Invasive Shothole Borers

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.

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Identifying Invasive Shothole Borers

Right now, ISHB can be found in San Jose and the peninsula, but they have been migrating northward. 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 our canopy in the Bay Area. 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

  • 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
  • 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 Urban Tree Management, 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 Urban Tree Management today.

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