Granulate Ambrosia Beetle: A Threat to Ornamental Trees

This exotic wood borer (Xylosandrus crassiusculus, subfamily Scolytinae), formerly called Asian ambrosia beetle, attacks both healthy and injured or stressed trees and shrubs. It also infests freshly cut wood.

Granulate ambrosia beetle was apparently eradicated from Oregon at least twice. Although periodically intercepted at border inspection stations of California, it is not reported as established in California. Its biology and management are similar to that of the Kuroshio shot hole borer and polyphagous shot hole borer, which are killing landscape trees in at least Southern California.

Identification

Trees severely infested with granulate ambrosia beetle may show symptoms of stunting (slow, undersized growth), delayed leaf emergence in spring, pale foliage, and premature leaf drop. An infestation of ambrosia beetles sometimes can be recognizable by the adults' distinctive excrement—solidified pinnacles of frass that resemble short round toothpicks protruding from bark. The fragile excrement structures are readily washed away by irrigation sprinklers or rain and may not be observed. It is thus possible to be unaware of an infestation of this beetle until the infested tree suddenly dies.

Certain other exotic ambrosia beetles also form frass protrusions on bark. These include the Kuroshio shot hole borer and polyphagous shot hole borer, which spread the Fusarium euwallacea fungus that kills trees of various species.

Adult granulate ambrosia beetles are stout-bodied, dark reddish brown, and darker red to blackish on top the rear. Adult females are about 1/10 inch long and males are somewhat smaller. Like other species of ambrosia beetles and bark beetles, such as oak ambrosia beetles, when viewed from above the adult's head is completely hidden by the pronotum.

Larvae are legless, white, and have a dark head. When disturbed or exposed, larvae commonly are C shaped. Beetle eggs, larvae, and pupae occur only under bark. They are not distinguishable in any simple way from those of other Scolytinae.

Life cycle

Ambrosia beetles develop through four life stages: egg, larva, pupa, and adult. Females bore into twigs, branches, or small trunks of woody broadleaves and excavate tunnels in the pith or wood. The beetles introduce symbiotic ambrosial fungi, and lay eggs that hatch into larvae that feed on the fungus. Larvae develop through four increasingly larger instars. Eggs, larvae, and pupae occur together in the tunnels. Females remain with their brood until the larva mature, and periodically expel frass from tunnel entrances to maintain good conditions under bark for the growth of larvae and their ambrosia-fungus food.

Damage

Granulate ambrosia beetle can infest at least 200 species of fruit and ornamental trees and shrubs. Hosts highly susceptible in the Eastern United States include azalea, Chinese elm, crape myrtle, dogwood, fig, golden rain tree, Japanese maple, liquidambar, magnolia, maple, ornamental flowering cherry, peach, pecan, persimmon, plum, redbud, and rhododendron.

Small branches and stems are most commonly attacked. Cankers may form at the site of attacks, sometimes resulting in the death of trees by girdling. Attacks on living plants commonly are near ground level on saplings or at bark wounds on larger trees. Trees severely infested with granulate ambrosia beetle may exhibit stunted (slowed) growth, delayed leaf emergence in spring, and premature leaf drop. Infestations commonly kill young trees and the pest is difficult to manage.

Solutions

Granulate ambrosia beetle occurs in the eastern United States and is spread when infested wood or nursery plants are moved. Avoiding the introduction, establishment, and spread of granulate ambrosia beetle are the primary management methods. Buy only pest-free plants from reputable local nurseries. Before purchasing plants from outside the local area or moving plants across county lines, contact the local county agricultural commissioner to learn whether any quarantines prohibit movement of the plants.

Do not move firewood or logs to other counties. Purchase firewood near where you will burn it and leave any unused wood on site rather than moving it or bringing it home. Moving firewood commonly spreads pests.

Keep trees in optimal health to minimize their likelihood of becoming borer infested. Plant species that are adapted to conditions at that location and do not require a lot of irrigation. Provide appropriate soil conditions 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).

Where the beetle is established, monitoring and management practices from the University of Arkansas (PDF) and University of Florida can be employed. If you find a tree with toothpicklike frass protrusions or suspected granulate ambrosia beetles in California, report this to the county agricultural commissioner.

Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).