Bark beetles are small, cylindrical insects, typically no larger than a grain of rice, that pose a severe threat to trees by tunneling into the inner bark layer. These pests cause widespread tree mortality, primarily targeting conifers like pine, spruce, and fir, but also attacking some hardwoods. The insects bore through the bark to lay eggs, and their larvae feed on the phloem and cambium. This feeding effectively girdles the tree, disrupting its ability to transport nutrients and water. While bark beetles naturally remove weakened trees, their populations can surge, overwhelming even healthy trees stressed by drought or other factors.
Recognizing Signs of Infestation
Early detection is important, as a successful attack often means the tree cannot be saved. One recognizable symptom is the presence of pitch tubes, which are small lumps of resin and boring dust pushed out of the entry hole as the tree attempts to repel the beetle. These resin masses may appear white, pinkish-brown, or reddish-brown. A large, visible pitch tube often indicates the tree successfully repelled the initial attackers.
Frass is another common sign, appearing as reddish or tan sawdust-like material created as the beetles bore into the bark. This fine material often accumulates in bark crevices or at the base of the tree. In later stages, the tree’s foliage begins to fade, turning yellow, then reddish-brown, often starting at the top of the crown. When adult beetles emerge, they leave behind tiny, BB-sized exit holes.
Cultural Practices for Tree Health and Prevention
The most effective long-term defense against bark beetles is maintaining tree vigor. Healthy trees can produce enough resin to physically repel initial attacks. During dry periods, provide supplemental, deep irrigation. Water should be applied slowly and infrequently to soak the soil to a depth of at least 12 inches. Reducing competition for resources, such as thinning overcrowded stands, allows remaining trees to access more water and nutrients, making them more resilient.
Avoid physical injuries to the trunk or roots, as damaged areas release chemical signals that attract the insects. When pruning dead or weak limbs, perform this work during the cooler, non-flight season, typically between November and March. This prevents attracting active beetles to fresh cuts. Proper debris management is also a preventative measure, requiring prompt removal or processing of slash and cut wood, which serve as breeding material.
Direct Treatment of Active Infestations
Once an infestation is confirmed, options for saving the attacked tree are limited, as insecticides are ineffective once the insects are established beneath the bark. Chemical control is primarily preventative. This involves applying a barrier spray containing insecticides, such as permethrin or carbaryl, to the uninfested bark of high-value trees. This treatment must thoroughly coat the entire trunk and large branches before the beetles begin their flight season in the spring.
Systemic insecticides, which are absorbed by the tree, have not been shown to prevent attacks or control established populations. For successfully attacked trees, non-chemical methods focus on controlling the local beetle population. Physical removal involves felling the infested tree and either debarking the logs on-site to expose the larvae to desiccation or processing the wood immediately.
Solarization is a physical method where infested logs are tightly wrapped in thick, clear plastic sheeting and left in direct sunlight. The goal is to raise the temperature under the plastic to at least 113°F, which is lethal to the developing beetles and larvae. For protecting nearby healthy trees, anti-aggregation pheromones can be used as a targeted repellent. These chemical signals, such as verbenone, mimic the scent of an overcrowded tree, deterring newly arriving beetles from attempting an attack.
Post-Treatment Tree Management and Debris Handling
When a tree is heavily infested and cannot be saved, prompt removal is necessary to prevent emerging beetles from attacking neighboring trees. Infested wood must be processed quickly, ideally within 30 days of cutting, to prevent adult beetles from completing their life cycle and flying out.
Acceptable disposal methods include:
- Chipping the wood into small pieces.
- Burying it under at least eight inches of soil.
- Burning it in accordance with local regulations.
Transporting infested wood, such as moving firewood, is strongly discouraged and often illegal because it risks spreading the beetle problem. After removing a dead tree, nearby healthy trees should be closely monitored for signs of secondary attacks. Preventative sprays or pheromone repellents can be applied to protect them. When replanting, selecting species not susceptible to the local bark beetle population reduces the risk of future infestations.