Tree Fruit Research & Extension Center

Bark Beetles in Washington Orchards

Bark Beetles in Washington Orchards

Bark beetles can be pests of all natural or orchard systems. Damage in forests can be devastating to natural ecosystems. Bark Beetles have been reported as pests of pome and stone fruit orchards in the Western United States for nearly a century. They generally attack weakened trees and can cause limb or even tree death if present in high enough numbers. Nutritionally stressed or previously damaged trees (for example sun scald or winter freezing) can provide an opportunity for beetles to colonize trees. Healthy trees have a natural defense against bark beetle attack. High sap flow from proper irrigation practices allows trees to mechanically flood out potential colonizers at the point of attack.

The repeated use of synthetic organic insecticides has likely helped control bark beetles in tree fruit orchards, and until recently they have been considered sporadic or localized pests. However, the reported incidence of injury from bark beetles in Washington cherry orchards increased during the late 1990s and early 2000s. Growers were reporting damage from bark beetles in healthy orchards, especially new plantings. Of particular concern were beetles feeding at the base of new buds or growing shoots, causing the buds or shoots to die. Continuous feeding from massive immigration weakened many young plantings, eventually killing the trees. It was believed that economic difficulties like those experienced in the Washington tree fruit industry during the late 1990s contributed to the problem due to an increase in neglected or abandoned orchards, which were serving as reproductive hosts for bark beetles.

It is particularly difficult to manage pests that immigrate from outside the orchard. This requires information on what are the potential hosts, as well as what triggers migration into healthy orchards. Growers need accurate life history descriptions of the damaging species, the best available monitoring tools, effective insecticides, and knowledge on the damage potential from invaders. The more information growers have available allows them opportunities to develop management practices that will protect healthy orchards from invasion by bark beetles.

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Life History

The dominant bark beetle found throughout central Washington is the shothole borer (SHB),Scolytus rugulosus. It is the SHB that is most often implicated in causing damage to healthy cherry trees. An ambrosia beetle (AB), Xyleborinus saxeseni, may be present in high numbers but generally isn't found attacking healthy trees. Rather the AB is more often associate with trees that are already in decline. In addition, many other wood-decomposing beetle species are often found infesting cutwood wood. Buprestids, bostricids and powderpost beetles are the primary families most associated with dry, dead wood, that is pruning piles or pushed out trees that had aged for one or more years. The SHB and AB appear to be the primary attackers of live and weakened trees.

Newly emerged SHB adults often feed at the base of buds, leaves or small twigs before they tunnel into the tree through the bark. They then excavate a narrow gallery running parallel to the wood grain. Each female deposits about 50 eggs in niches along the gallery wall. As eggs hatch, the larvae excavate slender tunnels, usually at right angles to the main egg -laying gallery. Adults are relatively long lived and care for the maternal galleries, cleaning debris and protecting the grubs from predators or parasitoids. When fully grown, about 6-8 weeks, the larvae construct pupal cells at the ends of the tunnels. When they emerge as adults they build exit burrows from the pupal cells to the outside. The common name of this beetle originates from the numerous exit holes that produce the shothole effect in the tree bark.

The AB overwinters as an adult, while SHB overwinters as a mature larva or pupa. Thus, AB adult flight occurs earlier than SHB. It is first detected as temperatures warm, allowing the adults to became active. Adult AB activity begins in late March or early April. A second peak of activity is seen in early June, with a third in July and early August. A slight increase in captures is often observed in September and early October. This could be prolonged activity of the 3rd generation, or possibly a 4th generation. AB adults are captured during the entire growing season. Two distinct periods of SHB activity occur in Washington. SHB adults are first captured in late April or early May and continue through June. A second adult flight is detected in mid- to late July and continues through August and into September. Adult SHB are trapped through the entire growing season. A small increase in captures occurs at the end of the year indicating that at least a partial 3rd generation is possible. The remainder of this site will focus on the SHB, the species most likely to cause damage to healthy orchards.

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Reproductive Hosts

The incidence of SHB attacks to healthy trees is generally linked to a migration of SHB adults from a reproductive host situated near a thriving orchard. Certain management practices have been characterized that increase the probability that an orchard will face a massive immigration from an outside host. SHB are able to reproduce in recently cut wood, or current year prunings. SHB may quickly colonize this wood, even while it is still green. Without the plants natural defense against colonization (sap flow), females are able to construct maternal galleries within which the grubs will thrive. Cherry growers that stack their cuttings to be used as firewood near the orchard are providing SHB with an ideal reproductive host. SHB will also reproduce in smaller wood, prunings, or brush piles near orchards. If these reproductive hosts are not removed (i.e. burned or shredded) SHB will be able to reproduce in that wood until it has dried for about 12-18 months old. Once that wood dries out, SHB may leave the host in a mass migration to healthy orchards.

During times of economic hardship, a higher incidence of orchards being removed from production or otherwise neglected may become more common. Pushed over orchards that are not destroyed provide ample reproductive hosts for SHB. Even after those trees are too old to be suitable for SHB reproduction, new growth from the roots can serve as a reproductive host for SHB. Clearly, trees that are neglected through improper horticulture management will eventually weaken to the point the SHB are able to successfully colonize them. SHB are generally considered pests of cherry trees, because of the dramatic response cherries have to injury, but SHB are able to colonize and reproduce in apple and pear, as well.

Once a host is too old for SHB reproduction, secondary decomposing beetles will colonize the wood. Many beetles will continue to reproduce in this wood, but are not considered pests of live trees. Powderpost beetles, bostricids, and buprestids are commonly seen emerging from older wood. The appearance of their grubs in woodpiles can lead growers to make false assumptions about the source of a SHB infestation. Typically, the bark becomes loosened on wood infested by these secondary decomposing beetles and when peeled sawdust may be readily apparent. Dry sawdust is not typically produced by SHB larvae.

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SHB are generally described as attackers of weakened or stressed trees, but they can be particularly troublesome pests when they migrate in mass to healthy orchards. Cherry trees have a natural plant defense against colonization from boring insects; large amounts of sap are pushed through the point of attack, flushing out potential colonizers and making it difficult to establish maternal galleries. The sap flow forms columns extending out from the limbs and is often the first sign that trees are under attack from SHB. Individual attacks are usually thwarted by heavy sap flow, but repeated attacks will stress limbs or branches leading to flagging or wilting foliage. Eventually the limb will become weakened resulting in "blind" wood. If the plant's defense system is compromised SHB colonization efforts may be successful and maternal galleries constructed. The hatching grubs excavate galleries beneath the bark and eventually stop the flow of water and nutrients by girdling the limb. The ultimate result is tree death.

SHB damage in healthy orchards is usually associated with a mass migration of newly emerged adults from a host located close to the orchard. We do not know how far SHB will migrate to infest orchards, but they will readily cover distances of 50 yards. The damage from this initial migration is usually constrained to a few rows adjacent to the host. SHB seem to spread out slightly along the border, but do not move more that a couple of rows into the orchard. The bar graphs below represent damage to individual trees along an orchard border. The relative size of the bars represents damage levels to individual trees. Monitoring and control efforts should be focused on determining the natal host responsible for the SHB infestation, usually piles of host wood near an orchard, and then protecting the area of the orchard immediately adjacent to that host. If control is neglected and SHB are able to colonize and reproduce in the weakened trees, migration is no longer the sole source of beetles and damage will extend further into the orchard thereby complicating management efforts.

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Monitoring programs for SHB should focus on identifying the reproductive host that is the source of infestation and tracking immigration into an orchard. Commercially available intercept style traps that are baited with an ethanol lure are useful for monitoring peak activity. Adult SHB are attracted to the ethanol odor and when they hit the intercept traps they slide down the trap into a collection vial. It is often possible to hang these traps between the host and the orchard border and measure SHB activity in that region. Yellow sticky traps will also work when placed directly in the suspected host or hung in border trees adjacent to the host.

Our experience is that higher trap captures are noted in the yellow traps than the intercept traps and not as many secondary wood decomposing beetles are captured. For that purpose yellow traps may be easier to read by non-entomologist. Regardless, all trap types are likely to trap many different insects and some level of skill is necessary to read them. Several traps spaced approximately 10 yard apart are necessary to adequately monitor orchards and outside hosts.

We have not established treatment thresholds with the monitoring systems, rather we suggest they be used as a presence/absence indicator. If SHB are trapped in the host, the host should be removed immediately. If SHB damage and/or immigrating adults are noted on orchard borders than some intervention is necessary.

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Best Management Practices

Many insecticides cause SHB mortality in laboratory bioassays, but residual control was limited with most of them. In other words, recently sprayed residues were toxic but mortality began declining after about 7 days. The repeated use of insecticides against other pests, primarily the cherry fruit fly, is likely sufficient to suppress damage in most commercial orchards, especially during the first SHB generation. Cherry orchards may become more susceptible to injury in the post-harvest period when insecticide programs for cherry fruit fly and leafroller have ceased and second-generation SHB adults are able to move into unprotected orchards. Additional insecticide efficacy trials are necessary to understand the full potential of each insecticide to control SHB under field conditions, but our experience is that most healthy orchards showing signs of damage are doing so despite a rigorous insecticide program.

Healthy cherry trees can repel initial colonization efforts by SHB adults by flooding attacked sites with sap. However, upon repeated attack even healthy trees will eventually become weakened, allowing successful colonization by secondary attacks from SHB. Our experience with SHB management indicates orchard sanitation is the most important factor contributing to a reduction in SHB populations and damage to healthy cherry trees. If recent feeding damage is noted on otherwise healthy trees, adult traps can be placed on the orchard borders or in suspected host areas to verify the source of infestation. Sanitation programs must include removing potential host material (weakened limbs or recent prunings) from within the orchard and eliminating any host material outside the orchard. SHB host material outside the orchard can be eliminated by burning or by thoroughly soaking the wood with an effective insecticide delivered by a handgun sprayer. The increased volume of water delivered by handgun applications appears to be an important factor in insecticide efficacy. We do not believe growers can rely on traditional insecticide applications via an air-blast sprayer to control infestations that originate from within the orchard, or protect orchard borders from massive immigration originating from a nearby heavily infested host.

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