Oak Tree Attacker

It’s becoming an all-too-frequent topic; there is a new exotic insect pest moving in our direction. This one is attacking oak trees, one of the most iconic features of our native habitats and created landscapes. It is the gold-spotted oak borer—Agrilus coxalis. The sturdy gold-spotted oak borer (GSOB) is a member of a group of small wood-boring critters whose larvae specialize in boring into woody trees or shrubs. They are in search of the sap-rich cambium layer just under the bark. Here, they burrow and feed, often girdling the limb or branch that they inhabit. This is fatal to that portion of the tree. If enough of the tree is compromised, the entire plant will die.

So far (hallelujah!), this new scourge is well to our south. It presumably started its journey north from somewhere in Mexico. It has been recorded in parts of Guatemala and Mexico for a long time. It was even collected in southeast Arizona since the 1800s. Recently, it has appeared in California. Of late, it seems to have moved into San Diego County, but no farther. Earliest positive identifications there are dated 2002, but latest ones, still in a narrow radius, are from 2008. In order to track and report incidence of GSOB, the U.S. Department of Agriculture Forest Service is enlisting the help of anyone and everyone to stop or at least slow the progress of this serious threat to native oaks.

At this point, the GSOB seems to prefer coast live oak (Quercus agrifolia), canyon live oak (Q. chrysolepis), and California black oak (Q. kelloggii). Unfortunately, these are the largest oaks and the dominant species in the southern regions of California. The pest’s impact on other species will hopefully never be known. One of the major transporters for this insect seems to be humans. The larvae lie dormant under the bark, normally through the winter, but also after the tree has succumbed to their depredation. Maybe they only kill a branch or two on a given tree that then falls to the ground. Gathering such wood is commonplace on public lands as well as in urban landscapes. Cutting and transporting this infected wood easily spreads the insect far beyond its own natural reach. Even if identification of an infestation isn’t positive, be warned that moving dead oak wood around the state may aid in the spread of this potentially devastating pest. There are no established guidelines for control of this pest, but it is strongly recommended that arborists, land managers, forest health specialists, and homeowners pay attention to these suggestions. Wood may still be used in the locale where it was collected, but if it is to be stored, it should be covered with thick, clear plastic sheeting or exposed to direct sunlight. This may kill GSOB larvae and pupae, but it has not yet been scientifically proven. If the wood is not to be burned soon, chipping it into one-inch or smaller pieces is the best method to drastically reduce its survival.

So what does it look like? What are the signs of infection? Eggs are probably (no direct observations, yet) laid in crevices of the bark. On hatching, the tiny larvae burrow in where they go through several stages, called instars (how poetic), feeding all the time on the nutrient-rich tree sap. This maturation may take most of one year or carry on to the next year (so much is still unknown about this particular species). There may be evidence of this larval presence in staining of the bark, either black regions or red blisters of sap oozing out from under the bark. Emerging adults leave a small (only about one-eighth of an inch) but recognizable D-shaped wound. This will probably only be revealed after a close inspection following some form of branch dieback or even excessive leaf drop. Not time to panic yet, but definitely time to learn the warning signs of a new pest on the block. Be vigilant!

November Tips

• Mulch: The very cool summer and fall probably retarded the breakdown of mulch, but check and replenish to a depth of three to four inches. It will retain moisture all winter and slowly release nutrients.

• There is still time to plant fall vegetables like carrots, beets, chard, and cabbage relatives from seed, but seedlings will probably be more successful because soil is cooling so early this year.

• The same applies for ornamentals like Iceland poppies, pansies, lobelia, and more. Purchase inexpensive starts at area garden supply outlets.