Beware the Tree Borers…

NanStermanTrees and shrubs across the region are under siege. Along every major street and in every neighborhood there are dead trees, browning trees, trees that are stressed and in decline. Much of this can be attributed to drought, whether directly or because drought-weakened trees are susceptible to pests, in particular to tiny beetles known collectively as “borers.”

These tiny, flying Asian beetles – no bigger than a sesame seed – cause big problems for more than 300 kinds of fruiting and ornamental trees and shrubs throughout the region. According to County of San Diego Entomologist Tracy Ellis, there are several kinds of shot hole borers, all lumped into a group referred to as “ambrosia beetles.”

What they do: Pregnant female beetles burrow through the tree or shrub’s bark and into its sapwood as they carve out tiny tunnels (known as “galleries”) to lay their eggs. Each beetle inoculates her tunnels with spores of a fungus that the developing larvae will feed on once they hatch from the eggs (all other stages of the beetles eat the fungus too). Once the fungal colonies begin to grow, the female returns to lay up to 10 eggs per tunnel. You might expect a female beetle to bore one or two tunnels in a tree; but those females are prolific! They can drill enough tunnels to girdle a plant, even enough to make a tree collapse! In the meantime, the tunnels interfere with the tree’s ability to move water and nutrients through its tissues, which spells death for a tree. And while the fungus is food for the beetles, it too is deadly for the tree. One way or another, the tree’s chances for survival are slim.

What to look for: An infected plant may look like it sprung a leak. Sometimes the plant tries to plug leaks by exuding crystal-like sugars to form, “sugar volcanoes,” large enough for you to see. Infected sycamore trees look as if they were shot with BBs. On some plants, branches develop dry or wet and oily, dark stains at the beetles’ entry points. Wood and leaves can become discolored and entire branches wilt, then die back. Ambrosia beetles infect at least three hundred kinds of trees and shrubs, including avocado, California sycamore, willows, several kinds of oaks, Camellia, Acacia, kentia and king palms, Jacaranda, and many more. Gold Spotted Oak Borers are very similar to ambrosia beetles but only attack oak trees whose trunks are eight inches or more in diameter.

Diagnosis: If you think you have a tree or shrub infected by borers, submit a photo or a sample of the plant to the San Diego County Plant Pathology Laboratory for free diagnosis (see below).

Prevention: Healthy, well-hydrated plants are more resistant to infection, while under-watered or otherwise stressed woody trees and shrubs are more vulnerable.

This is another reason to deep water trees and shrubs, even during drought.

Beetles travel on freshly cut wood so be very careful that any wood you bring onto your property does not have beetles:

  • If you bring in firewood, BE SURE the firewood has been aged (“seasoned”) long enough that the bark is dry and falls off.
  • If you bring in wood chips for mulch or any other reason, BE SURE the woodchips are no larger than an inch across and have dried for several months. BETTER YET, avoid the issue altogether by using aged mulch rather than fresh wood chips.

Treatment: There is no cure for badly infected plants; have them removed by a professional arborist who is trained to work with ambrosia-infected trees. Have the wood chipped to particles no larger than an inch; that appears to kill the borers. Have the entire rootball removed.

Websites to watch:

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