Invasive Shot Hole Borers Introduced in Southern California

Many species of ambrosia beetles (subfamily Scolytinae) colonize only recently dead trees or those that are already injured or stressed from other causes. However, the introduced Kuroshio shot hole borer (Euwallacea kuroshio) and polyphagous shot hole borer (Euwallacea whitfordiodendrus) are attacking and killing healthy trees in at least southern California and these pests are spreading. These invasive shot hole borers (ISHB) look identical, and additional look-alike ambrosia beetles (e.g., Euwallacea fornicatus) may also have been introduced into California.

Identification

The first obvious symptoms of ISHB infestation are patches of discoloration and oozing on bark, which can be dark, dry, water-soaked, or oily looking. These lesions are sometimes surrounded by wet discoloration (e.g., on oak and sycamore) or white crusty or powdery exudate (especially on avocado). Specific symptoms vary with the species of host (PDF).

An infestation sometimes results in solidified pinnacles of frass (beetle excrement) that resemble short, round toothpicks protruding from bark. The fragile excrement structures readily wash and weather away and may not be observed. The granulate ambrosia beetle and other exotic ambrosia beetles also can form frass protrusions.

Within bark lesions caused by ISHB are tiny holes about 1/30 inch in diameter made by the adult beetles. Scraping off bark around the holes reveals dark, discolored wood. Cutting the infected limb or trunk in cross-section reveals black to brown discoloration deep into the wood due to the Fusarium euwallacea fungus the beetles introduce.

Adult invasive shot hole borers are dark brown to blackish, cylindrical beetles 1/12 to 1/25 inch long. Larvae are white, legless, and have a dark head capsule. When disturbed or exposed, larvae are commonly C shaped.

Most hosts are susceptible to additional Scolytinae, such as oak ambrosia beetles and western bark beetle that infest oaks. For expert techniques on how to distinguish the beetle species and other invasive shot hole borers, see Southeast Asian Ambrosia Beetle ID. Generally, laboratory molecular tests (e.g., polymerase chain reaction, or PCR) must be used to discriminate the species.

Life cycle

Ambrosia beetles develop through four life stages: egg, larva, pupa, and adult. Adult females bore through tree bark, create galleries (tunnels), and introduce Fusarium euwallacea. The female lays eggs in the galleries and the Fusarium fungus grows and spreads throughout the tunnels. When the eggs hatch, the larvae eat the fungus.

Larvae develop through four increasingly larger instars. After pupating the new adult females mate with the males while in the galleries. Then pregnant females carrying the fungus exit the tree through the entry holes created by their mothers and reinfest the same tree or fly to seek new hosts.

Damage

Ambrosia beetle tunneling in combination with fungal growth injure the tree vascular system and weaken tree structure. As the beetles tunnel and diseased wood becomes more abundant, and subsequent generations of beetles reinfest the host, foliage yellows and wilts and branches die back, a malady called Fusarium dieback. Infestation of hosts in which the beetles can complete their development generally results in gradual death of the entire tree.

One or both of these beetles are known to feed and reproduce in California in about 60 tree species. For example, avocado, California sycamore, castor bean, coast live oak, cork oak, Fremont's cottonwood, and Persian silk tree can be infested with both Kuroshio shothole borer and polyphagous shothole borer. See the list of known reproductive hosts of these borers for the plants likely to be killed by them. Several dozen additional hosts can be infested and develop branch dieback.

Solutions

If you think invasive shothole borers or Fusarium dieback are affecting your trees, see Diagnosis & Management: Do You Have ISHB on Your Property? If by using the tool you determine your tree likely is infested by invasive shothole borers and Fusarium dieback, please report your situation to the local county agricultural commissioner.

No direct controls are known to prevent the beetles' attack, but taking preventive actions can at least avoid other tree maladies and reduce the spread of these pests. Plant species that are well adapted to conditions at that site and that do not require a lot of irrigation. Provide trees with a good growing environment and proper cultural care to make them less attractive to boring beetles. Provide appropriate soil conditions (e.g., prevent compaction) and sufficient space for roots to grow. Protect trees from injury, such as mechanical impact. Avoid excessive pruning, overwatering or underwatering, and the planting of inappropriate companion plants within the dripline (e.g., those with different irrigation needs than the trees).

Protect trees from invasive shothole borers and various other pests by not moving firewood. Purchase firewood near where you will burn it and leave any unused wood on site rather than moving it or bringing it home.

Prune off dead limbs and cut down dying trees and those with beetles infesting the main trunk. Properly dispose of infested wood. To prevent invasive shothole borers from emerging from cut wood and spreading to other trees, chip wood to pieces smaller than 1 inch, then solarize the chips. If an infested branch is too large to chip, cut it into firewood-length logs and solarize the logs under a clear tarp for several months. Avoid movement of infested firewood and chipped material out of the infested site.

To avoid spreading plant-pathogenic fungi on contaminated tools, after cutting a diseased tree clean and sterilize pruning tools before leaving the site or moving to work on other trees. See Best Management Practices for Disease in Oak Woodlands (PDF) for details on proper pruning and handling of tools and infested wood.

If trees are infested, systemic insecticides generally are ineffective for controlling ambrosia beetles. Prophylactic spraying of the trunk and limbs with a persistent, broad-spectrum insecticide such as carbaryl or permethrin could be used to protect uninfested trees in some situations. Applications would need to be repeated and a professional pesticide applicator must be hired to apply an effective product. Consult Pest Notes: Hiring a Pest Control Company to increase the likelihood of a satisfactory experience.

See the ISHB websites of UC ANR and UC Riverside for more information. Adapted from information on these websites and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).