By Keith Scheuer
From the mountains to the sea and throughout Southern California, millions of trees have succumbed to a mixed assault of insects, bacterial and fungal infestation, climate change, and insufficient water.
The numbers are staggering. The U.S. Forest Service reports that bark beetles had killed more than 102 million trees in the Sierra Nevada by the end of 2016. More than 200 kinds of bark beetles reside in California, about 20 of which are invasive. Most attack pines and other conifers, but some also go after broad-leaf trees.
In the Santa Monica Mountains, goldspotted oak borer beetles have destroyed several thousand native oaks, and approximately 30,000 more in San Diego County.
Another kind of beetle called the Kuroshio shot hole borer has destroyed almost 150,000 willows in Tijuana River Valley Regional Park. In many parts of Orange County, vast, leafy groves of alders, Liquidambars and sycamores are being decimated by the polyphagous shot hole borer, a close cousin of the Kuroshio. These two borers look identical but have differing DNA. Collectively, they are known as invasive shot hole borers.
These bugs have also invaded The Los Angeles County Arboretum and Botanic Garden in Arcadia. The Arboretum’s Frank McDonough estimates that more than 900 of its trees and shrubs have been attacked by borers, and that roughly 200 trees have been completely lost. The dead include 20 or 30 large Eucalyptus, destroyed by the Eucalyptus longhorned borer.
He says the climate of Southern California requires a more pragmatic sense of what can grow here.
“People are going to have to adapt a new aesthetic that uses plant material and trees that are more climate appropriate,” McDonough says. “The geology [in] places like the Los Angeles basin is such that the groundwater is over 100 feet deep and so impossible for trees to get their roots into. If you look at pictures of the L.A. basin before it was hugely developed it was a treeless plain.”
According to a study conducted by U.S. Forest Service researcher Greg McPherson, invasive shot hole borers threaten to kill nearly 38% of all trees in Los Angeles, Orange, San Bernardino and Riverside Counties, about 27 million trees total.
“Many of the trees we grow evolved in temperate climates and can’t tolerate the stress of drought, water restrictions, higher salinity level in recycled water, wind and new pests that arrive almost daily,” McPherson says.
Many kinds of killer
Beetles are just one group of culprits killing trees. Among many others are citrus psyllids, glassy-winged sharpshooters and palm weevils.
These and similar pests threaten a variety of native trees, decorative trees, and those cultivated for agriculture, including avocados, citrus and palms.
According to entomologists Paul Rugman-Jones and Richard Stouthamer of the University of California Riverside, the invasive Kuroshio and polyphagous shot hole borers most likely came from Vietnam and Taiwan and were first discovered in California in 2003 and 2013, respectively. Kuroshio are about as long as a grain of rice. Female polyphagous borers are the size of Lincoln’s nose on a penny, the males much smaller.
According to the UC Riverside researchers, these invaders have shown rapid population growth in California and elsewhere. In 2012, the polyphagous borers were confined to southern Los Angeles County, Orange County and the extreme southwestern edge of San Bernardino County. By 2016, their range extended from San Luis Obispo to the Mexican border.
These foreigners are not fussy about where they hang their hats.
Akif Eskalen, a plant pathologist at UC Riverside and his colleagues have identified 62 known reproductive hosts in California for invasive shot hole borers, including species of elder, maple, sycamore, willow, avocado, oak, cottonwood, alder, chestnut, holly, camellia, acacia, wisteria, fig, beech, poplar, buckeye, palm, coral, magnolia and jacaranda.
Different bugs kill trees differently. Bark beetles eat the inner bark. Borers dine on a fungus (fusarium dieback) that they introduce into host trees. The fungus disrupts the flow of water and nutrients to the host tree, eventually killing it.
Eskalen notes, “Trees may be more susceptible if they are already under stress due to other pests, diseases, or environmental conditions or are in close proximity to an existing infestation.”
Researchers with the Forest Service attribute the great slaughter of local trees by these insects to our low rainfall and mild winters in recent years. Too little water stressed the host trees and weakened their ability to resist predators; mild winters allowed the predators to survive and spread in far greater numbers than they otherwise would have.
“Among bark beetles and wood borers, higher than average winter minimum temperatures and drought are correlated with bark beetle outbreaks. Higher winter temperatures, and longer growing seasons are expected to become increasingly common into the future,” according to Forest Service experts Haiganoush K. Preisler, Nancy E. Grulke, Zachary Heath and Sheri L. Smith.
Eliminating these pests may be impossible, but some remedies have succeeded in limiting the damage.
McDonough of The Arboretum notes that preventive chemical spraying has shown some positive results but is expensive and only feasible if the tree has significant economic value.
Entomologists Rugman-Jones and Stouthamer and Mark Hoddle, head of the Center of Invasive Species Research at UC Riverside, state that certain biological remedies, such as parasitoid wasps from the invasive pests’ native regions, have been effective in controlling Eucalyptus longhorned borers, lerp psyllids and ash whiteflies. Other possible regimens include nematodes, parasitic flies and helpful fungi.
However, so far nothing has been found that can undo harm already inflicted. Once a tree has been severely hit, it’s probably a goner.
The expected long-term consequences of this gigantic die-off are dire – less shade, less beauty, less wildlife habitat, less carbon and pollutants removed from the air.
“If we cannot control the shot hole borer, it will kill all the sycamores in California,” says UC Riverside’s Eskalen. And according to Hoddle, “there will be no miraculous recovery of these urban ecosystems after the beetles are done with them.”