Beetle Could Destroy Many California Tree Species

By Laurel Busby
Staff Writer

The native box alders in Southern California are already gone, and if a tiny beetle isn’t stopped, experts say the California sycamores will be next.

The polyphagous shot hole borer, a type of ambrosia beetle, prefers the sycamore now that box alders have been eliminated, but also eats several species of native oaks and more than 48 other varieties of trees, including 20 native species, according to Akif Eskalen, a plant pathologist at UC Riverside who has been working on the issue for about five years. The beetle’s willingness to bore into and eventually kill numerous tree species makes it particularly damaging.

“This is dangerous; it has a wide host range,” said Eskalen, a plant pathologist who runs UC Riverside’s Eskalen Lab, which is not only studying the beetle, but working to find ways to stop it. “The beetle can produce thousands and thousands of beetles in one infected plant. Infested plants must be removed properly or they will infect nearby plants. We don’t want that beetle to move from one location to another location.”

Emil Chang and Natalie Queally (right) from the NASA Earth Observations DEVELOP program join Santa Monica Mountains Biologist Rose Dagit at Trippett Ranch in Topanga. Photo: Nina Trusso

Emil Chang and Natalie Queally (right) from the NASA Earth Observations DEVELOP program join Santa Monica Mountains Biologist Rose Dagit at Trippett Ranch in Topanga. Photo: Nina Trusso

The tiny one millimeter beetle, about the size of a chia seed, arrived in Los Angeles from southeast Asia in 2002, Eskalen told the Palisades News. As of this year, it had spread to seven other counties: Orange, Riverside, San Bernardino, San Diego, Ventura, Santa Barbara and San Luis Obispo. Aside from the sycamore, its current favored host trees are oaks, willows and cottonwoods.

The beetle, a fungus farmer, carries multiple fungi in its mouth and bores into trees to create tunnels that it then lines with the fungi to grow it as a food. Unfortunately, the fungus also prevents the travel of nutrients within the tree and eventually kills its host.

Rosi Dagit, a senior conservation biologist for the Resource Conservation District of the Santa Monica Mountains, has been studying the health of Topanga Canyon trees and others throughout the western Santa Monica Mountains.

In 2015, she initially began studying the drought’s effect on the trees, but she also gathers data on the shot hole borer beetle and other insects via lures placed throughout the western mountains. These lures trap insects that happen to be in those areas. Currently, the shot hole borers “are now pretty well established” in Calabasas, Dagit said, but they have yet to reach Topanga or the various creek areas.

Adult female shot hole borer is about the size of a Chia seed. Photos: Eskalen Lab

Adult female shot hole borer is about the size of a Chia seed. Photos: Eskalen Lab

“If they get into our creeks, that’s not going to be a pretty picture,” Dagit said. “If the creek trees go, it’s very hard for the creeks and the things that live in the creeks.”

A 10-week NASA project aided Dagit’s research by isolating the light spectrum of drought- and insect-damaged trees and tracking the changes through satellite data from 2010-16. The project provided initial data, but a more detailed analysis will be requested to provide a clearer picture of how many trees are affected.

“I don’t know how many acres or how many trees yet, but it looks very bad,” Dagit said of the initial analysis.

Greg McPherson, a research forester with the U.S. Forest Service, said the polyphagous shot hole borer beetle could kill as many as 27 million trees in L.A., Orange, Riverside and San Bernardino counties, or about 38 percent of the region’s 71 million trees.

“Itcouldbereallydisastroustolosealot of canopy at one time because canopy is so important to quality of life and the environment,” McPherson said. “It’s a manner of managing the insect rather than the insect managing us because then we just have to react and chop down the trees . . . Whether or not that happens depends on decisions that are made by people who control the resources to combat this kind of threat. Ultimately, it’s all of us, because it’s our community.”

Each year about nine new pests enter the metro area or a new one about every 45 days, and 30 percent of these insects are potentially damaging, said John Kabashima, a UC environmental horticulture advisor.

Part of the problem with the shot hole borer beetle in particular is that it is affecting such a wide variety of trees, including irrigated trees. Many pests tend to prefer drought-damaged trees or ones that live in oversaturated soil, so proper care of trees can make them resistant to many pests.

This sycamore has been infested by shot hole borer-fusarium. Photo: Eskalen Lab

This sycamore has been infested by shot hole borer-fusarium. Photo: Eskalen Lab

The shot hole borer beetle is also mainly targeting landscape trees, rather than agricultural trees, although it has affected some avocado trees, Kabashima said. However, most of the trees that are being infested are simply part of the urban landscape, and neither the state nor the country has a funded methodology to address this type of pest.

“The level of funding that we’re getting is a fraction of the millions that would be provided for an agricultural pest,” Kabashima said. “No one is responsible . . . It’s an orphan pest because it doesn’t fit into any particular category.”

Research into both pesticides and biological control is needed since there is currently no recommended way to stop the beetle, Kabashima said. Urban landscape pests also require both survey detection and a rapid response system in the same way that agricultural pests do.

“When we have that in place, it works quite well,” he noted. “With the polyphagous shot hole borer, we have none of that. It’s all being done on an ad hoc local level.”

Recently, State Assemblymember Lorena Gonzalez Fletcher introduced legislation, AB1530, that would amend the Urban Forestry Act to provide a state funding path for this type of insect, Kabashima said. However, AB1530 is still going through the legislative process.

This dying box alder tree was infected by the fungus left by the shot hole borer. Photos: Eskalen Lab

This dying box alder tree was infected by the fungus left by the shot hole borer. Photos: Eskalen Lab

Part of the difficulty in showing people the urgency of the problem is that the way the beetle proliferates is hidden inside trees, Kabashima said. Once a beetle enters a tree, it breeds and infects the tree internally. This process may take a couple of years, but when the tree dies, millions of beetles can be released into the landscape. This proliferation method can simultaneously cause a sudden loss of trees. For example, the insect took out 144,000 willow trees along the Tijuana River in San Diego County.

“Several million beetles can come just from one beetle,” he said. “It’s building up in Southern California. Once it builds up to a certain amount, it could explode into Northern California.”

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