Bark beetles are small, highly destructive insects that bore into trees, interrupting the flow of water and nutrients, often leading to rapid tree death. These pests are a serious threat to landscape trees and forests, capable of killing a healthy tree within weeks or months. Prompt identification and decisive action are necessary to control the population and protect surrounding host trees. This guide details how to recognize an infestation and implement effective control strategies.
Recognizing a Bark Beetle Infestation
Confirming the presence of bark beetles involves looking for specific signs on the tree trunk and at the base. The most common indicator is pitch tubes, which are small masses of resin and boring dust resembling popcorn on the bark surface. These tubes are formed as the tree attempts to push out the invading beetle with sap, and their color ranges from white or yellow to reddish-brown.
Another diagnostic sign is frass, a reddish-brown, sawdust-like material created by the beetles as they bore into the inner bark. Frass is often visible in bark crevices or accumulated at the base of the tree. If damage is advanced, removing a small section of bark may reveal characteristic gallery patterns etched into the inner wood, which vary by beetle species. Changes in the tree’s crown are also important, as the foliage will often begin to discolor, turning yellow, red, or brown as the internal damage progresses.
Before attempting treatment, assess the tree’s viability. Once a tree has been successfully attacked, especially if it has extensive crown discoloration, it is generally considered beyond saving. A tree with more than 50% of its crown showing signs of damage should be removed promptly. Timely removal of a heavily infested tree is necessary to prevent the spread of the infestation to nearby healthy trees.
Mechanical and Cultural Control Methods
When a tree is heavily infested, mechanical control focuses on eliminating the beetle brood inside the wood to prevent emergence and attack on other trees. The immediate removal and destruction of the infested tree, often called sanitation logging, is the most direct method to prevent spread. The felled wood must be treated quickly, as beetles can complete their life cycle inside the log and re-infest the area.
Infested wood can be destroyed by burning, burying, or transporting it to a specialized facility. Debarking the wood is highly effective because bark beetles feed on the phloem layer directly beneath the bark, eliminating their habitat and food source. Chipping the wood into pieces smaller than 2.5 cm can also kill the beetles and make the material unsuitable for reproduction, though fresh chips may temporarily attract beetles.
Solarization is another effective physical control method. This involves tightly covering infested logs with clear plastic sheeting and sealing the edges to the ground. This creates a solar oven, raising the wood temperature to lethal levels, often exceeding 74°C (165°F), which kills the brood inside. This technique is most effective during the hottest months when high temperatures can be sustained.
Cultural practices focus on increasing the tree’s natural defense mechanisms, making it less susceptible to attack. Proper watering is essential, especially during drought, as a well-hydrated tree produces more resin to pitch out boring beetles. Thinning dense stands of trees reduces competition for resources and improves the overall vigor of the remaining trees, making them more resistant to infestation.
Insecticide and Chemical Treatment Options
Chemical treatments are primarily preventative and protect uninfested, high-value trees from attack. Preventative sprays use residual insecticides like permethrin, carbaryl, or bifenthrin, applied directly to the entire bark surface until runoff. The insecticide residue remains on the bark, killing adult beetles as they attempt to bore into the tree.
Timing is important for these sprays; they must be applied before the adult beetles begin their flight and attack the tree, typically in late spring or early summer. Carbaryl (Sevin) has historically been effective, with some formulations protecting trees for up to two field seasons. Permethrin is another common pyrethroid insecticide used, often favored in urban settings for its lower toxicity to mammals and ease of use.
Systemic treatments involve injecting insecticides directly into the trunk or applying them to the soil, where the tree’s vascular system transports the chemical throughout the plant. Emamectin benzoate (EB) is a systemic insecticide effective as a preventative control against certain bark beetle species. Systemic injections are considered a safer, more targeted application method than spraying, reducing the risk of spray drift and non-target effects.
Pheromones are chemical signals utilized to manage beetle populations. Aggregation pheromones attract beetles and can be used in traps for monitoring flights or for mass trapping. Homeowners should use caution, as these may unintentionally attract too many beetles, potentially overwhelming nearby healthy trees. Anti-aggregating pheromones, such as verbanone, confuse the beetles and discourage them from attacking a tree, though they have a limited dispersal range.
Disposal and Long-Term Prevention
Once an infested tree is removed, final disposal of the wood is important to ensure the infestation does not spread. If the wood cannot be immediately chipped or solarized, it should be burned or hauled to a designated facility for safe processing. Untreated infested wood should never be stored near healthy trees, as emerging beetles will immediately target the nearest suitable host.
Long-term management focuses on maintaining the health and vigor of remaining trees to increase their natural resistance. This includes providing supplemental water during drought and ensuring trees have sufficient growing space. Avoiding physical damage to the trunk from construction equipment or lawnmowers is also important, as wounds create entry points and stress the tree. Monitoring high-risk trees regularly for early signs of attack allows for immediate preventative action, which is the most effective defense.