Goldspotted Oak Borer, Agrilus auroguttatus, is an invasive flatheaded borer that is moving into southern California from Mexico and Guatemala. Entomologists in California consider that this pest is simply extending its native range as it moves north. It was first identified in San Diego County in 2004. The rapid death rate of infested trees make the GSOB a catalyst in increased wild fire risk and also reduces wildlife habitat and food supply.
Research is being conducted to determine which insecticides might be effective against GSOB. There are no natural enemies of the pest known in the state of California. The USDA Forest Service does recommend the use of neonicotinoid treatments.
Other Treatment Practices
- If dead oak trees are taken down, landowners are asked to tarp the wood piles to restrict the ability of the adults to fly and infest other trees.
- If the wood piles are not tarped, it is also good practice to leave the wood onsite for more than 2 years before moving it- this will reduce the risk of moving GSOB in wood.
- If grinding or chipping wood, less than 1 inch particle sizes are recommended to eliminate the wood borers in infested wood.
- Cambistat improves the health of trees which improves their ability to withstand stress induced insect and disease attacks. For best results, Cambistat should be applied as part of an overall management strategy.
- Adequate water is a key factor in maintaining healthy trees. A slow, deep watering event once per week during dry conditions will help maintain soil moisture levels and minimize the stress that invites the two lined chestnut borer.
- Mulch is very beneficial for trees because it reduces competition with turf and moderates soil temperature and moisture levels. The addition of 2-4 inches of wood chips or shredded bark under the drip line can have a very beneficial effect.
- Avoid compacting the soil, changing the soil grade or water drainage pattern, damaging the bark, allowing significant amounts of defoliation by insects or anything else that may stress the tree. Anything that weakens tree health encourages borers.
- Promote health and vigor with, root collar excavation and prescription based fertilization practices.
High levels of insect control are expected. In areas with severe drought where soil applied insecticides may have reduced uptake and distribution a trunk injection should be considered.
Signs of Damage
- Premature twig dieback
- Thinning of the crown
- Woodpecker feeding
- Adult feeding on leaf margins
- Within the lower 8 feet of the main trunk and around root collar
- Black or dark red staining or bleeding on the bark
- D-shaped exit holes measuring 0.15 inches wide
- Dark colored frass filled larval feeding galleries under the bark in a meandering pattern
Photo: Center for Invasive Species Research
Photo: Wikimedia Commons
Trees At Risk
- Coast Live Oak (Quercus agrifolia)
- Canyon Live Oak (Quercus chrysolepis)
- California Black Oak (Quercus Kelloggii)
- Engelmann Oak (Quercus engelmannii)
- Single generation each year. Adults generally have a lifespan of several weeks.
- Adult females lay eggs on trees June through September.
- Larvae hatch within 2 weeks and bore through outer bark into phloem.
- They feed in a meandering pattern leaving dark frass filled galleries.
- Mature larvae (prepupae) remain in a nonfeeding stage from October to April – June of the following year.
- Adult emergence occurs mid-May to September.
- Peak flight activity is late June to early July.
- Once emerged, adults feed on foliage of trees and mate.
Always refer to product label for rates and approved uses. Some images courtesy of forestryimages.org or Wikimedia Commons. Use of the images does not imply endorsement of treatments.