The Invasive Shot Hole Borer (ISHB aka Polyphagous Shot Hole Borer), Euwallacea sp., is a boring beetle from the same family as bark and ambrosia beetles. ISHB is a non-native species originating from South East Asia, and was identified in2003 in Los Angeles County. Over 130 species of tree are susceptible to attack from ISHB, but the severity of injury can vary from single branch dieback to entire tree death. However, the ISHB can complete its life cycle in only about 20 tree species. Trees are not killed directly by the boring of the beetle, but by the symbiotic fungus the beetle carries with it, Fusarium euwallaceae. The fungus clogs the conductive tissues of the tree cutting off the flow of water and nutrients to the tree.
Bark sprays Bifen XTS or Tengard can be used as protectants to prevent ISHB from attacking its hosts. The use of Mectinite has yielded strong anecdotal reports from arborists managing this pest in Southern California, as well as in preliminary research trials. Applying in conjunction with trunk injected fungicide such as propiconozole has also been reported as having anecdotal success. Because ISHB can attack healthy trees, it is important to treat healthy appearing trees that are near known infestations. More research is needed to determine the effectiveness of the use of systemic insecticides/fungicides on this pest and its vectored pathogens.
Adequate water is a key factor in maintaining healthy trees. A slow, deep watering event once per week during dry conditions will help maintain soil moisture levels and minimize the stress on trees. Mulch is beneficial for trees because it reduces competition with turf and moderates soil temperature and moisture levels. The addition of 2 to 4-inches of wood chips or shredded bark under the dripline is recommended.
Timely removal of severely infested or ‘amplifier’ trees to prevent the spread (dispersion) of adult female beetles.
Signs of Damage
- Symptoms can vary depending on tree host.
- Vascular wilt symptoms, such as branch dieback.
- Wet staining on bark from leaking xylem fluid.
- White powdery exudate (sometimes referred to as ‘sugar volcanoes’), gumming, and/or frass (sawdust).
- Beetle entry and exit holes, ~0.85mm in diameter, can be located beneath or near symptoms.
- Secondary pathogen fusarium fungus vectored by beetle causes dark discoloration of the wood beneath the bark and around the beetle gallery.
- Scraping away the bark around the entry hole reveals brown to black staining or discoloration from Fusarium infection
Photo: Eskalen Lab, UC Riverside
Photo: Entomology Today
Photo: Natural History of Orange County, California
Trees At Risk
- Avocado (Persea americana)
- Big leaf maple (Acer macrophyllum)
- Box elder (Acer negundo)
- California sycamore (Platanus racemosa)
- Castor bean (Ricinus communis)
- English oak (Quercus robur)
- London plane (Platanus x acerifolia)
- Beetle is very small, measuring about 0.06”-0.08” long
- Brown to black in color
- Females are larger in size, can fly and disperse
- Female beetle bores into trees and inoculates galleries with spores of Fusarium spp. fungi
- After ~8 days of boring, females lay eggs in a pile at the end of the gallery.
- Larvae hatch after a few days and begin to feed on the fungal spores in the gallery.
- After a month, the larvae pupate and become adults – most are female. Males from the same offspring mate with females.
Akif Eskalen. Kabashima, John. Dimson, Monica. (2016). Polyphagous Shot Hole Borer and Fusarium Dieback Field Guide [Brochure]. CA: Author.
University of California IPM
Richard Stouthamer. The Polyphagous Shot Hole Borer, Euwallacea fornicatus, a New Invasive Pest in Southern California [Powerpoint]. Department of Entomology, University of CA, Riverside
University of California Riverside
United States Department of Agriculture
Always refer to product label for rates and approved uses. Some images courtesy of forestryimages.org or Wikimedia Commons. Use of the images does not imply endorsement of treatments.