Afflicted trees in Tijuana River Valley

More than 70,000 trees in the Tijuana River Valley are damaged — many are dying — from a Southeast Asian beetle that has also harmed more than two dozen North County avocado growers.

Trees in the riparian area next to the river bed and in the flood plain have had limbs fall off because of the infestation, and the area looks like the trees were savagely topped by a huge spinning blade. The San Diego Zoo, at the north edge of Balboa Park, is also in the early stages of the infestation, a University of California-Riverside professor said. Many of the dead or dying trees are willows and other native species.

The worst news: there is no way to eradicate the Kuroshio shot borer beetle, native to Vietnam and Thailand, which has been identified as the destroyer.

"If the boring doesn't kill the tree, the fungus will," said Akif Eskalen, who studies plant and tree infestations at the University of California-Riverside. "There is no cure yet, so now we have to learn to live with it."

Kuroshio shot borers get around — by car, typically

Avocado farmers in Escondido — and the California Avocado Commission — so far have funded all the research on the beetle, which carries a fungus that is the bugs' food source but is also a deadly poison for the 38 or so species of trees it infests, according to Eskalen.

"I don't have the manpower or funding to focus on riparian and landscape trees, and that is where it will show up next," he said. "This beetle scares me — every single riparian area is going to end up looking like the Tijuana River Valley."

In December 2013, the bugs showed up in the sycamore and golden rain trees along the Sycuan Casino and Resort golf course in East County. They lost hundreds of trees, Eskalen said. There were no sightings for months, and then it showed up in 2014 in avocado groves around Escondido, he said; about two dozen avocado groves are now known to be infested.

The avocado commission shifted into high gear and began funding research in hopes of controlling and eliminating the beetle. Meanwhile, the bug ran wild in Orange County as well. The University of California Irvine cut down more than 1000 trees, and an Orange County tree trimmer was killed when a shot-borer-compromised tree limb fell, Eskalen said.

The Tijuana River Valley infestation was identified about a month ago by an environmental consultant working on an unrelated project. The tiny bugs are barely visible, but the invaded trees respond to attack often by rushing sugar to the entry site so some trees under attack exhibit what's called "sugar volcanos" on the bark where the borer entered.

The Community Gardens' avocado trees are heavily infested, Eskalen said. "Judging by the level of damage, the beetles must have been there at least a year," he said.

Tijuana Slough Refuge manager Brian Collins, a wildlife biologist, called the infestation “shocking."

"The level of tree devastation from this exotic beetle is unprecedented," Collins said. "It is terrible news."

The bug doesn't have much capacity for flight, said Tim Spann, research project director for the avocado commission. The working theory of its spread to far-flung locations is that it has been spread by people unknowingly moving contaminated wood. "It doesn't fly, but it drives well," Spann said.

Eskalen said that most of the spread is easy to understand. "When you look at the map, you see a cluster," he said.

Growers and biologists want to get the word out to the public to help prevent the spread, he said.

"No one intentionally spreads an infestation, but it's impossible for homeowners and the public to follow every invasive species that shows up."

Eskalen's lab will test plants showing symptoms, he said. "If you have infested plants, confirm it and remove the plant. The wood must be chipped down to one-inch pieces and solarized — chipping controls 99 percent and solarizing controls the last 1 percent," he said. "And buy your firewood where you will burn it; don't haul it around."