Host Trees
MOB attacks at least 12 species of oaks. In California, it has been found infesting two species of white oak: most commonly valley oak and, to a lesser extent, blue oak. A single, very limited attack was found in a severely distressed California black oak.
Valley Oak
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The leaves of valley oak (above) are green, sinuate (alternatively rounded notches and lobes), up to 10 cm (3.9 in) long and up to 6 cm (2.4 in) broad. The leaf margin is smooth. The leaves of blue oak (below) have a blue-green tint leading to the tree's name. Leaves are deciduous, 3.8 cm (1.5 in) to 10.2 (4 in) long, and entirely or partially lobed.
Blue Oak
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In Europe, the beetles are known to attack a moderate range of trees in the oak and beech family, but that range could be much narrower or broader in California. They appear to prefer to infest trees that are already suffering from drought or other pests or diseases such as sudden oak death. California forests are periodically under stress from drought, fire, and disease, so they may be especially vulnerable to this beetle.