Invasive Shothole Borers

The Invasive Shothole Borer (ISHB) is a group of species of ambrosia beetles that feed on symbiotic fungi that they “farm” inside trees. These fungi infest the tree tissue, resulting in a disease called Fusarium dieback. For some trees, like the California Sycamore, this infestation may mean the death of the tree, and as the pest spreads throughout the Bay Area, its impact on our local ecosystems becomes an urgent concern.

Identifying Invasive Shothole Borers

Right now, ISHB can be found in the southern parts of the Bay Area, but they have been migrating northward and are becoming a concern in the North Bay. There are technically two cryptic species of ISHB present in California – both look identical and can only be distinguished through DNA analysis. One is called the polyphagous shothole borer (PSHB) and the other is the Kuroshio shothole borer (KSHB).

The females are ~2mm (0.08 in) long and range from brown to black in color. Males are smaller, only about ~1.6mm (0.06 in), and have no wings. Females can fly, whereas males cannot.

Invasive shothole borers live within channels (also known as galleries) that the females bore into the trunks and branches. The entrance holes are a clear initial sign that your tree has been infested by ISHB.

Why Invasive Shothole Borers Pose a Threat

The Invasive Shot Hole Borer is known to kill Sycamores and Valley Oaks, common native trees that make up a large portion of our local urban habitat. The death of these trees will drastically reduce the canopy in the broader Bay Area and California as a whole. There are no known natural enemies nor pheromone lures to help manage the pest, which means that without active treatment from experts, the pest will persist and continue to decimate our tree populations.

Which Trees Are Impacted

ISHB May Cause Tree Death

Acer buergerianum – Trident maple
Acer macrophyllum – Big leaf maple
Acer negundo – Box elder
Parkinsonia aculeata – Palo verde
Platanus racemosa – California sycamore
Platanus x hispanica – London plane
Populus fremontii – Fremont cottonwood
Populus nigra – Black poplar
Populus trichocarpa – Black cottonwood
Quercus lobata – Valley oak
Quercus robur – English oak
Ricinus communis – Castorbean
Salix gooddingii – Black willow
Salix laevigata – Red willow
Salix lasiolepis – Arroyo willow

Less Susceptible, But Still May Cause Branch Dieback

Acacia melanoxylon – Australian blackwood
Acacia spp. – Acacia
Acer paxii – Evergreen maple
Acer palmatum – Japanese maple
Acer saccharinum – Silver leaf maple
Aesculus californica – California buckeye
Ailanthus altissima – Tree of heaven
Albizia julibrissin – Mimosa
Alectryon excelsus – Titoki
Alnus rhombifolia – White alder
Archontophoenix cunninghamiana – King palm
Baccharis salicifolia – Mule fat
Bauhinia variegata – Purple orchid tree
Brachychiton populneus – Kurrajong
Camellia semiserrata – Camellia
Castanospermum australe – Moreton Bay chestnut
Casuarina equisetifolia – Australian pine tree
Cocculus laurifolius – Laurel leaf snailseed tree
Corymbia ficifolia – Red flowering gum
Cupaniopsis anacardioides – Carrotwood
Dombeya cacuminum – Strawberry tree
Erythrina caffra – Coast coral tree
Erythrina coralloides – Coral tree
Erythrina falcata – Brazilian coral tree
Fagus crenata – Japanese beech
Ficus altissima – Council tree
Ficus carica – Black mission fig
Gleditsia triacanthos – Honey locust
Harpullia pendula – Tulip wood
Howea forsteriana – Kentia palm
Ilex cornuta – Chinese holly
Jacaranda mimosifolia – Jacaranda
Koelreuteria bipinnata – Chinese flame tree
Liquidambar styriciflua – American sweet gum
Magnolia grandiflora – Southern magnolia
Magnolia virginiana – Sweet bay
Parkinsonia florida – Blue palo verde*
Parkinsonia x sonorae – Sonoran palo verde
Persea americana – Avocado
Platanus mexicana – Mexican sycamore
Prosopis articulata – Mesquite*
Pterocarya stenoptera – Chinese wingnut
Quercus agrifolia – Coast live oak*
Quercus chrysolepis – Canyon live oak*
Quercus engelmannii – Englemann oak*
Quercus suber – Cork oak
Salix babylonica – Weeping willow
Spathodea campanulata – African tulip tree
Tamarix ramosissima – Tamarisk
Wisteria floribunda – Japanese wisteria
Xylosma congesta – Dense logwood / Shiny Xylosma

What We’re Doing About It

At TreePro Professional Tree Care, we are doing everything we can to get ahead of the Invasive Shothole Borer infestation by educating our community. We also provide the appropriate treatment to get rid of the pest.

ISHB is best managed with the use of trunk-injected insecticides and fungicides at the base of trees. When trees die, they must be chipped and stumps ground to 1” in diameter for 100% control of the pest, including any surface roots. Woodchips must be disposed of at a recycling facility.

For more information, or if you suspect your trees are infested with Invasive Shothole Borers, contact the team at TreePro Professional Tree Care today.