• The Polyphagous Shot Hole Borer (PSHB) is a new pest in Southern California. This very small and hard to see boring beetle, from the group of beetles known as ambrosia beetles, drills into trees and brings with it a pathogenic fungus (Fusarium euwallacea), as well as other fungal species that may to help establish the colonies. The beetle is dark brown to black and tiny, with females between 0.07 and 0.1 inches long, and males even smaller, usually about 0.05 inches long. Pregnant females bore through the tree’s bark, creating galleries under the bark. They plant the fungus in these galleries, where it grows and spreads throughout a susceptible tree. The female then lays her eggs in these galleries and when the eggs hatch, the larvae eat the fungus. The larvae develop into adults in about a month. Many more of the larvae develop into females than males, and the females mate with the males (their brothers) while still in the gallery. The pregnant females then pick up some of the fungus in their mouths, and leave through the entry holes created by their mothers to start the process again.

  • Symptoms

    Symptoms of PSHB attack and fungus infection differ among tree species. The beetle produces a very precise, perfectly round, tiny (< 0.1 inches in diameter) entry hole in most trees. Infection with the fungus can cause a dry or wet and oily dark stain surrounding the entry holes, discolored wood, leaf discoloration and wilting, and dieback of entire branches. In box elders and avocados, a white crusty ring of sugar, also called a “sugar volcano” can be produced. Frass (wood dust from boring) may be produced, but because this can quickly dissolve in water it can be easy to miss. If the bark is scraped away, dark dead tissue may be found around the galleries.

  • When beetles attack a tree, there are several potential outcomes:

    1. The beetle is repelled and causes no infection. This has been observed in 20 species of trees. Investigators are trying to figure out what features of the tree might repel the beetle.

    2. The beetle drills into the tree and transmits the fungus, but doesn’t produce offspring. This has been observed in over 50% of the tree species attacked. Scientists don’t know the final outcome of this interaction. If the beetle has penetrated to the xylem, this could cause dieback of branches. Damage could also make the tree more prone to attack from other pest species.

    3. The beetle drills into the tree, fungus infects the tree, and the beetle produces offspring in the tree. This has been seen in about 8% of the tree species attacked, and these species are considered reproductive hosts of PSHB. Some trees seem to suffer mild symptoms like branch die-back, while others are killed outright.

    The 24 species listed by Dr. Eskalen [xiii] as reproductive hosts include

    • several maples (native and exotic): Box elder (Acer negundo), Big leaf maple (Acer macrophyllum), Trident maple (Acer buergerianum), Japanese maple (Acer palmatum), Evergreen Maple(Acer paxii);
    • several oaks (native and exotic): English oak (Quercus robur), Coast live oak (Quercus agrifolia), Valley oak (Quercus lobata) ,Engelmann oak (Quercus engelmannii) ;
    • several willows (native and exotic): Tortuosa (Salix matsudana),Weeping willow (Salix babylonica), Red Willow (Salix laevigata) ;
    • Other trees native to North America: California Sycamore (Platanus racemosa), Blue palo verde (Parkinsonia florida), Brea (Cercidium sonorae), Coral tree (Erythrina corallodendron), Titoki (Alectryon excelsus;)
    • non-native species important to agriculture or ornamental horticulture: Avocado (Persea americana), Mimosa or Silk Tree(Albizia julibrissin), Chinese holly (Ilex cornuta), Camelia (Camellia semiserrata);
    • And a widespread invasive species, Castor bean (Ricinus communis).

    Avocado, maples, coast live oak, and sweetgum are in very different botanical families (Lauraceae, Aceraceae, Fagaceae, Hamamelidaceae) -- so the host range could be quite large. Thirty three families contain both species that are hosts of the beetle and/or the fungus; and species that apparently do not host them. [xiv]

    Given the large number of tree types that can be infested by the beetle, and the growing list of species that support beetle reproduction, it appears highly likely that the disease complex could be transported in firewood. Firewood suppliers and customers, as well as regulatory agencies should take action to ensure that this does not happen.



    In late winter 2012, attention was drawn to a new insect/pathogen complex in Southern California as a result of damage to several backyard avocado trees in residential neighborhoods and a commercial avocado grove in Los Angeles County. The insect/pathogen complex is now known to be found over a wide area including most of Los Angeles County, parts of Orange and San Bernardino Counties; and, in late 2013, an outbreak in San Diego County. This outbreak is 60 miles from the nearest outbreak, and indications are that the presence in San Diego is the result of a separate introduction, not spread from Los Angeles.