Pine Bark Beetles and Needle Cast Disease
Red-Topped Pines in the Urban Forest
A Guide to Recognition and Roles of
Pine Bark Beetles and Needle Cast Disease
L.L. Hyche, Associate Professor
Department of Entomology
Among pines, condition of the crown, especially color of foliage, generally reflects the overall health and condition of the tree. A full, dark green crown is characteristic of a normal, healthy tree; a fading, yellowing crown commonly indicates a decline in tree vitality; and a red or straw-brown crown (red-topped) (Photo 1) usually, but not always, means that the tree is dead.
Any one of several things, or a combination thereof, may cause or contribute to development of red crowns among pines, e.g., old-age decline, mechanical injury, lightning, prolonged drought, insects, or disease. The most common and serious causes with which homeowners, arborists, and urban foresters have to contend are pine bark beetles and needle cast, a disease of pine needles. Successful attacks by either of these agents produce similar red crowns, but result in different consequences for the trees. As a general rule, bark beetle-infested pines with red crowns are dead, or will die, as result of beetle activity. In the case of red-top caused by needle cast disease, only the needles are dead; trees are alive and, in time, will usually recover. Consequently, correct identification of the cause is important in determining a course of action. Results of research at the Alabama Agricultural Experiment Station provide information and clues for determining the identity and importance of these two common causes of red-top among pines in the urban forest.
The principal bark beetles (Family Scolytidae) commonly associated with red-topped pines in Alabama are the Dendroctonus beetles (D. terebrans, black turpentine beetle and D. frontalis, southern pine beetle) and the Ips engraver beetles (I. calligraphus, sixspined ips; I. grandicollis, eastern fivespined ips; and I. avulsus, small southern pine engraver). These beetles commonly attack most native pines, and species of both Dendroctonus and Ips may be found developing in the same tree. Adults of the two genera can be easily differentiated by differences in external structure. In Dendroctonus (Photo 2), the head of the adult is visible when viewed from above, and the elytra (wing covers) curve down smoothly over the rear end. In Ips (Photo 3), the head is not visible from above, and the wing covers slant down abruptly at the rear, and each cover bears tooth-like spines along the outer edge. The immatures (larvae, pupae) of both bark beetle groups are much alike in form, structure, and appearance (Photo 4), and are not easily separated by casual observation.
The general mode of development is much the same for both Dendroctonus and Ips. Attacking adults bore through the outer bark into the phloem (inner bark) and construct galleries, or tunnels, in which the females lay eggs. Larvae, which are legless grubs, feed, develop, and pupate in the inner bark region. Subsequently, new-brood adults bore exit holes through the outer bark and emerge to start a new cycle. Adults are present outside the tree for only a brief period and are not often seen. Most of the life cycle is completed under the bark, thus, developing stages also are not visible. However, there are distinct, characteristic signs present on and in infested trees that reveal beetle presence and aid in identification of the species.
Black Turpentine Beetle
The black turpentine beetle (Photo 2) typically attacks the lower 6-ft. portion of the trunk (Photo 5). Resin accumulates on the bark at each beetle entrance hole to form a large (up to walnut size) lump referred to as a pitch tube. Fresh pitch tubes are soft and white, becoming reddish to gray from wood borings and age (Photo 6).
Once in the inner bark, adults construct a broad tunnel (Photo 6) in which the female lays eggs. Larvae feed in a group and consume patches of inner bark. When attacks are numerous enough to result in girdling of the stem, the tree dies and the crown turns red or straw-brown. If girdling is not complete, trees may survive and crowns remain green. However, in such cases, trees weakened by black turpentine beetle may be attacked and killed by other bark beetles.
Completion of the life cycle of the black turpentine beetle requires 2 1/2 to 3 months. If infestations are detected early, this beetle can be controlled and tree mortality prevented (see Prevention and Control).
Southern Pine Beetle
The southern pine beetle (Photo 7) is perhaps the most notorious bark beetle associated with red-top in pines. Attacks occur throughout much of the trunk, usually extending from 2-3 feet above ground level upward to the beginning of the crown. Pitch tubes form at each point of beetle entrance. Southern pine beetle pitch tubes (Photo 8) are small, 1/4 to 3/8 inch in diameter, and often numerous. Adults (one pair to each entrance hole) enter the inner bark and construct narrow tunnels in which females oviposit. Tunnels (Photo 9) are typically S-shaped and wind across the grain of the wood. Larvae feed and construct short individual tunnels in the inner bark; pupation takes place at the end of larval tunnels. New adults emerge through individual holes cut through the outer bark, leaving the bark with many “shotholes” (Photo 10). The life cycle is completed in about 30 days.
Southern pine beetle adults introduce a blue-stain fungus (Photo 11) which invades the sapwood of infested trees. Girdling by the beetle and action of the fungus cause tree mortality. Within about 2 weeks following initial attack, green needles begin to fade and turn yellow. By the end of the beetle cycle (about 30 days), crowns are usually fully red, and the new-brood beetles often have already vacated the trees. Pines successfully attacked by southern pine beetle cannot be saved; however, measures can be taken to reduce the chance of spread of beetles to uninfested trees (see Prevention and Control).
Ips Engraver Beetles
Ips engraver beetles are among the most common bark beetles found in red-topped pines, but typically, are purely secondary invaders. They attack and develop in freshly cut, newly killed, and/or dying trees; however, they are capable of contributing to the death of weak, low-vigor trees. Ips are often found in trees infested with black turpentine beetle and/or southern pine beetle.
Three species occur in Alabama. External structure (Photo 3) for all is typically that of Ips, but size and number of spines on the rear of each wing cover vary by species: the sixspined ips is about 1/4 inch long with six spines per wing cover; the eastern fivespined ips is about 3/16 inch long with five spines; the small southern pine engraver is about 1/8 inch long and has four spines on each wing cover.
The Ips male initiates the attack and is subsequently joined in the inner bark by one to several females. Trees attacked by Ips are weak and low in vigor, consequently, pitch tubes seldom form at entrance holes. However, reddish-brown boring dust collects in bark crevices (Photo 12) below entrance points and provides external sign of Ips beetle activity. In the inner bark, females construct long narrow individual galleries that run with the grain of the wood. Several galleries may lead away from a single entrance point resulting in a pattern described as I- Y- or H-shaped (Photo 13). Females lay eggs in niches along the sides of the galleries. Larvae construct individual tunnels in the inner bark (Photo 14) and, pupate at the end of tunnels. New adults emerge through the bark leaving numerous holes. The length of the development period varies; 18 to 22 days for the small southern pine engraver, 22 to 30 days for the fivespined ips, and 28 to 35 days for the sixspined ips. Ips, like southern pine beetle, introduce blue stain into trees they attack.
Other Insects Associated With Red-Topped Pines
Dying, freshly cut, and recently killed pines are attractive to many species of insects which have nothing to do with the death of the trees. These insects are there simply because such trees provide them with preferred food and ideal breeding sites. Characteristically, these insects are not found in living trees, thus their presence is an indication of tree mortality. Among the most common of these associated with dead/dying pines in Alabama are southern pine sawyer, ambrosia beetles, and pinhole borers. Each of these arrives rather promptly at “dead” trees, sometimes even before symptoms of decline are noticed, and each provides readily visible evidence of its presence.
The southern pine sawyer is a common longhorned (“longhorned” refers to the long antennae of the adults) wood borer regularly found at dead, dying, and/or bark beetle-infested trees. The adult (Photo 15) is 1 to 1 1/4 inches in length and mottled gray-brown in color. Antennae of males are 2 to 3 times longer than the body; those of females are about as long as or slightly longer than the body. Adult coloration blends with that of pine bark, consequently, beetles are easily overlooked. However, females, in preparation for oviposition, excavate crater-like egg niches in the outer bark (Photo 16), and thereby, provide readily visible evidence of sawyer presence.
Eggs are placed in the inner bark at the bottom of the niche. Larvae, called roundheaded borers, feed in the inner bark until nearly full-grown, then bore into the wood where they pupate. Full-grown larvae (Photo 17) are about 1 1/2 inches long. Sawyer larvae tunneling in the wood (Photo 18) produce a rasping sound that is clearly audible outside the tree or log.
Ambrosia beetles and pinhole borers constitute a group of several species of small (1/16 to 1/4 inch-long), cylindrical, reddish-brown or brown beetles (Photo 19), with similar wood-boring habits. Attacking adults bore through the outer bark directly into sapwood and heartwood. In the process, fine, white, powdery wood dust is pushed out and accumulates in bark crevices of the lower trunk and on the ground at the base of the tree (Photo 20). This is a sure sign of the presence of these beetles, and a good indication that the pine is dead.
The term needle cast refers to a fungus-caused disease of needles. Among Alabama’s native pines, loblolly, slash, Virginia, and shortleaf seem to be most often attacked. Diseased needles die and crowns of severely infected trees become “red-topped” (Photo 21), a condition characteristic of beetle-killed trees. However, in the case of needle cast, only the needles are dead and, with no other problems present, the tree will usually survive.
Crowns of infected trees usually begin to show conspicuous browning in winter (December to January); however, close inspection of these crowns will often reveal some live needles (Photo 21). As the disease progresses, crowns may become completely brown (Photo 21). During spring, dead needles fall, new needles develop and, in time, crowns become green again (Photo 21).
Needle cast may be caused by any one of several fungal species capable of infecting southern pines. Specific identification of the fungus and descriptions of the disease will require the services of a plant pathologist, and are beyond the objectives of this article. However, there are signs associated with red-topped pines that, by their presence or absence, can aid the non-specialist in determining the specific cause and whether the tree is dead or alive. In the case of a red-top caused by needle cast, some of the best signs implicating the disease are signs that are absent, i.e., absence of any sign of bark beetles, sawyers, ambrosia beetles, pinhole borers, or injury such as lightning strike. Elimination of insects or injury leaves needle cast disease as a possible cause.
Prevention and Control
Pine bark beetles prefer and develop best in injured or weak, stressed trees, which often describes many of the pines growing in the urban environment. Chances of having bark beetle problems can be reduced by maintaining uninjured, healthy, growing trees. Steps to control black turpentine beetle or to prevent the spread of southern pine beetle and Ips to uninfested neighboring trees are often necessary. Recommendations on control and management of pine bark beetles in the urban landscape can be obtained from Extension entomology specialists.
Control of needle cast in urban forest trees is not usually necessary; infected trees, if healthy otherwise, usually survive. Positive identification of needle cast fungi, and information on life history and control can be obtained from Extension plant pathology specialists.
Among southern pines, a red crown is typically associated with tree mortality and is commonly considered to mean that the tree is dead.
Successful attacks by pine bark beetles and severe infections of needle cast both produce red crowns characteristic of dead pines. Crowns of beetle-infested trees are red because the trees are dead or dying. Crowns of needle cast-infected trees, however, are red because the needles are dead; the trees are alive, and will usually recover.
In cities and thickly settled urban and suburban areas, removal of trees may involve considerable expense. Removal of red-topped trees on the assumption that all are dead may result in the needless cutting of living trees. Among the red-crowned pines commonly encountered in Alabama’s urban forest landscape, readily visible signs and symptoms are present that can be used to determine if trees are dead or alive, infested with bark beetles, or infected with needle cast disease. A little time spent on tree inspection may save trees and money.