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How to Manage Pests

Pests in Gardens and Landscapes

Bark Beetles Published 4/04 In this Guideline:
Identification Life cycle Damage Management About Pest Notes Publication Glossary

Bark beetles, family Scolytidae, are common pests of conifers and some attack broadleaf trees. Several hundred species occur in the United States. The most common species infesting pines in California are the western pine beetle (Dendroctonus brevicomis), engraver beetles (Ips spp.), and the red turpentine beetle (Dendroctonus valens). Cedar and cypress bark beetles (Phloeosinus spp.) attack arborvitae, Chamaecyparis, cypress, and redwoods. Oak ambrosia beetles (Monarthrum spp.) and oak bark beetles (Pseudopityophthorus spp.) attack oaks and certain other broadleaves including California buckeye and tanbark oak. Shothole borer (Scolytus rugulosus) attacks damaged trunks of many broadleaved tree species, including English laurel, fruit trees, and hawthorn. The European elm bark beetle (Scolytus multistriatus) feeds only on elms and vectors the Dutch elm disease fungus. Other common wood-boring pests in landscapes include clearwing moths (family Sesiidae) (for more information, see Pest Notes: Clearwing Moths, listed in Suggested Reading), longhorned borers (Cerambycidae), and roundheaded borers (Buprestidae).

IDENTIFICATION

Adults are small, cylindrical, hard-bodied beetles about the size of a grain of rice. Most species are dark red, brown, or black. Their antennae are elbowed and the outer segments are enlarged and clublike. When viewed from above, the head is partly or completely hidden by the thorax. They have strong, scooplike jaws (mandibles) for chewing. A buckshot pattern of holes is apparent on infested branches or on the trunks where the new adults have emerged. Larvae of most species are off-white, robust, grublike, and may have a dark head.

The species of tree attacked and the location of damage on the bark help in identifying the bark beetle species present (Table 1). On pines, for example, engraver beetles usually attack trees near the top, while red turpentine beetles attack pine trunks near the ground as well as below ground on the large roots. Engraver beetles are dark brown, cylindrical, and have a scooplike depression lined with stout spines at the end of the abdomen. Red turpentine beetles are larger than engraver beetles and reddish brown; their 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.

Peeling off a portion of infested bark to reveal beetle galleries is also helpful in identifying the beetle species present. Red turpentine beetle and western pine beetle adults usually pack about 60% of their egg-laying galleries with boring dust while engraver beetles maintain clean, open adult galleries. Larval galleries of all species are packed with sawdustlike boring dust called "frass" and most radiate out perpendicularly to the parent tunnels.

LIFE CYCLE

Females lay small, oval, whitish eggs at the interface of the bark and wood. After eggs hatch, the tiny larvae mine galleries that branch out from the egg-laying gallery. At first the larval mines are very small, but they gradually increase in diameter as the larvae grow. The winding pattern of these galleries is helpful in identifying a bark beetle infestation and in distinguishing among the different species. Pupation occurs in enlarged chambers at the ends of the larval tunnels or in the outer bark. Pupae are usually whitish and occur within or beneath bark. Adults can emerge at any time of year, weather permitting, but emergence is most common in late spring and again in late summer to early fall. After emergence, adults generally 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, the season of attack is usually longer and beetles have more generations per year in comparison with cooler locations.

DAMAGE

The important pest species of bark beetles mine the inner bark (the phloem-cambial region) on twigs, branches, or trunks of trees and shrubs. This activity often starts a flow of tree pitch in conifers and is accompanied by a sawdustlike material (frass). Frass accumulates in bark crevices or may drop and be visible on the ground or in spider webs. Small emergence holes in the bark with sap weeping out of the holes are a good indication that bark beetles have been present. Bark beetles commonly attack trees weakened or predisposed to infestation by drought, disease, injuries, or other factors that may stress the tree. Beetles can contribute to the decline and eventual death of trees but with a few exceptions usually are not the initial cause.

In addition to attacking larger limbs, cedar and cypress bark beetles feed by mining twigs up to 6 inches back from their tips, resulting in dead tips or "flags" hanging on the tree. The adult European elm bark beetle also feeds on twig bark before laying eggs. If the adult has emerged from infected elm wood, its body will be contaminated with Dutch elm disease spores. The beetle then infects healthy elms with the Dutch elm disease fungus during feeding; it is during this pre-ovipositional (before egg laying) feeding, which usually takes place in limb crotches, that the fungus is transmitted. Elms showing yellowing or wilting in spring are suspect and should be reported to the county agricultural commissioner.

MANAGEMENT

Except for general cultural practices that improve tree vigor, little can be done to control most bark beetles beneath bark once trees have been attacked. Prune and dispose of bark beetle-infested limbs. Promptly remove the entire tree if its main trunk is extensively attacked by bark beetles. Unless infested trees are quickly removed, large numbers of beetles can emerge and kill nearby host trees if they are weakened or predisposed by other factors. The exception is when pines are attacked by a few red turpentine beetles. Trees can often survive low-density attacks by this species. Valuable, uninfested host trees near infested trees may be protected from bark beetles by spraying the trunk with a persistent insecticide in spring; however, do not substitute preventive sprays for proper cultural care.

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 the damage from a few bark beetles. Rapid, vigorous growth encourages host resistance.

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 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 the tree’s susceptibility to bark beetle invasion.

Biological Control

Woodpeckers, several predaceous beetles such as the blackbellied clerid (Enoclerus lecontei) and trogossitid beetles (especially Temnochila chlorodia), a predaceous fly (Medetera aldrichii), and parasitic wasps are natural enemies of the western pine beetle, but rarely control it. Predators are more important in regulating bark beetle populations than parasites. When bark beetles attack and kill some trees, natural enemies are attracted and may eventually limit the infestation.

Cultural Control

Prevention is the most effective method of managing wood-boring insects; in most instances it is the only available control. Avoid injuries to roots and trunks, and protect trees from 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 to increase their 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. The specific amount and frequency of water needed varies greatly depending on the site and tree species (i.e., whether trees are adapted to summer drought or regular rainfall).

Properly prune infested limbs, and remove and dispose of dying trees so that 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 clear plastic in a sunny location for several months to exclude attacking beetles, and kill any beetles already infesting the wood.

Plant resistant species where bark beetles have been a problem. For instance, engraver beetles and red turpentine beetles do not attack redwoods or atlas cedars.

Chemical Control

Unless trees are monitored regularly so that borer attack can be detected early, any spraying is likely to be too late and ineffective. No insecticide kills larvae tunneling beneath the bark. Treatment must target the adults by spraying the bark so that they are killed when they land on trees and attempt to bore into the bark to lay eggs. If the tree was attacked during a previous year and no longer contains beetles because they have completed development and flown away, spraying that tree 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 implanted or injected through the bark or applied to soil beneath trees do not control or prevent attack by bark beetles.

Healthy specimen or high-value trees may be protected with an insecticide if they are stressed or near infested trees that are a source of beetles. Because each bark beetle species attacks only certain tree species (for example, pine bark beetles do not attack oaks and oak bark beetles do not attack pines) spray only healthy trees that are susceptible to the beetle species attacking nearby trees. It is not clear if products available to home gardeners can adequately prevent bark beetle attack. Most home gardeners also lack the high-pressure spray equipment and experience to effectively treat large trees. When hiring a professional applicator, discuss the specific pesticide to be applied.

Thoroughly drenching the main trunk with a pyrethroid (e.g., Astro or Dragnet) or the carbamate carbaryl can prevent new bark beetle infestations if applied when adults are flying. Be sure to use a product labeled for trunk applications and apply it at the proper rate for trunk treatments. Label rates for foliage treatments will not be effective. Effective products may not be available to home users, but are available to licensed pesticide applicators. Regardless of the insecticide used, mix only what you need. Apply the entire mix according to the label to avoid leftover insecticide, which should never be poured down a sink or storm drain. Take special care to keep pesticides from running off-site and into drains or waterways.

Remember that treatments must be applied to kill adults before they lay eggs. Treatment at any other time will not be effective. Spray the bark in spring when beetles begin to emerge, which is in early spring in warm areas of the state and late spring in cooler and high elevation areas. Depending on local conditions and the pesticide used, a second application may be needed several months later to provide season-long control.

The red turpentine beetle can have as many as three generations a year and engraver beetles can have up to five generations a year; apply the first spray for them about mid-February. Sprays made later will protect only against attack of later generations.

Insecticide sprays are not recommended against shothole borer and cedar or cypress bark beetles.

WARNING ON THE USE OF CHEMICALS

SUGGESTED READING

Dreistadt, S. H., J. K. Clark, and M. L. Flint. 2004. Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3359.

Koehler, C. S. 1987. Insect Pest Management Guidelines for California Landscape Ornamentals. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3317.

Marer, P. J., and M. Grimes. 1995. Forest and Right-of-Way Pest Control. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3336.

PUBLICATION INFORMATION

Pest Notes: Bark Beetles
UC ANR Publication 7421        

Authors: S. H. Dreistadt, UC IPM Program, UC Davis; D. L. Dahlsten, Biological Control, UC Berkeley; and T. D. Paine, Entomology, UC Riverside.
Produced by IPM Education and Publications, University of California Statewide IPM Program

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