Forest Health Protection and State Forestry Organizations March 2004 4.0 WEB Mar 2012 Bark Beetle General Biology Topics Life History 1 Mating Habits 2 Many species, but only a few are killers Adults and 3 Tables of 4 Bark beetles can be particularly Figure 1 illustrates the more Characteristics destructive pests with some rather common bark beetle species remarkable habits. Most belong to one encountered in the Interior West with a Tree Defenses, 5 subfamily of beetles, Scolytinae, and are reasonable facsimile of their galleries. Host Selection represented by some 600 species in the Tables 1and 2 list the characteristics of western United States. Here, we will the most common and destructive Other Guides address the more important and Dendroctonus beetles in the Mountain pine 4.1 commonly encountered species that Intermountain and Northern Regions. actually kill trees. Douglas-fir 4.5 Spruce Beetle 4.6 Western pine 4.7 Life History Red turpentine 4.2 usually branch out at right angles from Roundheaded 4.3 As a group, bark beetles have many interesting adaptations for their life the egg gallery. They feed within the Jeffrey pine 4.4 under the bark of trees. One feature, inner bark (cambium-phloem layer), which is distinctive to this family, is the (continued on page 2) Fir engraver 4.8 habit the parent beetles have of boring Pine engraver 4.10 tunnels under the bark for feeding and egg laying. Adults of most other insects Six-spined Ips 4.11 found under tree bark lay their eggs on Western balsam 4.9 the outside and the small larvae bore into the bark, or the female inserts the Pinyon Ips 4.12 egg through the bark with a strong ovipositor. Adult bark beetles bore Cedar bark 4.13 directly into the bark, construct a tunnel Field Guide and lay eggs. This behavior provides a strong identifying characteristic used to Management identify various species; such as, bark Guide beetle gallery patterns (See figure 1 on page 3). Key Points Many bark beetle adults construct Bark beetles are vertical galleries in which they feed and opportunists the female beetle deposits eggs. This gallery construction, in conjunction with They are the introduction of xylem-infesting generally fungi and feeding activity of larvae, attracted to less- eventually girdles and kills the tree. Bark beetle galleries generally include the egg vigorous hosts Following egg hatch, generally a 9-14 gallery along which the eggs are deposited, the larval day period, larvae feed and complete galleries which are mined by the developing larvae, and Multiple age class their development in galleries that the pupal cells in which the larvae pupate. trees and tree species diversity reduces impacts Page 2 Back to menu Bark Beetle General Biology 4.0 often lightly scoring the sapwood. A Larvae develop into pupae, few species complete larval typically in chambers formed at the end development and pupate in the outer of their feeding galleries. Pupae are bark. white, hairless, and reveal the Bark beetles undergo complete formation of adult features such as metamorphosis, passing through four eyes, legs, antennae, and wings. They Parent bark beetles stages of development during their life are approximately the size of the adult bore tunnels under cycle (egg, larvae, pupa, adult). All beetle (3-8 mm). the bark for feeding stages are at some time of the year Adult beetles are stout, found under the bark of their host tree cylindrical, have hard wing covers and egg laying. where developing beetles are protected (elytra). Their membranous flying from weather, parasites and predators. wings fold away neatly below the Small, white, shiny eggs are placed in elytra when not in use. Adults are niches along the side of the egg gallery. usually light yellow in color when first Once hatched, larvae are easily formed (callow adults), changing to recognized from their position and reddish-brown or black when mature. appearance. They are white, wrinkled, plump grubs with distinct heads, but no legs and are usually found in a “C”- shaped posture. They are found in galleries radiating from the main or central egg gallery. Mating habits influence gallery appearance Mating habits of bark beetles remain together in the egg tunnel while differ by species, the result being the female deposits her eggs. Since various and distinctive gallery patterns. there is no need to keep tunnels open, Many scolytids are polygamous—one monogamous beetles typically allow male beetle mating with two or more them to fill with boring dust. females. Each female constructs her Dendroctonus beetles are well known own egg gallery, starting from a central for this habit. Most Dendroctonus (nuptial) chamber beneath the bark. beetles found in the Rocky Mountain This results in a forked or radiating area have relatively straight egg tunnels pattern of egg galleries, as exemplified oriented with the axis of the tree and by beetles of the genus Ips. (See vary in length up to 36 inches. A illustration at left.) It appears that notable exception to this gallery pattern beetles mate frequently, so a common is the circuitous gallery constructed by A nuptial chamber may be feature in the polygamous-type gallery D. brevicomis. Individual larval constructed by a male that system is that the egg tunnels are clean. galleries are generally oriented mates with more than one These open runways allow free and perpendicular to the egg gallery and female. As the females each easy access to all branches of the may extend from either side, usually in produce their egg gallery gallery. Apparently the male beetle alternating patterns. Some of the away from the nuptial does most of the work in cleaning the smaller genera of monogamous beetles tunnels. maintain clean galleries. In the other mating arrangement, only one female is associated with each male. Monogamous beetles mate and 4.0 Bark Beetle General Biology Page 3 Back to menu Gallery patterns and adults of some important bark beetles Page 4 Bark Beetle General Biology 4.0 Back to menu Table 1. Identification of Dendroctonus species— stands and hosts. Host Tree Bark beetle Characteristics of Susceptible stands Location in Tree Ponderosa Mountain pine beetle Even-aged second-growth stands with BA >120 Trunk of trees up to 6” pine Dendroctonus ft2; old-growth trees with high risk rating (dbh diameter ponderosae >10”) and poor sites Ponderosa Western pine beetle Normally found in trees weakened by drought, Trunk of tree pine Dendroctonus brevicomis lightning, fires, etc. Under epidemic conditions can be found in unmanaged stands. Lodgepole Mountain pine beetle Old-growth stands (>80 years) with trees above 8” Trunk of tree up to 5” pine Dendroctonus dbh, and stand BA > 120ft2 in the lower elevation diameter ponderosae zone for lodgepole pine White pine Mountain pine beetle Larger diameter western white, limber, and Trunk of tree species Dendroctonus whitebark pines occurring in dense stands, dense ponderosae stem clusters. Engelmann Spruce beetle Infestations start in logging slash, windthrow, and Along the bole up to 6” spruce Dendroctonus rufipennis damaged trees. Susceptible stamds consist of dbh, in shaded areas of BA>150, average spruce dbh >16”, more than 65% down trees particularly of the overstory composed of spruce. along the underside of tree boles. Douglas-fir Douglas-fir beetle Stressed trees weakened by drought, fires, etc. Along the tree bole up to Dendroctonus BA>250ft2, dbh>14”, age>120, high percentage of 5” dbh. pseudotsugae host trees. Windthrow creates favorable habitat for beetles. All pine Red turpentine beetle Generally found at base of trees infested with other Attacks of beetle found Dendroctonus valens bark beetles and in stumps in a logging operation. in lower 10 feet of tree Table 2. Identification of Dendroctonus species— beetle Name of Insect Size of Color of Adult Insect Life Cycle Time of Attacking New Adult Tree Mountain pine beetle 1/5”-1/4” Black 1 year July through August Western pine beetle 1/8”-1/5” Black 1-2 generations per Spring to Fall with some year overlap between flights Jeffrey pine beetle 1/4”-3/8” Black 1 year July through August Spruce beetle 1/4” Dark reddish- 1-2 years June and July brown to black Douglas-fir beetle 1/5” Reddish to dark 1 year Early spring, extending brown to black into summer Red turpentine beetle 1/4”-3/8” Reddish 1 to 2 per year Mid-summer 4.0 Bark Beetle General Biology Page 5 Back to menu Beetles gang up to overcome a tree’s defenses Although bark beetles least a portion of it to stop this derive important advantages from pitching before they can establish a breeding and feeding beneath the brood. Gallery construction, bark of their hosts, the tree does attraction of other beetles, and have a method to defend itself from introduction of fungi are all attacks. Attacking beetles are often elements associated with beetles in danger of being drowned in their ability to overwhelm the trees’ galleries by tree pitch. In fact, the natural defense. beetles must kill the host tree or at Bark beetles use powerful attractants to produce mass- The importance of pitch attacks that overwhelm the original focus trees and adjacent trees in close proximity. The effect of pitch on the amounts of pitch. Successfully beetles influences their choice of a attacking beetles mix pitch with host tree. Research has shown that boring dust and push it out of the beetles are more tolerant of pitch entrance hole, forming what are produced by their natural host trees called "pitch tubes" on the outside than they are of pitch from other of the bark. Pitch tubes, then, tree species.