June 8, 2018
by Carol Bornstein
Trees in Southern California have had a particularly tough time these past few years due to the ongoing drought. Below-normal rainfall stresses trees in gardens as well as natural areas, potentially increasing their susceptibility to harmful insects and diseases. One unwelcome pest wreaking havoc and causing much concern among just about everyone involved in landscaping and forest management is the invasive shot hole borer (ISHB) and its associated disease, Fusarium Dieback.
ISHB is actually two closely related beetles (Polyphagous and Kuroshio) in the genus Euwallacea that look virtually identical. First detected in 2003 at Whittier Narrows, these tiny brown borers — about the size of a sesame seed — have spread rapidly throughout the Los Angeles area and have now been found in eight other counties in California. Entomologists aren’t certain how these beetles from southeast Asia made it into California (probably in shipping materials) but the damage they are causing is increasingly evident. Some trees are dying at alarming rates, especially in riparian woodlands (moist zones along rivers and creeks). Sadly, several trees in the NHMLA Nature Gardens and in Hancock Park are showing signs of infestation, and we recently removed two unhealthy trees.
What exactly is going on? First, a female beetle bores into a tree. The tiny, perfectly round holes are roughly the size of a ballpoint pen tip. Next, the beetle tunnels into the wood and introduces a trio of fungi (Fusarium euwallaceae, Graphium euwallaceae, and Paracremonium pembeum). She then lays eggs in the tunnels, also called galleries. After hatching, the emerging larvae eat the fungi and in about a month become adult beetles. The adult males and females (technically brothers and sisters) mate inside the galleries, the impregnated females pack some fungi in their mouths, and then leave through the entry holes, crawling up the same tree or flying off to another tree to start the process all over.
The entry holes are somewhat difficult to spot but there are other, more conspicuous signs of trouble if the fungi are present. Look for white or brown staining, gummy brown ooze, or a powdery white substance on the bark near the entry hole. Sometimes there is reddish-brown frass (insect poop) mixed with sawdust. The symptoms differ among tree species and the damage is initially internal. The microscopic fungi clog up the tree’s conductive tissues, impeding the flow of water and nutrients that are essential to healthy growth. This results in wilted or discolored leaves, branch dieback, or death of the entire tree.
A fitting analogy to this destructive insect/disease duo is mosquitos that transmit diseases like malaria, dengue fever, and West Nile virus to humans. And just like humans, different kinds of trees have different reactions to the beetle/fungi combo. To date, over 250 tree species have been attacked by ISHB. The good news is that some trees are able to repel the beetles. Other trees become infested and infected, but no breeding occurs. The third possibility is infestation, infection, and breeding and there are now 64 species of these host trees. Exotic species include avocado, fig, and London plane as well as California natives such as western sycamore, big-leaf maple, oaks, willows, and black cottonwood.
Researchers at the University of California have been diligently studying ISHB and Fusarium Dieback, looking at both chemical and biological controls. They have also produced some excellent online references for landscape professionals and home gardeners on how to identify the beetle, what to do with infested wood (don’t transport it!), which trees are affected, and helpful photos of the symptoms. See www.pshb.org and click on Handouts and Resources or Diagnosis and Management for the most up-to-date information.
Some trees in L.A. appear to be recovering after sealing off the fungal infection. Researchers are monitoring this promising development and hope to learn more about this phenomenon. Although they can’t yet predict how trees will adapt or succumb to ISHB and its fungal partners over time, researchers can and do tell us that our best defense is to keep our trees healthy. For an established tree, that means avoiding under- or over-watering and excessive pruning, minimizing soil compaction and disturbance in the root zone, and maintaining a 2-3 inch layer of mulch.
Trees offer so many tangible and intangible benefits to our communities. When we help them, we help ourselves and local wildlife, too.