Tree Killers: Bark Beetles and Their Control
Bark beetles are one of the most destructive insect groups in the world. While most of the damage occurs on forest land, trees in the urban landscape are also at risk of attack by bark beetles. Bark beetles (subfamily Scolytinae) are a group of small, mostly black to brown beetles that are very difficult to identify. The largest member of this group is the red turpentine beetle, which reaches a maximum length of 8 mm. Despite their small size, bark beetles can work together to overcome host trees (mass attack), killing them outright, or killing portions of the tree.
Usually, by the time people realize they have bark beetles, it is too late to save the tree. All control tactics for bark beetles are proactive, and begin by keeping trees stress free. Trees should be properly planted in the appropriate site, given optimal water and fertilizer, and kept free from injury. For example, blue spruce should not be planted with drought tolerant plants (xeriscapes) because they will not receive enough water, becoming stressed and more susceptible to attack. Planting rows of the same species of tree provides easy access to host material for bark beetles as they move from one tree to the next. Planting a diverse landscape limits pest movement. Where necessary, use chelated iron, and avoid injury to root systems during construction or other projects.
Infested trees should be removed and chipped or debarked immediately to prevent emerging adults from killing more trees. Do not cut down infested trees and keep the firewood near susceptible trees. A preventive insecticide spray can provide protection to trees that have not been attacked but are near beetle-killed trees.
Preventive insecticides should be applied before beetles emerge in spring or early summer. Typically, spring applications should occur once temperatures are consistently over 50°F. At this temperature, many bark beetles continue development under the bark or emerge to find new host trees. Properly timed and applied insecticides will kill bark beetles as they chew through the insecticide-soaked bark, preventing successful attack. Once beetles are under the bark, no insecticide treatment will save the tree. To date, no systemic insecticides have been proven effective at preventing bark beetles from killing trees.
For pines and spruces, formulations of carbaryl (Sevin SL), bifenthrin (Dragnet, Masterline Plus C, Astro), and permethrin (Onyx) are effective at preventing bark beetle attack. Carbaryl offers 2 year protection, while the pyrethroid insecticides last 1 year. Avoid using lawn and garden products with these active ingredients, as they may not be as effective as the products listed above. For fruit, nut, and other ornamental hardwood trees, active ingredients such as spinosad, endosulfan, chlorpyrifos, carbaryl, and permethrin can effectively prevent successful attack. Always make sure that the site of application is listed on the insecticide you select. In most cases, commercial applicators with high pressure sprayers are needed to propel insecticides high up on the main stem and branches.
Beetles collected from infested trees may be sent to the Utah Plant Pest Diagnostic Lab (UPPDL) for identification. When submitting a bark beetle sample always include host tree information, and a picture of the galleries if possible (remove bark with a hatchet). Proper identification of bark beetles can lead to precise prevention tactics for other at-risk trees.
-Ryan Davis, Arthropod Diagnostician