Searching for natural enemies of the goldspotted oak borer in Arizona

By CISR Team |
Vanessa Lopez
Vanessa Lopez University of California, Riverside

Written by:
Vanessa Lopez (Ph.D. Candidate, UC Riverside)
Mark Hoddle, Ph.D. (Biological Control Specialist and Principal Investigator)

Photos by:
Mike Lewis and Vanessa Lopez


The goldspotted oak borer (GSOB), (Agrilus auroguttatus) (Coleoptera: Buprestidae),is a recent pest of native oak trees in southern California.  This fairly small beetle (about 10 mm long and 2 mm wide) is native to the mountains of southern Arizona, and was likely introduced into the Cleveland National Forest in San Diego County, California through the unintentional movement of infested oak firewood.  GSOB was first collected in San Diego County in 2004, but was probably introduced into the area several years earlier.  This beetle has killed more than 22,000 native California oaks in a relatively short time period, and continues to spread into new areas within southern California.  In fall 2012, GSOB was found in Idyllwild, Riverside County, about 80 miles from the GSOB infestation epicenter, This find resulted in immediate eradication efforts by local government agencies including CAL Fire and the US Forest Service.  GSOB was also detected in 2010 in Marion Bear Memorial Park, San Diego County, about 65 miles from the GSOB epicenter and has likely established here. Movement of infested oak firewood out of the Cleveland National Forest probably moved GSOB into these new areas.

In southern California, GSOB aggressively attacks larger (more than 12 cm in diameter) coast live oak (Quercus agrifolia), California black oak (Q. kelloggii), and canyon live oak (Q. chrysolepis), and can even attack and kill healthy trees.  Trees infested with GSOB die from repeated larval feeding damage to the cambial layer, which is the water and nutrient transportation system of the tree.  Although adult GSOB feed on oak foliage, adult feeding does not result in tree injury.

In southern Arizona, where this beetle is native, GSOB is not a pest.  In its native habitat, this beetle infests stressed tress, and can actually be very difficult to find.  Since GSOB occurs at much lower population levels in Arizona, there may be natural enemies that have evolved to efficiently find and consume these beetles.

Developing a classical biological control program for GSOB in California

Since 2009, research towards the development of a classical biological control program for GSOB in California has been ongoing.  The goal of classical biological control is to restore balance to an ecosystem by reuniting an invasive pest with its host specific co-evolved natural enemies.  To find natural enemies that have co-evolved with and only attack GSOB, we’ve conducted several surveys throughout the home range of this beetle, and are continuing our search in summer, 2013.

Searching for egg parasitoids of GSOB

Surveys for egg parasitoids of GSOB were initiated in summer 2012 using field deployed GSOB egg masses to attract and potentially become parasitized by these tiny wasps.  For eight weeks, GSOB egg masses were set out and collected every seven days in the Santa Rita Mountains, Pima County, Arizona and in the Cleveland National Forest, San Diego County, California.  Surveys in Arizona resulted in the collection of the first known egg parasitoid of GSOB, a Trichogramma species that is a currently undescribed generalist parasitoid wasp which likely attacks eggs of a lot of different species.  No egg parasitoids were found in California.  Additional GSOB egg parasitoids surveys are planned for this summer (2013) in Arizona.

Continuing surveys in summer 2013

GSOB has been collected from several mountain ranges across southern Arizona including the Chiricahua, Dragoon, Huachuca, Santa Catalina, and Santa Rita Mountains.  To find field sites for the 2013 GSOB egg parasitoid surveys, we took a road trip through southern Arizona to search for active populations of GSOB.  GSOB densities in Arizona are naturally very low, so this beetle can be difficult to find in its native range.  Trees showing symptoms of decline (crown die-back, patches of pale and dry leaves, and bark staining) were ground checked for evidence of GSOB infestation (D-shaped exit holes, larval feeding galleries, and GSOB life stages), but the majority of death or decline in the oaks we encountered was due to fire damage.  However, we did encounter several sites with GSOB activity in the Chiricahua, Dragoon, Huachuca, and Santa Rita Mountains which we will use in summer 2013 to search for GSOB egg parasitoids by hanging GSOB eggs onto infested oak trees.

Ultimately, if we can find a host specific egg parasitoid of GSOB eggs it may be a candidate for eventual release in California to help control this invasive pest. So far our surveys suggest that GSOB eggs in California are not attacked by parasitoids and this could be one reason why populations of this pest are so high in the Cleveland National Forest when compared to forests in Arizona. Restoring an egg parasitoid from Arizona with GSOB in California may help reduce the densities of this pest and alleviate some of the damage it causes.


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