Identifying Symptoms and Damage

Identifying Signs of ISHB attack and Fusarium Dieback (FD) in Trees

Because ISHB is just a tiny beetle which spends most of its life inside the tree, it can be difficult to find, let alone identify the beetle itself. If you’re unable to find a ISHB beetle on the bark surface, start looking for small (ball point pen tip-sized), round entry-holes on the bark. In addition to entry-holes, trees under attack can exhibit a variety of other signs of ISHB/FD infestation, like staining, gumming, presence of frass (thin sawdust), and sugary exudates (see descriptions below). Each tree species react differently to ISHB attacks, so the combination of symptoms can be unique to each host tree species (see the Field Guide to the right for guidance). Finally, there are other beetles and disease-causing pathogens that can cause signs similar signs to ISHB/FD; use this lookalike pests guide to help determine if ISHB/FD is a likely cause of the tree’s problems or another pest and/or pathogen may be to blame. Once the correct identification of the problem is made, you can use this handout and this management assessment tool to choose the most appropriate management action.

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Entry-Hole

PSHB entry-hole on Mimosa/Silk tree (Monica Dimson / UC Cooperative Extension)

PSHB entry-hole on coast live oak (Monica Dimson / UC Cooperative Extension)

The typical entry-hole to a PSHB gallery is perfectly round and about 0.85 mm (less than 0.04 inches) in diameter, or about the size of the tip of a ballpoint pen. The abdomen of the female beetle can sometimes be seen protruding from the entry-hole. It may also be plugged or covered by sap, exudate, or frass (sawdust from boring) from the tree. If sending photos to a specialist for identification, include a picture with a pen for scale.

Symptoms of Attack

Staining on box elder in response to PSHB attack (Monica Dimson / UC Cooperative Extension)

Gumming caused by PSHB activity on Goldenrain tree (Monica Dimson / UC Cooperative Extension)

Sugary exudate caused by PSHB on avocado (Akif Eskalen / UC Riverside)

Frass caused by PSHB activity on California sycamore, which also typically responds by staining (Monica Dimson / UC Cooperative Extension)

Entry-holes are typically accompanied by attack symptoms, which are the host tree's visible response to stress. These symptoms (and examples of species that produce them) include:

Staining: may be wet and dark OR dry and light-colored (e.g. sycamore species, oak species)
Gumming: thick resin that sometimes pushes the beetle out of the gallery (e.g. silk trees, Koelreuteria species like Goldenrain tree)
Sugary exudate: may form white, powdery "volcanoes" (e.g. avocado)
Frass: produced by the beetle's boring activity, may be present on any host tree depending on the infestation level (long "matchsticks" of frass have been observed on box elder, coral tree, and willow species)

Some of these symptoms may be washed away or obscured by rain or irrigation water.

Branch Dieback

Healthy sycamore, right, and PSHB-infested sycamore with dieback, left (Akif Eskalen / UC Riverside)

Advanced infestations lead to branch dieback and overall decline. Watch for beetle attacks concentrated on a branch or in the branch collar (the point of attachment between the main trunk and a branch)—infestations in this area can lead to limb failure.

Signs of Infection

Fusarium stains red willow wood around a PSHB gallery (Akif Eskalen / UC Riverside)

Fusarium-infected CA sycamore wood (Monica Dimson / UC Cooperative Extension)

Galleries stained by Fusarium infection, seen on a cut coast live oak branch (Akif Eskalen / UC Riverside)

Fusarium causes dark discoloration of the wood beneath the bark and around the beetle gallery. Lightly scraping away the bark around the entry-hole will reveal the dark brown to black staining.