INVASIVE SPECIES: Tree-infesting beetle found in Riverside
The polyphagous shot hole borer, an invasive species from Asia, attacks more than 40 different kinds of trees. There is no known way to treat the pest.
UCR researcher Akif Eskalen has been monitoring the riverbed near Martha McLean-Anza Narrows Park. Six months ago, he said, there was no visible sign of the tiny intruder. There is still no sign of the beetle itself, but two weeks ago, Eskalen found willow and cottonwood trees peppered with the holes the borer makes as it eats its way into trees.
“Look at this,” Eskalen said, pointing to a tree trunk with dark tear-drop spots on it, “willow, willow, willow, all infested. This is a forest and almost all of them are infested. We have a very big problem and this problem is spreading very fast.”
Eskalen, a plant pathology professor, said the riparian habitat along the river is serving as a corridor through which the beetle is entering the Riverside area.
“I knew this was a vulnerable area,” he said. “I knew the southern part of the river was infested in Orange County.”
How fast that infestation will move is unknown. Scientists are still in the dark about much of the beetle’s habits since it lives out of sight nearly its entirely life. In fact, generations of beetles breed inside the trees they inhabit, never seeing the light of day.
But the scope of the infestation is troubling to Eskalen.
In the Tijuana River Valley, near San Diego, he said, the beetle is estimated to have attacked 140,000 trees, 500 of which have died. A similar scenario is possible here, he said.
The sesame-seed-sized beetle doesn’t actually kill the host tree itself, its food does. When the beetle bores into the heart of the tree, it brings its lunch with it. On its body is the Fusarium fungus, and possibly two other fungi. The fungi grow inside the tree, serving as a food source for the bug, but also, eventually, cutting off the tree’s water transport system and killing the tree. The process takes two to three years.
Eskalen said the challenge in combating the beetle is getting to it. Topical chemicals won’t reach the pests inside and, as of yet, no systemic pesticides – which can either be injected or taken up by the roots – have been approved. One such chemical is close to being approved for the market, Eskalen said, but it is specific to avocado trees.
Avocado trees are a favorite of the beetle. It was the infestation of avocado groves that first alerted scientists and pest-management experts in 2012 when the bug was first reported in California.
“As soon as I found this, I went to the Avocado Commission,” Eskalen said, seeking funding to face down the threat. “Since then, we have been chasing this beetle. Every week we’re learning something new about it.”
That includes what it likes to eat, how far it will fly and what might slow it down. While Eskalen and his fellow researchers – UCR entomologist Richard Stouthamer and UC Santa Cruz grad student Shannon Lynch – have found some promising pesticides, he believes fighting the beetle with biological controls is the only effective way to handle it.
The pest has also been found in De Anza Park in Ontario.
“This beetle is not going anywhere,” he said. “From now on we are going to have live with (it). We cannot eradicate this beetle.”
Eskalen said he and Stouthamer began field-testing a biological agent two weeks ago, a bacteria that kills the Fusarium fungus. The two men will make their fifth trip to Asia this week, visiting Taiwan where they are working with forest service scientists to identify other possible biological controls. So far, a wasp that attacks the beetle has been identified, but it has not been approved for release into the Southern California environment.
Already, the beetle is going beyond the river area. Eskalen pointed out several sycamore trees in the park that were littered with pockmarks from the beetle. He’s seeking more funding to continue trying to find a solution.
“This is a threat throughout Southern California,” he said. “We need to do more research.”
What To Do
If you suspect one of your trees may have been attacked by the polyphagous shot hole borer, here’s what you can do:
On the web: Go to eskalenlab.ucr.edu and click on the tab for the bug.
If symptoms match: Take a clean knife and peel the bark away from one of the holes, exposing the hole. Place a ball point pen’s tip near the hole for reference and take a photo.
Contact researchers: Send the photo to UCR researcher Akif Eskalen’s lab. If it looks like a shot hole borer attack, the lab will contact you