Bark beetles, family Scolytidae, are common pests of conifer trees (such as pines) and some attack broadleaf trees. Over 600 species occur in the United States and Canada with approximately 200 in California alone. The well known species of bark beetles are: the European elm bark beetle S. multistriatus and the large elm bark beetle S. scolytus, which like the American elm bark beetle Hylurgopinus rufipes transmit Dutch elm disease fungi (Ophiostoma); the engraver beetles, the red turpentine beetle, and the western pine beetle in California;

Bark beetles are so-named because the best known species reproduce in the inner bark (living and dead phloem tissues) of trees. Some species, such as the mountain pine beetle (Dendroctonus ponderosae), attack and kill live trees. Most, however, live in dead, weakened, or dying hosts. Bark beetles are ecologically and economically significant. Outbreak species help to renew the forest by killing older trees. Other species aid in the decomposition of dead wood. However, several outbreak-prone species are known as notorious pests and only a few very aggressive species are known to be the sole cause of tree mortality

Bark beetle have hard bodies, small, cylindrical shape and are the size of a grain of rice. Most species are brown, black or dark red. Their antennae are visibly elbowed with the outer segments enlarged and clublike. When viewed from above, the head is partly or completely hidden by the pronotum (the top of the body part behind the head). Bark beetles have strong mandibles (jaws) for chewing. A buckshot pattern of holes is apparent on the bark surface of infested branches or trunks where the new adults have emerged. Larvae of most species are off-white, robust, grublike, and may have a dark brown head.

Infestation Signs

Most of the time bark beetles attack trees that are already weakened by physical damage, drought, disease, smog, or other factors that may put a stress on a tree. Healthy trees may put up defenses by producing resin or latex, which may contain a number of insecticidal and fungicidal compounds that can kill or injure attacking insects, or simply immobilize and suffocate them with the sticky fluid. Under outbreak conditions, the sheer number of beetles can however overwhelm the tree's defences, and the results can be disastrous for the lumber industry.

Bark beetles mine the inner bark (the phloem-cambial region) of trees and shrubs on branches, twigs or trunks. This activity often starts a flow of tree sap in conifers, but sometimes even in hardwoods like elm and walnut. The sap flow (pitch tube) is accompanied by the sawdustlike frass created by the beetles. Frass accumulates in bark crevices or may drop and be visible on the ground or in spider webs. Small emergence holes in the bark are a good indication that bark beetles were present. Removal of the bark with the emergence holes often reveals dead and degraded inner bark and sometimes new adult beetles that have not yet emerged.

The species of tree attacked and the location of damage on the tree help in identifying the bark beetle species present.

On large pines, for example, engraver beetles usually attack trees near the top. Their presence is indicated by piles of dry boring dust pushed out on the bark surface. Engraver beetles maintain clean and open adult galleries under the bark.
Engraver beetle adults make shorter, compact gallery patterns that are made up of 3 to 4 egg galleries emerging from an open cell in the center. The larvae feed individually in mines much like the western pine beetle.

Red turpentine beetles attack the lower portion of the trunk. They can even colonize near the root collar and exposed roots and continue to mine under the bark below ground on the large roots. Red turpentine beetles presence is indicated by large, pinkish brown to white pitch tubes (a mixture of pine sap and beetle boring dust that appears on the lower trunk). The adults adults usually pack about 60% of their egg-laying galleries with a sawdustlike boring dust called “frass,”. Red turpentine beetle adults mine out wide cavelike galleries that progress down along the stem. Their larvae feed as a group in generally the same direction as the gallery.


Biology

Bark beetle females lay small, oval, whitish eggs just beneath the outer bark. After the eggs hatch, the tiny larvae mine galleries that branch out from the egg-laying gallery. At first the larval mines are very narrow, but they gradually increase in diameter as the larvae grow. Pupation occurs within or beneath the bark in enlarged chambers at the ends of the larval tunnels. Pupae are usually plump and whitish. Adults can emerge at any time of year, if they are fully developed and the temperatures are high, but emergence is most common in late spring and again in late summer to early fall. After emergence, adults may re-infest the same tree or, in most cases, disperse to attack susceptible trees elsewhere. Most bark beetle species have two or more generations a year in California, depending on temperature. At warmer locations (such as lower elevations away from the coast), the season of attack is usually longer and beetles have more generations per year in comparison with cooler coastal or high-elevation locations.

Control

Except for general cultural practices that improve tree vigor, little can be done to control most bark beetles once trees have been attacked. Because the beetles live in the protected habitat beneath the bark, it is difficult to control them with insecticides. If trees or shrubs are infested, prune and dispose of bark beetle-infested limbs. If the main trunk is extensively attacked by bark beetles, the entire tree or shrub should be removed. Unless infested trees are cut and infested materials are quickly removed, burned, or chipped on site, large numbers of beetles can emerge and kill nearby host trees, especially if live, unattacked trees nearby are weakened or stressed by other factors. Never pile infested material adjacent to a live tree or shrub.

Cultural control

Plant only species properly adapted to the area. Learn the cultural requirements of trees, and provide proper care to keep them growing vigorously. Healthy trees are less likely to be attacked and are better able to survive attacks from a few bark beetles. Where bark beetles have been a problem, plant nonhost trees. For instance, engraver beetles and red turpentine beetles do not attack redwoods or atlas cedars. A mixture of tree and shrub species in planted landscapes will reduce mortality resulting from bark beetles and wood borers.

Reduce Tree Stress. Pay particular attention to old, slow-growing trees, crowded groups of trees, and newly planted trees in the landscape. Large nursery stock or transplanted trees, notably oaks and pines, can become highly susceptible to bark beetles or wood borers after replanting. Transplanting success depends on the tree species and its condition, appropriate tree and site selection, characteristics of the planting site, the season of the year, the transplanting method, and follow-up care. Stresses placed on a tree caused by poor planting or planting at the wrong time of year, lack of proper care afterwards, or the planting of an inappropriate species for the site will increase a tree’s susceptibility to bark beetles or wood borers.

Prevention is the most effective method of managing bark beetles and related wood-boring insects; in most instances it is the only available control. Avoid injuries to roots and trunks, damage and soil compaction during construction activities, and protect trees from sunburn (sunscald) and other abiotic disorders. Irrigation may be important during dry summer months in drought years, especially with tree species that are native to regions where summer rain is common. Also, dense stands of susceptible trees should be thinned (complete removal of some of the trees) to increase the remaining trees’ vigor and ability to withstand an attack.

Irrigate when appropriate around the outer canopy, not near the trunk. Avoid the frequent, shallow type of watering that is often used for lawns. A general recommendation is to irrigate trees infrequently, such as twice a month during drought periods. However, a sufficient amount of water must be used so that the water penetrates deeply into the soil (about 1 foot below the surface). The specific amount and frequency of water needed varies greatly depending on the site, size of the tree, and whether the tree species is adapted to summer drought or regular rainfall.

Properly prune infested limbs, and remove and dispose of dying trees so that bark- and wood-boring insects do not emerge and attack other nearby trees. Timing of pruning is important; avoid creating fresh pruning wounds during the adult beetles’ flight season. Do not prune elm trees from March to September or pines during February to mid-October. Do not pile unseasoned, freshly cut wood near woody landscape plants. Freshly cut wood and trees that are dying or have recently died provide an abundant breeding source for some wood-boring beetles. Tightly seal firewood beneath thick (10 mil), clear plastic sheets in a sunny location for several months to exclude attacking beetles, and kill any beetles already infesting the wood. To be effective, solar/plastic treatment requires vigilance and careful execution. It is important to keep wood piles small, use high-quality clear plastic resistant to UV (ultraviolet light) degradation, and thoroughly seal edges and promptly patch holes to prevent beetles from escaping.

Biological Control

When bark beetles attack trees, natural enemies are attracted to feeding and mating bark beetles. The two main groups of natural enemies are predators and parasites. Predators are more important in regulating bark beetle populations than parasites. Natural enemies are unlikely to save an infested tree, but they can reduce bark beetle population size, thereby reducing the number of nearby trees that are attacked and killed by bark beetles. The release of predators and/or parasites into sites infested with bark beetles has not been an effective tactic to suppress bark beetle populations.

The following natural enemies attack the western pine beetle, but rarely control it: woodpeckers, several predaceous beetles such as the blackbellied clerid (Enoclerus lecontei) and a trogossitid beetle (Temnochila chlorodia), a predaceous fly (Medetera aldrichii), snakeflies, and parasitic wasps.

Behavioral Control

Bark beetles locate mates and attract or repel other individuals of the same species by emitting species-specific airborne chemicals called pheromones. Pheromones are naturally occurring chemicals that are widely used as baits to monitor bark beetles by attracting them to traps. These baits are especially important for detecting invasive species. Professional foresters have sometimes controlled or suppressed small local populations of bark beetles by using attractant pheromones in traps, and repellent pheromones and other behavioral chemicals to deter beetles from valuable trees. Some behavioral chemicals are being used experimentally on an area-wide basis to protect stands of forest trees. The interactions among host trees and beetles and their pheromones are complex and often poorly understood. Researchers are refining the reliability of pheromone-based management techniques. Behavioral chemicals are currently recommended for use only by specially trained professionals familiar with bark beetle management.

Chemical Control

Unless trees are monitored regularly so that bark beetle attack can be detected early, any chemical spray application made once the beetles have aggregated and penetrated the bark is likely to be too late and ineffective. Treatment must target the adults by spraying the bark so that beetles are killed when they land on trees and attempt to bore into the bark to lay eggs. Chemically treating trees that have been previously attacked will provide no benefit and could kill beneficial insects. Seriously infested trees, or trees that are dead or dying due to previous beetle attacks, cannot be saved with insecticide treatments and should be removed. Systemic insecticides, meaning those that are implanted or injected through the bark or applied to soil beneath trees, have not been shown to prevent attack or control populations of bark beetles. Although new systemic products are being investigated, they are not currently recommended for bark beetle control.

Highly valued, uninfested host trees may be protected by spraying their bark with a persistent, registered insecticide labeled as a preventive spray for bark beetles. Look for signs of recent infestation to help decide whether preventive spraying of nearby, lightly attacked or unattacked trees may be justified. Spraying a persistent insecticide on valuable, uninfested host trees near infested trees may be warranted to protect uninfested host trees from bark beetles. However, do not substitute preventive sprays for proper cultural care. The infestation status of a tree can be determined by inspecting the trunk or limbs for fresh pitch tubes or frass; peeling a small portion of the outer bark from the trunk or limbs and looking for signs of adult beetles or larvae; and inspecting the foliage for yellow or yellow-green needles or leaves. Frequently the infestation is diagnosed after the beetles have vacated the tree.