Don't give this beetle a ride: A new threat to Placer County oaks

Drive through Placer County in late summer and you may notice something unsettling: oak trees with browning leaves, dead branches or thinning canopies. Much of what we see is the normal result of summer heat and drought stress. But there's another reason to look more closely. The Mediterranean oak borer (MOB), Xyleborus monographus, is a tiny invasive beetle that can kill mature oak trees. It has recently been found in Placer County. MOB has been seen in Penryn, with evidence of infestations in Rocklin as well. Its appearance along the Interstate 80 corridor is especially concerning because one of the primary ways invasive wood-boring insects travel is with us, via firewood and other infested wood. A tiny beetle with a dangerous partner At only about 1/8 inch long, MOB is easy to overlook. Female beetles bore into an oak and create branching tunnels, called galleries, where they introduce fungi carried on their bodies. The beetles and their larvae feed on these fungal "gardens." Together, the beetle and fungus damage the tree's water-conducting tissues. Branches begin to die. A severe infestation can eventually kill the tree. Valley oaks, prominent in the lower elevations of Placer County, are particularly susceptible. Blue oaks, common as we climb into the foothills, can also be attacked. Is it MOB or just summer stress? A brown oak does not necessarily mean MOB. Look for several clues. Look at the canopy. Drought and heat stress often affect much of a tree uniformly. MOB may first appear as individual dead or rusty-brown branches, sometimes called "flagging," while surrounding portions of the canopy remain green. Because MOB may begin high in the crown, early symptoms can be easy to miss. Look at the bark. The beetles make tiny, round entry holes, about 1/16 inch across. Fine, pale boring dust may collect in bark crevices or on branches. Sap may sometimes ooze from the tree. These symptoms can also be caused by other insects or diseases. Look for galleries. If a dead or suspect branch has already been cut, examine the wood beneath the bark. MOB creates distinctive, crowded tunnels that intersect and branch in a trellis-like pattern, often with dark staining. Don't give MOB a ride There is currently no established chemical spray or injection treatment that will save a heavily infested tree. That makes containment particularly important. Dead and infested wood should be managed appropriately. Don't transport untreated oak wood to another location, including wood chips. Beetles can continue to emerge from cut wood. Infested material can be chipped, and solarizing wood chips beneath heavy plastic can help prevent surviving beetles from escaping. Firewood deserves special attention. An infested log can carry MOB into an entirely new neighborhood or woodland. Remember: Buy it where you burn it. Better yet, keep oak firewood as close as possible to where the tree grew. Watch, report and don't panic Don't assume every browning oak has MOB. Many environmental conditions, native insects and diseases produce similar symptoms. Instead, look for the combination of unusual branch dieback particularly at the crown of trees, tiny boring holes, boring dust and characteristic galleries. If you suspect MOB, take photographs and report the suspected infestation. UC Agriculture and Natural Resources provides information on identifying MOB and directs reports of suspected infestations to the California Department of Food and Agriculture. Our oaks are one of the defining features of Placer County's landscape. Learning what MOB looks like, reporting suspicious trees and making sure we don't give this destructive beetle a ride can help protect them. For more information, visit the UCANR Mediterranean Oak Borer website, https://ucanr.edu/site/mediterranean-oak-borer.