1970a) consists of two named subspecies. On the average, this species is slightly smaller than I. integer and the frontal ridge is lacking or inconspicuous. The galleries are constructed under the bark of the main bole and the pattern is very similar to that of I. paraconfusus. The typical pattern consists of three egg galleries from 10 to 35 cm long issuing from the nuptial chamber. I. plastographus maritimus Lanier occurs along the California coast in Monterey pine, Bishop’s pine and the shore form of lodgepole pine (Trimble 1924). It is not often primary in its attacks, usually being associated with I. mexicanus and Dendroctronus valens in the killing of trees weakened by fire or other causes. This subspecies may have up to five generations annually. The typical species, I. plastographus platographus (LeConte), occurs in the Sierra, Cascade, and northern Rocky Mountains, principally in lodgepole pine (Bright and Stark 1973). It is a secondary and often is associated with I. pini and Pityogenes. Some new adults emerge in the fall and construct irregular feeding galleries in fresh material. No eggs are laid in these galleries which may penetrate the sapwood 6 to 12 mm.

Group IV (Hopping 1964) contains two species in the West. They are four-spined Ips. The spines of the female are about equal in size; the second and third are connected at the base by a curved ridge. Pines are the principal but not exclusive hosts.

Ips bonanseai (Hopkins) is a Mexican species that occurs northward into southern Arizona and New Mexico. In appearance and habits it closely resembles I. pini.

The pine engraver, Ips pini (Say) (Sartwell et al. 1971), is a transcontinental species, one of the commonest bark beetles in North America, and at times a serious pest. Long known as I. oregonis (Eichhoff) in the West, it is distributed extensively in the interior forests of the Western States and Provinces where it breeds in almost any species of pine. It is most commonly found attacking and killing ponderosa, Jeffery, and lodgepole pine. Large numbers develop in such host material as windfalls, freshly cut logs, pieces of slash over 5 cm (2 in) in diameter, and in the tops and limbs of trees killed by Dendroctonus beetles. When suitable host material is plentiful, they frequently develop in such numbers as to become aggressive in their attacks on healthy living trees. Damage often follows droughty spring weather. Outbreaks are usually of short duration and seldom last more than one season. The most frequent damage is in the killing of young replacement trees from 5 to 20 cm (2 to 8 in) in diameter and the top-killing of older trees. During outbreaks, group-killing becomes widespread. In ponderosa pine the group resemble those killed by Dendroctonus ponderosae. The gallery pattern beneath the bark identifies the culprit.

The adults are reddish brown to nearly black and from 3.5 to 4.2 mm long. A typical sample of their work shows three or four egg galleries forking from a central nuptial chamber and running more of less longitudinally with the grain of the wood for a distance of 13 to 25 cm (5 to 10 in). There may be anywhere from one to seven females to each male, with as many egg galleries radiating from one nuptial chamber. There are from one to five generations of this species a year, depending on the locallity and the length of season. The parent adults often emerge and make a second and even a third attack, a habit that results in a confusing overlapping of broods. During late summer large numbers of beetles may attack and mine extensively under the bark without producing brood. The resulting galleries are mazelike. The winter is spent almost exclusively in the adult stage, either under the bark or in forest litter. Preventing these beetles from becoming too numerous through timely slash disposal and thinning of dense immature stands will do more to prevent damage than the application of control measures after damage has occurred.

Group V (Hopping 1964) contains one species. It is four-spined and unique in that the structures of the antennal club are straight and transverse. Pines are the only hosts.

Ips perroti Swaine (Reid 1955) extends westward to northern Alberta on pines. It is not recorded in the Western United States. In lodgepole pine slash in Alberta, this species has one generation with two broods annually. The larvae have the unusual habit of developing in heavily blue-stained pockets adjacent to the egg galleries.

Group VI (Hopping 1965a) are four-spined Ips with the third declivital spine being enlarged and conical at the outer end in both sexes. The species are 3.5 to 5.2 mm long and are stouter than most Ips. The front of the head is not swollen as in Group VIII. Two species attack spruce; one, pine.

Ips hunteri Swaine is recorded from spruce in the Southwestern United States. The adult is 3.5 to 3.9 mm long and resembles the male of I. pilifrons. The gallery system usually consists of two egg tunnels extending in opposite directions from the entrance and is oriented at random. The biology is not reported.

Ips perturbatus (Eichhoff) (= hudsonicus (LeConte), = interpunctus (Eichoff)) (Gobeil 1936) is abundant in northern coniferous forests. White spruce is its principal host. I. perturbatus breeds abundantly in logging and right-of-way slash and in tops of Dendroctonus-killed trees. In Alberta and British Columbia it sometimes kills trees in seed strips adjacent to logging operations. In Eastern Canada there is one generation annually. The females construct two sets of egg galleries the first year and sometimes a third set the following year. Winter is spent exclusively as adults in forest litter.

Ips woodi Thatcher apparently occurs throughout the range of limber pine, its only host. The biology is not recorded.

The species of Group VII (Hopping 1965b) are four-spined Ips that differ from all others in that the front of the head of the female is smooth, polished, and practically without punctures.

Ips borealis Swaine (= swainei R. Hopping) is commonly associated with I. perturbatus in white spruce, its principal host. The egg galleries are narrower than most other Ips within its range. Apparently this is a secondary species.

Group VIII (Hopping 1965c) until recently was ranked as the most confusing group of Ips. Discovery that females of the “species” in this group are of several types (Lanier and Oliver 1966) dispelled the confusion and reduced the named western species from 10 to 2 (Lanier in correspondence). They are spruce-infesting, four-spined Ips. The lower portion of the front of the head of the female is usually swollen and often bears a brush of hairs.

Ips pilifrons Swaine (= sulcifrons Wood, = utahensis Wood) is reported to be the most abundant species of Ips in Engelmann spruce in Colorado. There it attacks recently down trees, and has been credited with depriving Dendroctonus rufipennis of favorable breeding places in windthrown timber, thereby reducing the threat of a spruce beetle outbreak. In Arizona it has caused scattered top-killing of mature spruce.

Ips tridens (Mannerheim) (= amiskwiensis G. Hopping, = engelmanni Swaine, = dubius Swaine, = interruptus (Mannerheim), = semirostris G. Hopping, = yohoensis Swaine) (Bright and Stark 1973) is notable principally for its many aliases. They came about because of females are of several forms. I. tridens attacks only weakened and down trees, hence is not economically important.

Group IX (Hopping 1965d, Lanier 1970b) contains seven species, six of which are western. All attack only pines. Three of the western species are serious pests. The members of the group are unique in being five-spined. Usually the third and largest spine is notched on the lower side. The adults range in size from about 3.0 to 5.5 mm long.

The pinon ips, Ips confusus (Leconte) is appropriately named. Because of taxonomic confusion, most of the published information regarding “I. confusus” in California and Oregon actually concerns I. paraconfusus. The real I. confusus breeds primarily in pinyon pines and at times kills them extensively in Southwestern States. Such outbreaks commonly and quickly develop in trees that are injured or uprooted as in land clearing for range improvement. Damage is most significant in National Parks and Monuments, around homes, and on other areas of intensive use. Three and sometimes four generations are produced annually. From November through March the adults hibernate in masse under the bark of standing trees, mostly in the basal portion of the bole (Chansler 1964). Prompt disposal of slash to prevent population buildups is highly desirable where pinyon occurs extensively.

Ips cribricollis (Eichhoff) (= cloudcrofti Swaine)is a secondary enemy of ponderosa pine in New Mexico. Southward it attacks various other pines in Mexico. Ranging in length form 3.1 to 3.9 mm, it is a smaller replica of I. confusus.

Ips hoppingi Lanier (Lanier 1970b) occurs on Mexican pinyon in southern Arizona and southward in Mexico. Its habits are not recorded.

The Arizona five-spined ips, Ips lecontei Swaine (Massey 1971), closely resembles I. confusus but differs in that the first and second teeth on the declivity are more widely separated. Ponderosa pine is the principal host of I. lecontei. It is ranked as the most destructive pine bark beetle in central and southern Arizona where it frequently kills sapling and pole-sized ponderosa pine extensively in groups up to 100 or more. This species has the reputation of being strongly attracted to freshly cut pine, even pine lumber, and attacking trees nearby.

Males are polygamous; there is an average of three galleries per nuptial chamber. The upper bole is attacked first; then the lower bole is filled in either by this or other species of Ips or by Dendroctonus. There are three generations annually. The adults overwinter in colonies forming irregular galleries under the bark of the bole and usually occurring in greatest numbers between 1.5 and 3.0 m (5 to 10 ft) above ground level. Outbreaks are of short duration, hence, it is more practical to prevent them than to try to control them. Prompt disposal or treatment of slash over 7.6 cm (3 in) in diameter produced in spring or early summer is necessary to prevent outbreaks of this beetle.

Ips montanus (Eichhoff) (= vancouveri Swaine) resembles I. confusus, but it is slightly larger and considerably less aggressive. I. montanus usually attacks decadent, weakened, and down western white pine, its principal host, and commonly occurs in association with Dendroctonus ponderosae. The gallery pattern is radiating, longitudinal type with three to five short egg galleries extending up and down the tree from the nuptial chamber. Two generations per year apparently is the rule.

The California five-spined ips, Ips paraconfusus Lanier (Bright and Stark 1973, Lanier 1970b, Struble 1966, Struble and Hall 1955) closely resembles I. confusus. They differ in hosts and range. Most of the published information concerning I. paraconfusus will be found under the name of its look-alike kin. The California fivespined ips occurs from southern Oregon to southern California west of the summit of the Cascade and Sierra Nevada Mountains. All species of pines within this range may be attacked. In the central Sierras, ponderosa pine is especially susceptible. This bark beetle kills saplings, poles, young trees up to about 65 cm (26 in) in diameter and the tops of even larger trees. During spring it breeds in fresh slash and recently down trees. During summer the broods that develop in such material often emerge and kill nearby living trees. Outbreaks characteristically build up and subside in 1 year. At times young ponderosa pine is killed extensively in groups up to 500 or more trees. Top-killing by I. paraconfusus sometimes stimulates attacks by Dendroctonus brevicomis and thus contributes to outbreaks of this destructive beetle.

The egg galleries comprise from three to five nearly straight tunnels radiating from a central entrance chamber. The typical pattern consists of three galleries in the form of a tuning fork or an inverted “Y”. Males attack first. When an attack is successful, a potent attractant is produced which leads to mass attack and characteristic group-killing. Attacks are started early in the spring and from two to five generations of beetles may develop during the summer.

In the northern part of the range, at an elevation of about 1,000 m, there are usually two summer generations which develop in fallen logs and a third, or overwintering generation, which develops in standing trees. At lower altitudes and in the southern part of the range there are from three to five summer generations. Most of the beetles overwinter in the adult stage under the bark of recently killed trees. In midsummer and fall, saplings and poles may be attacked and the phloem extensively fed upon by adults that produce no brood.

Some attempts have been made to control outbreaks of this beetle by salvage logging, burning, or chemically treating infested trees during the winter and early spring. Usually such methods are not warranted, as outbreaks are brief and sporadic and can be avoided if roadway, powerline, or other slash created in late winter to early summer is burned or lopped and scattered where it will be fully exposed to the sun. Such precautions are especially important in years showing a marked deficiency in spring precipitation.

Group X (Hopping 1965e) stands apart as the only six-spined Ips. It contains two species, both of which occur in the West on pines. Neither is economically important in the West. The adult range from about 4.0 to 6.5 mm long.

The six-spined ips, Ips calligraphus (Germar) (Ciesla 1973, Lanier 1972), is transcontinental species, consisting of two subspecies. The typical form, I. calligraphus calligraphus (Germar) (Wood and Stark 1968), occurs principally in the East but also in California where it presumably was introduced. In California it is a secondary enemy of ponderosa pine, attacking trees that are down or those dying from attacks of more destructive insects such as Dendroctonus brevicomis, Melanophila californica, and Ips paraconfusus. The thick-barked portions of the bole are preferred. The gallery pattern consists of two to six egg galleries extending 25 to 38 mm up and down the tree from the nuptial chamber. Though the pattern is similar to that of I. pini, the galleries are wider and etch the wood deeper. In California there are four generations annually. The subspecies, I. calligraphus ponderosae Swaine, occurs extensively in the West, principally on ponderosa pine. It is somewhat larger than the typical form of the species. The biology is not recorded.

Ips interstitialis (Eichhoff) is a southwestern species that closely resembles I. calligraphus in appearance and presumably in habits.

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