The beetles, which are slightly larger than a pinhead, also introduce a fungus that blocks water movement from the roots to the top of the tree. Healthy, vigorously growing trees are best equipped to withstand beetle attacks because of their active resin systems that deter beetle colonization. When the beetle bores into a healthy tree, resin exudes through the wound and may prevent the beetles' entry. Consequently, the beetles prey on water-stressed trees, taking advantage of drought conditions that severely weaken natural defenses. Many trees are weakened by rocky soils, excessively sandy soils or mechanical injuries to roots or trunks caused by construction.
Drought exacerbates the problem, attracting more beetles and causing more intense infestations.
Once bark beetles have successfully colonized a tree, it cannot be saved, Cain said. Homeowners should also check green firewood because bark beetles emerging from infested firewood account for many tree losses near homes and in urban settings. If firewood has been stored for at least one season, the beetles won't be present, but homeowners should take precautions with green firewood collected or purchased in summer when temperatures are high, Cain said. Infested firewood should be stacked in direct sunlight and covered with clear plastic in piles no larger than 4 feet by 4 feet by 4 feet, which creates a greenhouse that can kill the beetles. Tree thinning in overcrowded wooded areas is also a good precaution against bark beetles, because it reduces the need for trees to compete for scarce water.