Tree Threats Update


Our region’s precious trees are still under attack, threatened by various “bad guys.” Here’s an update from last year’s report.

Shot Hole Borer Beetle
Oaks, willows, palo verde, and California sycamore are just a few of the trees susceptible to infestation by shot hole borers, which continue to spread across the region, according to Beatriz Nobua –Behrmann Urban Forestry Advisor for UC Cooperative Extension. According to Nobua –Behrmann, each beetle bores a tiny hole into a tree’s bark and inserts a bit of fungus. The beetle actually eats the fungus, not the tree, but while a tree might tolerate a small number of beetles, but as the population grows, all those holes eventually kill the tree. By the time you notice that a tree is dying, it could be home to thousands of beetles. Once it dies, each of those beetles (and their offspring) spread out to find new trees to infest.

The best shot hole borer defense is a good offense. Check your trees for tiny holes in the bark, stained bark, gum on the bark, and so on. Mild infestations, Nobua-Behrmann says, can be treated with sprays or tree injections.

One caution: Since shot hole borers can find their way into your garden in firewood and in freshly chipped wood mulch. Nobua-Behrmann recommends that all firewood be kiln dried or properly solarized (heated by the sun’s rays) before you bring it onto your property. Only accept any woodchips (aka “arborist chips”) that are smaller than one inch across. For information about the trees most susceptible to shot hole borers, how to recognize an infestation, and what to do if you find shot hole borers in your trees, visit opens in a new

South American Palm Weevil
Have you ever seen a Canary Island palm (Phoenix canariensis) whose fronds flop backwards? Collapsed fronds are a sure sign of the deadly South American palm weevil. Other signs are yellowing fronds, holes, tunnels, and frass at the base of fronds.  Once infected, a palm becomes a weevil breeding ground. The black, inch and a half long weevils can fly up to 15 miles in a day. They can also travel from palm to palm on poorly cleaned pruning shears, saws, and other tools.

As soon as you see the oddly floppy fronds, It is important to cut down infested palms as soon as you see the oddly floppy fronds, and don’t hire the guy around the corner to do the work unless he or she is a certified arborist and disinfects their tools as they work.

Please don’t move live palms out of infested areas — weevils can move with them. If you suspect that South American palm weevils are in your area, you can trap them using a pheromone-baited bucket trap. Directions are at opens in a new

To learn more, attend the September 4th meeting on palm weevil management put on by University of California Cooperative Extension in Escondido. Contact Sonia Rios sirios2@ucanr.educreate new email for information.

To learn more about this pest and to report collapsing palms, visit opens in a new

Citrus Green Disease (HLB)
Experts say it is just a matter of time until deadly citrus greening disease (HLB) reaches San Diego County. It is already infecting orange, lemon and other citrus trees in LA, Orange, and Riverside Counties. The USDA describes it as “one of the most serious citrus diseases in the world.”

Citrus greening is caused by a bacterium that is carried from tree to tree by the tiny Asian citrus psyllid (ACP).

HLB bacteria interferes with the tree’s ability to move nutrients. Leaves on infected plants develop irregular yellow splotches or mottling (yellowing from nutrient deficiencies are have more distinctly even patterns). Fruits stay small, grow lopsided, and taste bitter. Some fruits remain partially green even when ripe. There is no cure.

ACP is in our region, but so far, none of the local psyllids are infected with the deadly HLB bacteria, thanks to an aggressive program by the California Department of Food and Agriculture (CDFA) for releasing tiny parasitic wasps that kill Asian citrus psyllids.

In addition, there is an HLB quarantine that prohibits the movement of citrus fruits, leaves, and trees out of regions of Orange and LA counties. Please, do your part to keep citrus greening from coming into San Diego County by following the quarantine guidelines posted at opens in a new

If you see or suspect that a tree has HLB, report it at opens in a new

Garden expert, designer, educator, and author Nan Sterman specializes in low water, sustainable, and edible landscapes. Nan hosts, co-writes, and co-produces opens in a new windowA Growing Passion, a show that explores the power of plants in our world. Nan is also author of California Gardener’s Guide vII, Waterwise Plants for the Southwest, and Hot Colors, Dry Garden. More information is at opens in a new