Ambrosia beetles are wood-boring insects known for a unique symbiotic relationship with fungi. These tiny beetles do not consume the wood itself, unlike many other borers. Instead, the adult female beetles excavate tunnels, called galleries, deep into the sapwood and heartwood of a host tree. They introduce fungal spores they carry in specialized pockets to inoculate the wood. This cultivated fungus, known as ambrosia fungus, serves as the sole food source for both the adult beetles and their larvae as they develop within the gallery system.
How Ambrosia Beetles Select a Host Tree
Ambrosia beetles primarily target trees that are already weakened by stress. This vulnerability is often caused by environmental factors such as drought, flooding, frost damage, or physical injury. Physiological stress causes the tree’s internal environment to change, leading to the production of ethanol within the woody tissues.
Ethanol is a byproduct of anaerobic respiration, occurring when tissues are deprived of oxygen, such as during waterlogging. The odor of this ethanol acts as a powerful chemical signal that attracts the female ambrosia beetles to a susceptible host. The ethanol scent also plays a role in the fungal garden, as the ambrosia fungus utilizes the alcohol to grow and thrive while suppressing competing microorganisms.
Commonly Targeted Hardwood and Softwood Species
Ambrosia beetles are generalists with a broad host range, attacking hundreds of woody plant species. They often prefer thin-barked, deciduous trees, including both ornamental and fruit-bearing varieties. Susceptibility relates more to the tree’s stress level than its specific species.
Hardwoods
Among hardwoods, maples (such as Japanese and red maple), dogwood, redbud, and crape myrtle are common targets. Other susceptible deciduous species include oaks, magnolias, sweetgum, and Chinese elm. Fruit and nut trees are also highly vulnerable, with frequent attacks reported on pecan, peach, plum, cherry, fig, and avocado. The beetles usually target the trunks and branches of young trees or the smaller branches on mature specimens.
Softwoods and Exceptions
While hardwoods are most frequently reported as hosts, ambrosia beetles will also attack softwoods and conifers, especially when stressed. Species like pine, spruce, and fir can be affected. The redbay ambrosia beetle is an exception, primarily attacking trees in the Laurel family (such as redbay and avocado) even when they appear healthy, transmitting the lethal laurel wilt fungus.
Identifying the Tell-Tale Signs of an Attack
The clearest indication of an active ambrosia beetle infestation is the presence of “toothpick-like” strands protruding from the bark. These fragile structures are formed from the beetle pushing frass (a mixture of fine sawdust and excrement) out of its entry hole as it tunnels. The frass strands are often white or light brown and can be up to an inch and a half long, easily breaking off with wind or rain.
If the frass tubes are washed away, the entry holes themselves provide evidence of the attack. These holes are very small, typically less than 2 millimeters in diameter, appearing as tiny, perfect circles that resemble pencil-lead punctures. Fine sawdust may also accumulate near the base of the trunk or in the bark crevices.
Other signs of internal damage include sap oozing from the entry points and the presence of dark fungal staining around the hole. Visible symptoms in the canopy, such as wilting or dying leaves and branch dieback, often appear later, indicating the vascular system has been compromised by the beetle’s tunneling and the fungus.
Protecting Vulnerable Trees from Infestation
Because ambrosia beetles are strongly attracted to stressed trees, the most effective defense is preventative care and maintaining overall tree health. Ensuring trees receive proper irrigation, especially during periods of drought, minimizes water stress. Applying mulch helps regulate soil moisture and temperature, further reducing stress.
Preventing mechanical damage to the trunk and roots is also necessary, as wounds can invite beetle attack. For high-value, susceptible trees, chemical control may be used as a preventative measure. Pyrethroid insecticides can be applied as a bark spray during the early spring flight period, preventing the females from boring into the wood.
Once the beetles have successfully tunneled inside the tree, however, chemical treatments are generally ineffective because the insects and their fungal gardens are protected within the wood.