MOB creates tunnels (or galleries) in the trunks and branches of host trees. It initially attacks the crown of the tree, where it will kill individual limbs. Infestation can continue over several growing seasons, eventually invading the main trunk and killing the entire tree. Infested trees can become a hazard when their limbs and upper trunk are weakened by the widespread galleries. This can lead to premature failure, especially if combined with heart rot.
In California, this beetle-fungus complex will be introduced to species of oaks to which it has not previously been exposed, and damage to some of these species could be more severe than that observed in the beetle’s native range. Oaks are important components of forests and woodlands in California. Valley oak, which is already known to be attacked, is widely distributed in California and is an important component of oak woodlands. Oaks provide habitat for threatened species, have historically been used by Native Americans for food and other purposes, and are important landscape trees that add to the value of property.