Oak ambrosia beetles—Monarthrum spp. and
Western oak bark beetle—Pseudopityophthorus pubipennis
Oak ambrosia beetles, Monarthrum dentiger and M. scutellare, and the western oak bark beetle, Pseudopityophthorus pubipennis, infest injured and unhealthy California buckeye (horse chestnut), tanoak, and true oaks. These Scolytinae are commonly found in oaks dying from Phytophthora ramorum, the cause of sudden oak death (SOD).
The first obvious symptom of infestation by these beetles is fine, powdery, boring dust and excrement (frass) that accumulates on the bark surface and around the tree’s base. Adult beetles push the boring dust out of small holes (each about 1/20 inch diameter) in the bark, and the holes may be visible on the surface of infested branches and trunks. The beetles can be discriminated:
- Oak ambrosia beetles produce whitish frass around their tunnel openings. The frass is whitish because ambrosia beetles bore deeply under bark, mostly in the pale-colored heartwood and sapwood.
- Western oak bark beetle adults produce reddish frass on the surface of bark. Their frass is dark to red because they bore mostly in oak’s reddish inner bark (phloem).
The adults are small, dark brown to blackish, cylindrical beetles about 1/10 to 1/6 inch long. The larvae occur under bark and are white, legless, and have a dark head capsule. When disturbed and exposed the larvae commonly are C shaped. Be aware that oaks can also be infested with other similar-looking Scolytinae including the invasive shothole borers Kuroshio shothole borer and polyphagous shothole borer.
Ambrosia and bark beetles develop through four life stages: egg, larva, pupa, and adult. Except for the adults, all stages are found only under bark.
In northern California the beetles have about two generations per year. Most first-generation adults emerge from hosts and fly during about March. The second generation emerges in about September. The adult beetles reinfest the same tree or move to find new hosts. Because beetles develop at different rates and from eggs laid at various times, some adults may be flying and seeking stressed hosts to lay their eggs in anytime from about March through October. Each beetle spends most of its life in the larval stage. The second (fall) generation of beetles feeds and overwinters as larvae in the host.
Oak ambrosia beetles. Ambrosia beetles tunnel and breed beneath bark, but they feed on fungi they introduce and grow in the galleries (tunnels). Male Monarthrum species tunnel through the bark and about 2 inches deep into sapwood. The female joins the male, mates, and introduces the ambrosia fungi. Both sexes excavate 2 to 4 diverging galleries about 2 to 6 inches deep into heartwood and sapwood. The female lays eggs in niches in the sidewalls of the galleries.
The emerging larvae lengthen the egg niches and feed on ambrosia fungi that grow there. Larvae feed for 3 to 4 months, then pupate into adults that emerge through the same entrance holes made by parent beetles.
Western oak bark beetle. Pseudopityophthorus pubipennis is a true bark beetle; the larvae chew and feed on phloem and outer xylem tissues. Although it does not feed on fungi, western oak bark beetle can spread plant pathogens, such as the Geosmithia pallida fungus that causes foamy bark canker on coast live oak.
Female western oak bark beetles chew a tunnel about 2 inches long between the bark and wood and lay eggs there. After hatching the western oak bark beetle larvae mine upward and downward with the grain into the inner bark. Mature larvae pupate just beneath the bark surface, then as new adults chew individual, new holes through which they emerge.
These beetles do not attack otherwise-healthy trees that are vigorous. They attack stressed, dying, and recently dead hosts. Their tunneling injures trees' vascular system and weakens tree structure. Once trees become attacked by these beetles they have a greatly increased risk of structural failure (e.g., limb drop, trunk breakage) and are likely to die prematurely.
Prevention is the only available control. Plant only species well adapted for the conditions at that location. Plant properly, protect tree roots and trunks from injuries, and provide good cultural care. Avoiding inappropriate irrigation is particularly important. California buckeye, tanoak, and native true oaks are adapted to summer drought. They are readily damaged by frequent irrigation beneath their canopies, such as when turfgrass is planted underneath them.
Remove hazardous limbs whenever they appear, but otherwise avoid pruning during the season when egg laying adult borers are active (March through October). Prune out and dispose of borers restricted to a few limbs. Remove and dispose of dying trees to prevent boring insects from emerging and attacking other hosts.
Do not pile unseasoned, freshly cut wood near woody landscape plants. Freshly cut wood or trees that are dying or recently dead provide an abundant breeding source for many wood-boring pests. Solarize recently cut wood to kill most beetles infesting it. For more information, see A Field Guide to Insects and Diseases of California Oaks and the Pest Notes: Bark Beetles.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Reddish and white, beetle-boring dust on bark.
Ambrosia beetle adult and white, boring dust.
Ambrosia beetle adult.
Bark beetle galleries exposed (bottom).