Polyphagous Shot Hole Borer
PSHB in CaliforniaThe Polyphagous Shot Hole Borer (PSHB) is a new pest in Southern California. This boring beetle, from the group of beetles known as ambrosia beetles, drills into trees and brings with it a pathogenic fungus (Fusarium euwallacea), as well as other fungal species that may to help establish the colonies. The PSHB attacks many species of trees, but some trees are resistant to the fungus it carries. The beetle is dark brown to black and tiny, with females between 0.07 and 0.1 inches long, and males even smaller, usually about 0.05 inches long. Pregnant females bore through the tree’s bark, creating galleries under the bark. They plant the fungus in these galleries, where it grows and spreads throughout a susceptible tree. The female then lays her eggs in these galleries and when the eggs hatch, the larvae eat the fungus. The larvae develop into adults in about a month. Many more of the larvae develop into females than males, and the females mate with the males (their brothers) while still in the gallery. The pregnant females then pick up some of the fungus in their mouths, and leave through the entry holes created by their mothers to start the process again.
There are several potential outcomes of a beetle attack.
- Beetle is repelled with no infection. This has been observed in 20 species of trees. Investigators are trying to figure out what features of the tree might repel the beetle.
- Beetle drills into the tree and transmits the fungus, but doesn’t produce offspring. This has been observed in over 50% of the tree species attacked. We don’t know the final outcome of this interaction. Often leakage of xylem fluid is noticed on the trunk and branches. Maybe nothing bad will happen to the tree, but the tree could suffer if the xylem vessels are clogged up, which could cause dieback of branches. Damage could also make the tree more prone to attack from other pest species.
- Beetle drills into the tree, fungus infects the tree, and the beetle produces offspring in the tree. This has been seen in about 8% of the tree species attacked, and these species are considered true host of PSHB, and include box elder, coast live oak, and avocado. Interestingly from a natural resources perspective, it also includes invasive plants like castor bean and tree of heaven (Ailanthus). Some trees seem to suffer mild symptoms like branch die-back, while others are killed outright.
The pest in California
The PSHB seems to have originated in South East Asia, possibly Vietnam. At first, researchers identified it as the Tea Shot Hole Borer (Euwallaecea fornicatus), which it very closely resembles, but DNA evidence points to it being a new, as yet unnamed species in the same genus. The symbiotic fungus may also be a new unnamed species, in the genus Fusarium, which is commonly associated with ambrosia beetles. The PSHB was first found at Whittier Narrows in Los Angeles County in 2003. From 2003-2010 the beetle was found on a few ornamental trees, then in 2010 it was the presumptive cause of the death of a large number of box elder street trees in Long Beach. In 2012 the beetle was collected from a backyard avocado tree in South Gate, and from several tree species at local botanical gardens. It now appears to be established in Los Angeles, Orange, and Riverside Counties, and is expanding its range in San Diego County. A single beetle was caught in Santa Cruz County in 2014.
It attacks a wide range of native, ornamental, and horticultural trees, and has caused severe damage in avocado groves in Israel. As of fall 2014, it’s been found to attack over 200 species of trees in the LA area. Thirty-three tree species have been confirmed as reproductive hosts including the native riparian species Coast Live Oak (Quercus agrifolia), California sycamore (Platanus racemosa), Fremont cottonwood (Populus fremontii), and red willow (Salix laevigata), and white alder (Alnus rhambifolia) and 52 species (about 57%) of the most common street trees in the area, including box elder (Acer negundo), liquidambar (Liquidambar styraciflua), and Japanese wisteria (Wisteria floribunda).
For a recent distribution map for the pest, click here. For the latest information about the impacts of PSHB/Fusaium complex on various species including a list of reproductive hosts in Southern California from Extension Plant Pathologist Dr. Akif Eskalen, click here.
Protect your trees and local habitat from a variety of pest species by avoiding moving infected wood around – use firewood locally.
PSHB has been found to attack healthy trees, but as always a good defense against disease is to keep trees in optimal health. Healthy trees are also more likely to recover more quickly from an attack. Choose trees that are appropriate for the site and don’t require a lot of additional water. Provide appropriate soils and access for roots to grow and expand. Avoid excessivepruning, over- or under-watering, and planting inappropriate companion plants within the dripline. If trees are infected, systemic insecticides generally are poor for treating ambrosia beetles. Prophylactic spraying of the bark could be used to protect uninfected trees in some situations. Sterilize pruning tools between uses to avoid spreading the fungus. This handout can guide you through deciding when to remove an infested tree, and how to handle the wood waste.
Chipping and solarizing/tarping infested wood can help to limit the spread of the beetle/fungus complex. Wood should be chipped to pieces smaller than 1".
R. Stouthamer, P. Rugman-Jones, A. Eskalen, J. Kabashima, M. Dimson, A. Gonzalez, G. Arakelian, D. Hodel, S. Drill
Department of Entomology & Department of Plant Pathology and Microbiology,UniversityofCalifornia,Riverside
Los AngelesCountyAgricultural Commissioner/Weights and Measures