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Fall 2024

Invasive beetle known for killing trees spotted in the Bay Area

by Dorothy Abeyta

Figure 1. Young branches dying. Photo courtesy of UCANR.
Figure 1. Young branches dying. Photo courtesy of UCANR.
The headline above appeared recently in the SF Chronicle, on Facebook, X, and local news sources, but the Invasive Shot Hole Borers (ISHB) have been in the Bay Area since last year when they were spotted in north San Jose. The first beetle was detected in Los Angeles County in 2003 and the second beetle twin (for lack of a better term) was detected in San Diego County in 2014. They have spread north to San Luis Obispo County and east to Riverside and San Bernardino counties. The most recent find in Santa Clara County is a leap up the coast, which points out that the mode of spread appears to be artificial, i.e. spread by humans transporting infested firewood and green waste. This beetle is non-native and therefore, has no natural enemies to keep it in check. Preventing it’s spread is the number one method for control. The state has implemented “Buy It Where You Burn It” and “Don’t Move Firewood” campaigns directed at residents to make them aware this invasive insect IS a problem and urge them not to spread the problem.

The beetle-fungus complex (explained below) has a very wide host range of over 70 species, including California native valley oak, sycamore, and boxelder trees that are very susceptible. Many common landscape trees, like Japanese maple and sweetgum, are also very susceptible. Commercial avocado orchards are also a high risk.

Figure 3. Size of male (L) and female (R) beetles. Photo courtesy of UCANR.
Figure 3. Size of male (L) and female (R) beetles. Photo courtesy of UCANR.
These tiny beetles are about the size of a sesame seed (Fig 3). The beetles bore into the tree, carving tunnels deep into the wood where they lay their eggs. However, it’s not the tunneling that kills the tree. The real damage comes from a fungus the female beetles carry and deposit in the tunnels. This fungus grows inside, serving as food for the young larvae when they hatch. But as it spreads, the fungus invades the tree's water transport system, cutting off its ability to move water and nutrients to the leaves and branches. Over time, the tree weakens, declines, and eventually dies. As the larvae eat the fungus, they molt and form a new generation of male and female beetles. Once the young female mates, she will fly off with a supply of the fungus on the sides of her mouth and start the cycle again on a different or, more often, the same tree, if it is still alive.

Usually, the first symptoms of this beetle-disease infestation are young branches dying (Fig. 1). When the dying branches are viewed closely, symptoms of oozing and weeping of tree sap (Fig. 2), and sometimes what looks like sawdust left behind as the female beetle drills into the wood can be observed. The oozing symptoms can look different on different tree species.

Figure 2. Oozing on trunk (L to R): Native sycamore, Coast live oak, Avocado. Photo courtesy of UCANR.
Figure 2. Oozing on trunk (L to R): Native sycamore, Coast live oak, Avocado. Photo courtesy of UCANR.
Hence, the University of California Agriculture and Natural Resources (UCANR) prepared an online photo gallery of symptoms available to the public. All you need to do is type in the name of the tree and photos of the symptoms specific to that tree pop up. 

What distinguishes this beetle from the many other similar beetles are distinctive signs of the Invasive Shot Hole Borer-disease infestation, including:

  1. perfectly round entry holes about the size of a ballpoint pen tip (Fig 4), and 
  2. deep tunnels into the wood in contrast to the beetles that bore tunnels just under the bark.

Figure 4. Round entry holes the size of a ballpoint pen. Photo courtesy of UCANR.
Figure 4. Round entry holes the size of a ballpoint pen. Photo courtesy of UCANR.
Early detection is key to treatment and management of this beetle-fungus disease complex. Early detection allows for treatment options like pruning to remove the pest. Pesticide treatments may be an option. Removing the source protects the rest of the trees in your landscape and in your community. If you see symptoms of branches dying, look it up using the online tools to see if the symptomatic tree or any trees on your property are susceptible. If the answer is YES, go the next step and look closely at the branches for signs of the beetle.

Be on the lookout! Help save our trees. If you see dead branches, examine your trees closely. There are many websites to help you with initial identification (see resource list below), including photos, lists of susceptible trees, and a link to report the find to the UCANR research teams. You can ask your knowledgeable Master Gardeners, too. Before reporting infestations, take the ISHB Detection Assessment to see if your tree is suffering from ISHB damage. And most important of all DON’T MOVE FIREWOOD!

Trunk showing how deep into the trunk the ISHB tunnels. Photo courtesy of UCANR.
Trunk showing how deep into the trunk the ISHB tunnels. Photo courtesy of UCANR.

 

RESOURCE LIST

VIDEO – AN INTRODUCTION TO ISHB:
https://ucanr.edu/sites/pshb/

DISTRIBUTION MAP:
https://ucanr.edu/sites/pshb/pest-overview/ishb-fd-distribution-in-california/

ISHB Reproductive Hosts:
https://ucanr.edu/sites/pshb/pest-overview/ishb-reproductive-hosts/

Polyphagous shot hole borer (DONTMOVEFIREWOOD.org):
https://www.dontmovefirewood.org/pest_pathogen/polyphagous-shot-hole-borer-html/

Identifying signs and symptoms on different tree species:
https://ucanr.edu/sites/pshb/diagnosis/#identifying-signs-and-symptoms

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