The California Avocado Commission is funding research on the beetle and avocado groves infested by a closely related Thousands of trees across Southern California are dying because of an invasive beetle for which there is no remedy.
The polyphagous shot hole borer is attacking hundreds of tree species and can reproduce in 41 of them, including some of the trees most favored by gardeners, city landscape designers and universities, such as maples, elders, cottonwoods and oaks.The first infections were discovered in 2003 in the Whittier Narrows in L.A. County, and the beetle was implicated for the mass deaths of box elder trees in Long Beach in 2010. In Riverside County, the beetle has attacked trees near Martha McLean-Anza Narrows Park along the Santa Ana River and in De Anza Park in Ontario. Now, in Mason Regional Park near UC Irvine, the sycamores and willows also look sickly, with black stains caused by the beetle shotgunned across their trunks.“We’re in an epidemic now in Southern California. We let it get away from us. This beetle fell through the cracks,” said John Kabashima, a former UC Cooperative Extension advisor who is working on beetle issues. “We don’t normally see this reaction from these trees, because we don’t have a pest like this.”There’s no known cure and, and the pricetag for dealing with the pest is quickly mounting since it costs about $1,000 to remove a tree. Many homeowners don’t have that kind of money.
The potential damage to crops, especially avocados, is more severe. The California Avocado Commission is funding research on the beetle and avocado groves infested by a closely related beetle in San Diego County are being monitored.The shot hole borer infests riparian trees that grow near waterways – often the same types of trees citydwellers and landscape architects favor. Pregnant females land on trees, burrow holes the size of a pen tip through the bark, and dig tunnels.The beetles bring with them a fungus that feeds on the tree’s cambrial layer – between the bark and the wood. The beetles feed on the fungus, which actually does the job of killing the trees. It takes two to three years from infection to when the tree starts to die, even though the shotgun wounds appear much sooner. Multiple generations of the minuscule beetles will live and mate in a tree until it dies, when tens of thousands of beetles take off to find new trees.The beetle arrived in the U.S. in wood products from Southeast Asia. A monoculture of nonnative trees grown for wood products got hit by the beetle, which flourished in those uniform stands of trees.“It could have come in on a wooden pallets for all we know. There could be hundreds of beetles in a pallet,” Kabashima said. In San Diego County, along the Tijuana River, 100,000 willows have been struck by a closely related beetle. “It’s like Armageddon,” said Kabashima, who worries that the same thing could happen in the riparian areas of the Santa Ana River.
Another beetle -- the goldspotted oak borer -- is attacking stands of oak trees already weak, thirsty for water after four years of drought, in L.A., Riverside, San Diego and Orange Counties. More dead trees raise the risk of wildfire during what is starting out as a summer of record-breaking heat.
Together, the two beetles are creating “a contiguous swath of dead trees,” Kabashima said.
Kabashima worries that fires that feed on trees in wild areas will more easily jump to urban areas because of the dead trees there. Normally, well-watered trees along creeks and river beds act as an impediment to fire. But if those trees are hit by the beetle and die, the fire defense fades.
In a small glade in Orange County’s Weir Canyon, next to a 3-foot-wide stump, a coastal oak tree lies on the ground, hacked apart and stripped of its bark.
Hatchet in hand, Kabashima chips at the top layer of a 5-foot-long log. Painted blue spots mark the tiny D-shaped exit holes where the beetles have burrowed their way out of the tree. Underneath, he uncovers a warren of black trails through which a small BB might roll.
The snaking spaghetti trails surround the tree, dug by the goldspotted oak borer. Orange County Fire Authority crews then cut the tree down and apart, their chain saws working like scalpels to remove a tumor from Orange County’s emblematic coast live oak woodlands.
“We’re not sure we caught it in time,” Kabashima said.
The goldspotted oak borer, originally from southeast Arizona, is already rampant across San Diego County, where it was discovered in 2008. Scientists believe the beetle arrived near El Cajon in San Diego County in the mid-1990s – a hypothesis they developed by examining tree stumps. It started to spread, aided by humans who transported the beetle in infested firewood.
The beetle soon made it to coastal San Diego County and La Jolla, then Idyllwild in Riverside County in 2012, then Orange County in 2014. In 2015, it showed up in northeast Los Angeles County, in Green Valley.
Response to the goldspotted oak borer was, at first, minimal. The U.S. Department of Agriculture considered it a native species, since it lived in other parts of the U.S.
“It’s becoming more and more clear there’s a need to address the native oaks and sycamores down here,” said Brian Norton, a battalion chief for the Orange County Fire Authority.
“We need to stop transporting firewood from one area to another.”
In its native home of Arizona – a place it never left for hundreds or possibly thousands of years – the beetle doesn’t pose much of a problem since another insect that flourishes during monsoonal storms controls the beetle population. There is no such control here in Southern California.
Forests across Southern California are stressed by four years of drought.
In Orange County’s Weir Canyon, the canopies are thin and meager, and the green leaves are scant enough that gray bark in the upper reaches shows through. A thick mat of grass – three crops from the winter rains – lies golden and crisp on the forest floor. Many of the trees stand tall, but they’re dry and dying.
George Ewan, a wildland defense planner for the Orange County Fire Authority, maneuvers his truck down a bumpy gravel road. If the canopies are green, he said, they’re OK; if they’re gray, the trees are in serious trouble.
“There is a combination. Is it drought or beetle, or that the drought has weakened the trees so the beetles can hit it?” Ewan said.
Ewan samples shrubs weekly and tests them to see how dry they are. In June, the plants were already reaching critically dry levels – months ahead of normal, in September or October.
Dead trees ignite easier than live ones, and the trees act as fire ladders. This enables flames to reach higher and throw embers farther afield. The trees also burn longer and hotter than smaller fuels, such as grass or shrubs, and their radiant heat can be so intense that it pushes nearby shrubs and grass to their flash points.
In the San Bernardino and Sierra Nevada Mountains, trees are perishing en masse because of native bark beetles that have flourished during four consecutive years of drought and heat.
Since 2010, at least 66 million trees have died across the state, according to aerial surveys. And the problem is ramping up: While 40 million of those trees died between 2010 and October 2015, 26 million died in the southern Sierra Nevada alone between October and June.
“Drought is definitely a big stressor, particularly consecutive years of drought,” said Kevin Turner, who works on beetle issues for the UC Cooperative Extension. “Trees that normally pitch insects out can’t produce enough sap to push them out.”
Those native beetles are also attacking conifers, white firs and pines at higher elevations in Southern California forests, but not at nearly the same rate as farther north, in the Sierra Nevada. The vast stands of dead trees can fuel raging wildfires.
The beetles release a pheromone that draws other beetles to the tree until it succumbs, Turner said.
“That pretty much goes on until the drought ends.”