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The headwaters of the Santa Ana River Watershed begin in the San Bernardino Mountains with forested areas receiving over 90% of the precipitation that we rely on downstream. Because of this, and the numerous other benefits to our ecosystem, the health of our forests is a very important natural resource for our agency to consider. Forest health can be defined differently depending on what you are managing the forest for, but generally speaking it is good to have forests with mixed species of trees and a diversity in age of trees.

There are many factors that can harm forest health including native and invasive pests, wildfire, diseases, and extreme weather events such as drought and large storms. Some of the ways to protect our forests include:

  1. Monitoring forests for evidence of disease and infection

  2. Managing fuel load to protect forests from large crown fires

  3. Preventing the movement of firewood that can carry pests and diseases into our communities.  

GSOB: Threatening our Native Oaks

The goldspotted oak borer (GSOB), Agrilus auroguttatus, is a flathead beetle that is native to southeastern Arizona but was likely introduced to California via infested firewood. Now, native oak woodlands throughout the state are at risk due to the rapid rate of spread and aggressive feeding behavior of this pest. Adult beetles are approximately 0.4 inches long and 0.08 inches wide and capable of flight. Mated females lay eggs on mature oak trees, and upon hatching, the larvae burrow below the bark where they feed, creating long, meandering galleries that disrupt nutrient transport. The beetle completes one generation each year, and while some trees are capable of withstanding several years of attack, the aggressive feeding by successive generations of GSOB eventually kills the tree if left untreated.


Goldspotted oak borer eggs are too small to be visible, and the larvae stay hidden in the galleries beneath the bark of the host tree until they pupate into adults and emerge. Because of their shape, emerging adults form a distinctive 0.15-inch-wide D-shaped exit hole, so identifying these holes is the primary method for detecting host trees. Other visible external symptoms of GSOB infestation include thinning of the tree canopy, which progressively worsens over time, as well as wet brown, black or red stains on the exterior of the tree. (via Irvine Ranch Conservancy. View more information here)

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Confirmed Areas of Infestation Within Our District
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If you have any further GSOB questions, including concerns about oaks on your property or our FREE GSOB surveys, please contact:


Phone: (909) 255-0587

Forest Health Specialist


Do you want to communicate to your community the importance of GSOB prevention? Contact us for FREE outreach materials!
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