Oak Tree Protection and Invasive Insects

Oak trees are a long-lived tree which can live many hundreds of years. Oak trees support a complex ecosystem with many species, including humans. They are symbols of strength, nobility, resistance and knowledge. In Native American communities individual oak trees of great size and longevity are culturally important, considered sacred. and used as spiritual centers for important tribal gatherings.

Carbon sequestration is the process of capturing and storing atmospheric carbon dioxide. It is one method of reducing the amount of carbon dioxide in the atmosphere with the goal of reducing global climate change. Oak trees and other hardwoods are very effective at sequestering carbon dioxide to fight climate change.

Oak trees and native forests are under attack in Southern California. Extreme weather and climate events in recent years have resulted in severe droughts and devastating wildfires, which have killed or stressed trees and decimated forests.

The Rincon Oak Tree Monitoring and Restoration Program consists of annual monitoring, treatment of lightly infested, high value oak trees, removal of heavily infested oak trees, and the planting of new oak tree seedlings to replace the oak trees lost to invasive insect attack.

Invasive Insects of Concern

Invasive beetles are also killing native oaks in several areas of Southern California, including San Diego County. Susceptible oaks include coast live oak, which is the predominant species of oak tree found on the Rincon Reservation .

Gold Spotted Oak Borer (GSOB)

Gold spotted oak borer (Agrilus auroguttatus) is a beetle that is native to Southern Arizona and Northern Mexico. In its natural habitat the beetle isn’t very destructive. In Southern California, however, the situation is very different – oak trees are dying rapidly. In areas where the invasive pest has become established, it’s killing 80 to 90 percent of the mature oaks – a dieback that’s fundamentally changing the landscape and the ecosystem the oaks support.

It is believed that GSOB arrived in a load of firewood that someone brought from Arizona. GSOB larvae feed beneath the bark of oaks, and damage the vital tissues of the main stem and larger branches. Generally, a few years after initial infestation, trees decline and die from the damage caused by multiple generations of this aggressive beetle.

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. The beetle prefers to attack larger diameter (>18′′ diameter breast height (DBH)) coast live oak, Quercus agrifolia, and represents the most significant insect threat to red oak species in southern California.

Although the Goldspotted Oak Borer was first identified in San Diego County in 2004, it wasn’t until 2008 that oak deaths were linked directly to them. By 2010, they’d killed more than 20,000 oak trees growing in forests, parks, and urban areas in San Diego County. GSOB infestation was first detected on the Reservation of the Rincon Band of Luiseño Indians in 2018. Additional areas, and existing areas with expanding numbers of infested trees, were detected during surveys conducted on the Reservation in 2020 and 2021.

Invasive Shot Hole Borer (ISHB)

Invasive shot-hole borers (ISHB) have invaded Southern California and are currently threatening tens of thousands of trees throughout the region. This invasive species complex is quickly reaching epidemic proportions and has widespread environmental, economic, and aesthetic implications for the region.

Experts believe the beetles were introduced into San Diego County via products and/or shipping material from Taiwan. ISHB was confirmed to be attacking avocado trees in several commercial groves in north San Diego County in 2014 as well as several species of landscape trees in a single area in east San Diego County. In 2015, a large-scale infestation was identified in the southwestern area of the county at a public regional park. Tree species that ISHB will attack include Coast Live Oak.

According to the University of California Agriculture and Natural Resources website, ISHB beetles tunnel into tree trunks and branches creating channels called galleries. In the process, they introduce fungi that grow in the galleries and later serve as the food source for ISHB larva. The fungi also destroy the food and water conducting systems of the tree, eventually causing stress, dieback and ultimately the death of the tree.

No ISHB have been found to date within Reservation boundaries; however, one live ISHB was found in a live trap outside of, but close to the boundary of the Reservation. The Rincon Environmental Department ensures annual surveys are conducted to monitor for ISHB presence with the Reservation.

Go back to Rincon Environmental Department (RED) Page.