Layman's Guide to the Polyphagous Shot Hole Borer Beetle (PSHB)

The PSHB was first detected in Southern California in 2003. The insect was not thought to be problematic. In 2012, it was deemed problematic, but by them it was too late as its numbers had skyrocketed. It is initially from S.E. Asia/India.

The difference between the PSHB and the other boring beetle known to the canyon (Goldspotted Oak Borer) in that the PSHB transmits a fungus in addition to boring into the tree and affects many trees other than the Oak. They do not eat the wood, but rather the fungus they produce. Both the fungus and the tunnels they produce (aka Galleries) are what kills the tree over time. Woodpeckers have also been known to transmit the fungus from tree to tree.

The females are the ones that carry the fungi. Females can fly and are much larger than the males. The beetles also mate with siblings, which is not good. The below is a photo of the female, magnified many times. They are actually about the size of a large flea.

PSHB

They have currently been identified in 75 species of trees, including many that exist in our canyon. Of these 38 species are reproductive hosts. At UCI, they seem to prefer the Sycamore tree. These are just some of the trees they prefer:

  • Box Elder
  • Alder
  • Sycamore (beetle attacks the tree trunk)
  • Live Oak (beetle attacks the the limbs-pruning the affected branches will eliminate the beetle)
  • Maple
  • Avocado

They only reproduce in some of the tree species.

The telltale signs of the PSHB are tiny holes in the tree. If you scrape away at the hole (below image) you will see the black residue which is a sign of the PSHB, although the live oak trees do not necessarily exhibit staining. You can also look for sawdust around the tree's base. This is produced by the beetle pushing the wood out of the tree.

Stained wood from shot hole borer fungal infection on Mexican sycamore.

At this time, the PSHB has not been identified in the Cleveland National Forest. However, it has spread throughout the Irvine area. It is only a matter of time before we are affected.

UCI and UCR are experimenting with various treatments for the beetle. It is a difficult problem to treat because the beetle burrows deep into the tree, making it hard to kill. They have had some success with pesticides (ONYX) but if the tree is too infested, the best treatment is to remove it. Trees that produce a lot of sap are better able to fight of the infestation.

What can you do?

  • Monitor your trees for activity. If you think you have an infestation, the FS would like to know.
  • Corrective pruning. If a limb is infected, remove it. If you do remove it, do NOT transport the wood outside of the forest. Chip it into tiny pieces or burn it. If you cannot do either, try solarization. Cover the wood with plastic and allow it to heat up in the summer sun. This will kill the beetles.
  • Tree Removal. It's probably best to check with the FS if you need to remove an entire tree that has become infested.

Remember, our planet loses 15 billion trees per year. Only 5 billion are replaced so it is important to save all the trees that we can.

More Information:

Regional Beetle Infestation Prompts Removal of UCI Trees

Polyphagus Shot Hole Borer Beetle- UCI

Polyphagus Shot Hole Borer Beetle - UCR