June 2019 June 18, 2019 Vol. 6, Issue 2
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Invasive Shot Hole Borers: Not all Ambrosia Beetles are the Same
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Many ambrosia bee
tles
colonize r
ecently dead trees, but some attack stressed trees that
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Invasive Shot Hole Borer Adult. Photo by: Curtis Ewing, CALFIRE
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are still alive and a fe
w species attack healthy trees. The invasive shot hole borers (Euwallacea spp.) are an important invasive ambrosia beetle that attacks many species of healthy trees. The Invasive Shot Hole Borers (ISHB) in California consist of two physically identical species, the Polyphagous Shot Hole Borer (PSHB, Ewallacea whitfordiodendrus) and the Kuroshio shot hole borer (KSHB, Ewallacea kuroshio
), originating from Southeast Asia. These beetles are very small, about the size of a sesame seed and are believed to have been accidentally introduced into California on wood products and/or packing material. They attack and reproduce in many types of trees including commercial avocado groves, common landscape trees and native trees found in urban, wildland and riparian environments. ISHB was first identified in Los Angeles County in 2012 and since then, the infestation has spread to six other Southern California counties: Orange, Riverside, San Bernardino, San Diego, Santa Barbara and Ventura.
Upon infestation, tree
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Crown dieback and staining on California Sycamore caused by ISHB infestation. Photo by: Kim Corella, CALFIRE
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death is caused not from direct beetle injury, but rather by the fungi they carry in their specialized mouth parts. Both species of ISHB carry three different fungi in these mouth parts, and these fungi are the sole food source for these beetles. When the beetles make galleries in the host tree, they inoculate with these fungi. The larvae and adults then feed on the fungi, forming a symbiotic relationship between the fungi and the beetle. One kind of fungi that they carry, Fusarium, causes the disease known as Fusarium Dieback. The fungus destroys the vascular system of the tree, which causes branch and crown dieback, and in many cases tree death. Currently there are 64 confirmed species of trees in which these beetles can successfully grow and complete their life cycles. The susceptible tree species include box elder (Acer negundo) California sycamore (Platanus racemose), oaks (Quercus lobata, Q. robur), willows (Salix gooddingii, S. laevigate. S. lasiolepis), maples (Acer buergerianum, A. macrophyllum and cottonwood (Populus fremontii, P. trichocarpa). For a more complete host list please visit
www.phsb.org
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Symptoms of ISHB attack vary with the tree
species attacked, but there are commonalities:
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Female abdomen protruding from entry hole on California Sycamore. Photo by: Kim Corella, CALFIRE
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- A small beetle entry hole (0.8 mm wide), about the size of a medium ballpoint pen tip, that the beetle makes when excavating their galleries.
- Wet staining, gumming, white powdery exudate and/or frass, associated with the beetle entry hole.
- Branches in the tree crown may die back. Crown symptom could be a sign of a severe infestation and the tree should be checked closely for ISHB entry holes.
Manag
ement of landscape trees requires regular monitoring, early detection survey for infested trees, and managing infested wood properly. Only infested trees should be treated. Trees infested with ISHB can rapidly become hazardous, as limbs and branches break and fall due to pathogen colonization and mechanical damage from beetle galleries. So far there are no effective preventative treatments, so best management practices are essential to control the spread of these insects and their associated disease fungi.
When an infested tree is found, disposal of infested wood is essential because ISHB can survive in cut wood for weeks or even months. The best disposal method is to chip and treat the infested wood -- chipping infested wood to 1 inch or less will kill 95% of the beetles. The chips should then be treated by solarization, composting or kiln-drying to ensure all beetle and fungal propagules are eliminated. Untreated chipped material should only be used in areas that are already heavily infested with ISHB.
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Forest Management Task Force
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Healthy forests provide many environmental, health and economic benefits and the members of this task force strive to increase the health and resiliency of our forests. Please visit the
California Forest Management Task Force for more information.
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Tree Mortality and Future Wildfires
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When buying firewood for the up coming camping season this summer, remember to buy wood sourced local to where you will be using it, helping to minimize the spread of pests and diseases -
Buy It Where You Burn It. For a list of local firewood dealers, go to
firewoodscout.org.
Sincerely,
The California Forest Pest Council
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