Invasive shot hole borers (ISHB) are two closely related species of small, non-native, beetles that bore into trees. ISHB introduce fungi that cause a tree disease called Fusarium dieback (FD). The ISHB-FD pest-disease complex is responsible for the death of thousands of trees in Southern California and poses an imminent threat to the integrity of our urban and natural forests.
Invasive Shot Hole borers attack a wide variety of tree species including avocados, common landscape selections, and California native species in urban and wildland environments.
It’s been a huge problem in Southern California the last 10 years and has finally made to Northern California.
Over 78 species of trees and shrubs have been identified worldwide as ISHB-FD reproductive hosts (trees in which the fungus can grow, and the beetles can reproduce), including many California native and common landscape tree species. Another 247 species in 64 plant families are attacked but the beetles cannot successfully reproduce in them. Among the confirmed reproductive hosts, the combined effects of the beetles and the fungal pathogens have different impacts. On some, they cause tree decline and branch dieback, while on others they cause tree death. In some cases, hosts only get infested with ISHB-FD in the margins of preexisting canker infections caused by other wood canker fungal pathogens.
The characteristics of ISHB attack and fungus infection differ among tree species. The beetle commonly attacks the main stem and larger branches of trees and shrubs, but injury can be found on twigs as small as 1 inch in diameter. The beetle produces a very precise, perfectly round, tiny (< 0.1 inches in diameter) entry hole in most trees. Wet staining and discoloration on the bark of the main stem and branches are early symptoms of beetle attack. Depending on the tree species attacked, ISHB injury can be identified either by staining, gumming, or a sugaring response on the outer bark. Infection with the fungus can cause leaf discoloration and wilting, dieback of entire branches, and tree mortality.
The most significant damage occurs from May through October so if ISHB is found near or on trees, treatments should be made immediately.
Generally, the best seasons for injection are spring and fall, since the best uptake occurs when trees are actively transpiring, but treatments can be made at any time of the year when there is good soil moisture to encourage translocation of the systemic formulations.
The most significant damage occurs from May through October, so if ISHB is found near or on trees, treatments should be made immediately.
Generally, the best seasons for a trunk banding with a systemic insecticide is in Spring and Fall, since the best uptake occurs when the trees are actively transiting.
I strongly suggest an Imidacloprid systemic insecticide soil drench in February. It can be done any time of the year when there is good soil moisture to encourage translocation of the systemic formulations. This has been proven to discourage new borer activity.
In a perfect world it would have been done as a preventive measure, before the tree is infested.
If you see any signs or symptoms of Invasive Shot Hole Borers on your trees, call to schedule a Consultation appointment with our Certified Arborist / Plant Health Care Specialist for the appropriate treatment.
“An ounce of prevention is worth a pound of cure.”
-Larry Van Groningen
ISA Certified Arborist, TRAQ Qualified
Plant Health Care Specialist