We have seen an increasing number of disease pathogens and insects attacking many tree species throughout our country. In Southern California, the gold spotted oak borer, (GSOB), (Agrilus auroguttatu), first introduced in San Diego County between 1990-2000 has spread into Riverside County and is expected to spread throughout California. This pest specifically targets the Coast Live Oak and Black Oak, (Quercus agrifolia, Quercus kelloggii), both oaks comprising a major part of our red oak forests throughout California.
The adult female beetle lays eggs on host trees mainly June through September, the eggs hatch into larvae which bore through the outer bark and feed on the phloem and xylem, the interior vascular system of the tree. After several months, the larvae mature into pupae, which eventually bore from the vascular tissue just under the bark, and may be observed on the outer bark of infested trees April through July. Adult emergence follows and the beetles feed on the foliage of the infected trees throughout summer, mating and then repeating the life cycle.
Larval feeding on the cambium, the vascular system of the tree, causes damage to the oak tree. Extensive feeding disrupts the water and nutrient movement throughout the tree vascular system, resulting in crown and branch dieback. Repeated, severe infections may girdle the tree, resulting in tree death. To date, thousands of red oaks throughout the state have succumbed to this non-native pest.
Research continues in an attempt to identify biologic predators to control the GSOB, to date no commercially effective predators have been developed. State and county registered commercial pesticide applicators are having limited success, mainly by following the protocol established for another known pest, the emerald ash borer. Homeowners using common over the counter insecticides have limited or no success at controlling the GSOB.
More about the GSOB is available at UC Davis Research on GSOB
It Gets Worse!
As if the GSOB wasn’t bad enough, a new destructive pest has emerged within San Diego County, the polyphagous shot-hole borer, (PSHB), yet another wood boring beetle. Unlike the GSOB that targets limited species of oaks, the PSHB beetle has infested 286 tree species and poses a major threat to landscape trees and orchards throughout Southern California. This nasty pest threatens the avocado industry, and our beloved natives such as the Coast Live Oak and California Sycamore, (Platanus racemosa).
The beetle was first found in Whittier Narrows in Los Angeles in 2003 and has been steadily moving southward into Orange, Riverside and San Diego Counties. The beetle drills into the tree bark, injecting the Fusarium fungus into the tree. While certain trees are resistant to the fungus, many are not. The fungus clogs and blocks the xylem cells of the vascular system; resulting in branch dieback and in severe cases, tree death.
Like the GSOB, this beetle is non-native, most likely originating from Southeast Asia or Africa. While the GSOB is more likely to attack trees in poor health, the PSHB is known to attack healthy trees. Currently, there are no control methods available to contain this invasive pest.
How are These Pests Spreading?
The main vector for the spread of both woodborer beetles is through the use of firewood. As infected tree branches and entire trees die, the tree and branches are cut and in many instances, used for firewood. This is especially true for trees in the backcountry of San Diego and Riverside Counties and where open space lands are cleared and grubbed for development and construction.
The beetle or larvae in dead or dying infected trees are still alive when the tree is felled. Cutting the tree into firewood does not remove the pest, which may remain active on or under the bark. Firewood is sold and transported from the infection site and moved throughout the community, spreading the beetle with it. Unknowing property and homeowners, land developers and grading contractors cutting, selling, buying and transporting infected wood are the main culprits for the spread of both of these extremely damaging pests.
Homeowners removing dying trees may unwittingly spread the disease by disposing the dead material into their trash or dumping. Sapling trees planted at nurseries may be infected, then sold to homeowners who plant infected trees in the yards, increasing the spread of the insect and disease.
Since the insect is an exotic pest, California trees have not co-evolved with the insect and have no means to protect themselves from the beetle. Learn more about this pest at Polyphagous_Shot_Hole_Borer/.
Recommendations on Handling Firewood
Did you recently dispose of a diseased tree? Did you purchase and import firewood from another part of the county? If you answered yes, you may have inadvertently spread these pathogenic insects. Here are some suggestions to prevent the spread of these infectious insects.
- Don’t move firewood more than 50 miles from where it was cut.
- Buy firewood from known, local sources; avoid buying firewood from a “guy” cutting trees from properties in rural areas.
- Buy it where you burn it.
- Advise construction and developer clients on proper means to treat wood from cleared and grubbed projects via de-barking, chipping and grinding to under two inch size material.
- Heat treat wood by tarping and solarizing cut wood to limit the spread of the disease causing organisms.
- Season or allow firewood from cut trees to dry more than two years to reduce the threat of transporting invasive insects.
Remember Smokey the Bear?
Not only can you prevent forest fires, you can also prevent the spread of infectious insects and disease killing our street and landscape trees, forests and crops by your awareness of the problem. We all need to be educated about these diseases, spread your knowledge, not the disease!
Here are some articles on these pests and firewood as a vector:
Brennan, Deborah Sullivan. New Beetle Threatens Arbors, Orchards. Professional Tree Care Association (PTCA) of San Diego Newsletter, January 2014.
Reducing the Spread of Invasive Insects and Pathogens in Cut Wood. Western Arborist Magazine, Summer Issue, 2012.