The Goldspotted Oak Borer (GSOB) (Agrilus auroguttatus) continues to kill native oaks in several areas of Southern California. Susceptible oaks include coast live oak (Quercus agrifolia), canyon live oak (Q. chrysolepis), and California black oak (Q. kelloggii). In many cases, GSOB has damaged or killed mature oaks valued for their beauty, wildlife habitat, and shade. Areas with large numbers of native oaks are particularly at risk. Unfortunately, oaks that are injured over several years from multiple generations of the GSOB often die.

Although GSOB was first identified in San Diego County in 2004 it wasn't until 2008 that oak deaths were linked directly to GSOB. By 2010, GSOB killed over 20,000 oak trees growing in forests, parks, and urban areas in San Diego County. Later infestations occurred in Idlyllwild (2012), Orange County (2014), and Los Angeles County (2015). The three most recent outbreaks have all occurred in San Bernardino County. The first occurred in Oak Glen in 2018 followed by infestations in California black oaks in the Sugarloaf area of Big Bear in August 2019 and in Wrightwood in early November 2019.

The GSOB is native to southeastern Arizona where it is not destructive to otherwise healthy native oaks. This may be due to natural enemies and/or resistant oak species that have co-evolved with GSOB. Damage. Damage occurs from larval feeding on the vascular (water and nutrient conducting tissues) system inside trunks and branches. Infested trees have black stained bark and may ooze sap underneath red bark blisters. Adult beetles leave a distinctive D-shaped exit hole.

Damage from GSOB adults feeding on leaves is not a major concern. Insect Identification. GSOB larvae are about 0.8 inches long, white and legless with two pincher-like spines on the end of their abdomen. Adult GSOB are smaller (about 0.4 inch long) and are mostly black with six gold spots on their forewings. Soft-bodied pupae resemble adults in size and shape and are found in the outer bark from late spring to early summer.

 Prevention is important since there are no known control methods once trees become infested with GSOB. Keeping infected firewood onsite is the most effective way to stop its spread. Wood should never be moved offsite since this is the major method by which GSOB is spread. No known natural enemies have been identified and insecticides are not generally effective. Monitoring susceptible trees species and identifying and reporting new infestations early are both important.

If you believe there is an infested oak on your property please submit photos of the entire tree, a close up of a leaf (to confirm the species), and a close up of the surface of the bark on the main trunk. If possible, include a photo of an unsharpened #2 pencil tip next to any visible exit holes since are both around .15 inches wide. https://ucanr.edu/sites/gsobinfo/Help_Monitor/Report_Goldspotted_Oak_Borer_Symptoms/ A team of scientists from UC, California Department of Forestry and Fire Protection, CALFIRE and the U.S. Forest Service and other agencies are working collaboratively to reduce the devastation from this insect and identify effective biological control agents.

[Originally published in the Fall 2018 issue of the Green Bulletin. Modified slightly from original.]

Invasive insects and diseases are threatening numerous tree species throughout the country. Impacts include tree mortality, destruction of forest and urban habitats, and other significant changes in forest ecosystems due to the decline or elimination of tree species. Many of these pests can be transported in inadequately-processed wood, including firewood and discarded wood debris left behind from tree care operations (Figure 1). Preventing the spread of these pests to new areas is critical for protecting valuable forest resources. Invasive forest pests of concern include those established in other parts of the country (e.g. emerald ash borer and Asian longhorned beetle), as well as those with limited distributions in California and other areas of the western U.S. (e.g. invasive shot hole borer, goldspotted oak borer, various bark beetle species, and the pathogens responsible for pitch canker and sudden oak death).

Properly managing cut wood can effectively reduce the risk of spreading these threats to new areas. Arborists and tree care workers can help by using best management practices (BMPs) for cut wood.

BMPs need to be broad for a number of reasons:

• Some species, such as shot hole borers (Figure 2), can infest over a hundred tree species.
• They are intended to prevent the spread of a number of invasive pests.
• These pests are located in many different areas and their distributions are not definitively known.
• Incipient or outlier infestations may not be reported, making it less certain which areas may be free of invasive pests.
• It can be difficult to determine if a particular tree or wood cut from it is infested (or contaminated) with an invasive pest, including trees that appear healthy.
• Wood can become infested after it is cut from a tree.
• Even insects and pathogens that are native to a particular region of the U.S. can become serious pests when moved to an area where they don't exist. Goldspotted oak borer and pitch canker are good examples.

Basic Recommendation - Avoid moving unprocessed wood

The safest approach is to assume that all untreated wood can harbor invasive pests and therefore should not be moved from the local area. Utilize or dispose of untreated wood near the site where it was cut. Firewood outreach campaigns like “Don't Move Firewood” and “Buy It Where You Burn It” encourage this approach.

Untreated cut wood should not be left on the curbside for free pick-up (Figure 3) as there is no way to control where the wood is moved to. If wood is to be left with the landowner, talk to them about the danger of spreading invasive pests in infested wood. Handouts explaining the risk can be found at the California Firewood Task Force's website firewood.ca.gov/outreach.html and includes a Firewood Question and Answer Factsheet and Firewood Postcard. These can be printed and distributed to landowners.

Treatments to Discourage or Destroy Invasive Pests in Wood

• Grinding wood to a 1-inch minus chip size greatly reduces but does NOT eliminate all shot hole borers (SGB). Therefore, composting chips originating from near or within known SHB infestation areas is recommended. Wood should be chipped before being transported to a biomass or composting facility more than 50 miles away. If that is not possible, the receiving facility should promptly chip the material upon arrival.
• Heat treatment of infested firewood to a core temperature of 160° F for a minimum of 75 minutes has been shown to eliminate most insects and diseases.
• Complete removal of bark from branch and trunk rounds can eliminate certain insects that feed solely in or under the bark, such as bark beetles and some wood borers. Debarking does not eliminate insects that burrow in the wood or pathogens infecting the wood.
• Seasoning (drying) firewood on site for at least 2 years can reduce the risk of transporting most invasive insects. Similarly, wood that is cut from trees that have been dead for greater than 2 years is unlikely to contain living invasive insect pests, though invasive pathogens may still be present.
• Never treat firewood with an insecticide or pesticide. Doing so could result in exposure to toxic fumes when the wood is burned.

Invasive pests associated with firewood in the western U.S.

The following insects and tree diseases are found in limited areas of the western United States and represent forest health threats to hardwood and conifer species throughout the nation. Compliance with federal and state regulations is required to move wood from quarantine areas. Adhering to BMPs will limit the spread of all invasive pests and help protect our urban and native forests.

Federally quarantined species:

Sudden oak death, Phytophthora ramorum

Non-quarantined invasive species:

Invasive shot hole borers, Euwallacea spp.

Goldspotted oak borer, Agrilus auroguttatus

Redhaired pine bark beetle, Hylurgus ligniperda

Mediterranean pine engraver, Orthotomicus erosus

Balsam wooly adelgid, Adelges piceae

Pitch canker disease, Fusarium circinatum

Thousand cankers disease, Geosmithia morbida

General recommendations for handling and using firewood

• Don't move firewood long distances from where it was purchased. Use wood from local sources: “Buy it where you burn it.”
• Ask about the firewood you are buying. Is it from a local source (less than 50 miles preferred) or has it been treated to eliminate or reduce pests (e.g. heat treated or seasoned for 2 or more years)?
• When purchasing firewood for camping, buy an amount that can be completely burned during your stay. If firewood is left over, leave it for the next camper; don't take if home with you.
• Plan on cutting firewood within 50 miles of where it will be used.
• Seasoning of green wood (letting it dry) for at least 2 years or using wood from trees that have been dead for at least two years can reduce the danger of transporting most invasive insects threatening trees.
• Note that cutting standing trees can be dangerous, especially dead trees which often are unstable. Only cut standing trees if you have the proper equipment and training to do it safely.

Online Resources

Don't Move Firewood dontmovefirewood.org/

APHIS aphis.usda.gov/newsroom/hot_issues/firewood/index.shtml

USDA Forest Service na.fs.fed.us/firewood/

CA Firewood Task Force firewood.ca.gov/

Yosemite National Park nps.gov/yose/naturescience/forest-pests.htm

Buy It Where You Burn It firewood.ca.gov/pressreleases.html

- Reprinted with permission, courtesy of the California Firewood Task Force

Shot hole borers are tiny insects the size of a sesame seed that don't look particularly harmful, but don't let their diminutive size fool you. Two of these borers are invasive—the polyphagous shot hole borer and the Kuroshio shot hole borer. They carry pathogens and are spreading them throughout southern California. Together, the borers and the fungi are a deadly combination that are killing many trees. Trees affected include avocado, sycamore, white alder, box elder, cottonwood, and willow.

The two shot hole borers are nearly identical in appearance, and both have a symbiotic relationship with several pathogenic fungi. The female borers lay eggs which introduces fungi into trees. The fungi grow and provide food for larvae, which tunnel through the tree. This disrupts the flow of water and nutrients and causes branch dieback, canopy loss, and eventually tree mortality.

Dead or declining trees may cause public safety hazards from falling limbs, increased fire danger, and increased risk of flooding due to blocked waterways. Dying and dead trees also decrease property value, and result in habitat loss for endangered birds and other wildlife.

How Can You Help?

Prevent the spread of these damaging pests in several ways.

• Buy it Where You Burn It. Buy firewood in the same county where you plan to burn it.
• Leave wood at home when visiting a park or going camping.
• Purchase and use wood from local sources for both home heating and outdoor recreation.

To learn more about these borers and the diseases they carry, read the leaflet A Devastating Threat to California Trees and visit UC ANR webpage on Invasive Shot Hole Borers.

Two closely related Ambrosia beetles (Euwallacea sp.) have been identified in commercial avocado groves in California.  The polyphagous shot hole borer (PSHB), detected in Los Angeles, Orange counties and recently in Ventura county, and the Kuroshio shot hole borer (KSHB), detected in San Diego and recently in Orange and Santa Barbara counties, are morphologically indistinguishable, but genetically distinct.  Already widespread in a variety of reproductive host trees common in the urban landscape (including box elder, willow, several maples, oak and sycamore species), the beetles represent a significant threat to trees in both landscape and agricultural settings.  Adult females construct galleries in the xylem system of host trees, where they cultivate symbiotic fungi (Fusarium, Paracremonium and Graphium spp.) as a food source for their developing young.  The fungi are taken up by progeny females in specialized organs within their mouthparts, and transported to other sites within the same tree, where new colonies are established, or to newly colonized hosts.  The galleries compromise the structural integrity of infested trees, which can represent a serious safety hazard in urban environments, and disrupt the flow of water and essential nutrients within the xylem.  In addition to the physical damage, the fungi extract nutrients from the xylem system, further depriving the tree of nutrients essential for healthy growth and fruit production.

An effective biological control agent is not yet available to manage the SHB in California, and so management for now must rely on the use of chemical pesticides.  The control of Ambrosia beetles and their associated fungi using

chemical pesticides is complicated because of their location inside the host trees.  The application of insecticides to the external surfaces of trees, where the beetles must first alight prior to boring, has the potential to kill beetles by contact activity, and they may also have the potential to control emerging young adults before they can re-infest the trees. 

The drawback of surface treatments is that multiple applications are often required because of the relatively short duration of efficacy.  In addition, once the beetle burrows inside the tree, surface treatments are become ineffective.  One possible solution to this problem may be the use of systemic pesticides, and scientists at UC Riverside are evaluating the use of both systemic insecticides and fungicides in a 2-pronged attack against the symbiotic system. 

Systemic pesticides are mobile within the xylem system of plants, and the fungicides could potentially target the fungi growing in the xylem and deprive the beetle larvae of a food source.  The insecticides would prevent the beetle from establishing galleries within susceptible tree hosts, and prevent the survival of beetles and their offspring already present within trees.  The big problem with systemic pesticides is getting sufficient concentrations of chemicals to the areas within the trees where the beetle and fungus occur.  Although there are exceptions, most systemic treatments are administered to the soil for uptake through the roots.  However, in mature avocado groves, the high organic matter content of the soil can prevent effective absorption by roots because the pesticide becomes bound to organic components within the soil.  Trunk injection of pesticides directly into the vascular system of trees eliminates the potential for binding of pesticides within the soil, and increases the amount of active ingredient inside the tree available to impact the beetle/fungal system.  Systemic pesticides must be formulated for trunk injection and so careful evaluation is needed to ensure optimal efficacy.  Trials are being conducted with the assistance of avocado industry and grower collaborators in areas where the SHB has been recorded.  The chemicals are injected into the trees using commercially available equipment, and the movement of the active ingredients is then monitored over time in wood core samples taken at different heights of the trees.  Two methods are being used to confirm the presence of the chemicals.  Insecticides are being quantified using ELISAs that are specific for the active ingredients under investigation.  Wood cores taken from trees treated with fungicides are placed in direct proximity to the fungal pathogens growing on agar plates to determine if growth of the fungus is inhibited.

The investigations are still at an early stage, but the researchers are optimistic that they will develop effective control strategies for the SHB that growers can incorporate into their overall pest management programs.  Laboratory based bioassays have been used to identify several pesticides that are toxic to the beetle and fungi.  The objective of the field trials is to determine whether these chemicals can be utilized as trunk injection agents for the protection of avocado trees. Anyone interested in finding out more about the SHB should go to the web site maintained by Dr. Akif Eskalen at:

 http://eskalenlab.ucr.edu/avocado.html