The beetles have arrived, this time carrying fungus instead of droves of fanatical teenagers.
An invasive species from Southeast Asia, the polyphagous shot-hole borer beetle brings with it fungi (Fusarium euwallacea) that can devastate trees as it spreads from within, causing “Fusarium dieback,” or branch dieback, which in time can kill the tree.
The beetle has been spotted in green waste in Ojai, but it’s in Santa Paula along the Santa Clara River corridor that the beetle has generated the most concern over fear that it could devastate the avocado industry.
Vast swaths of San Diego have been affected by the beetle, leaving behind “thousands of dead willows,” says Sabrina Drill, Ph.D. Drill is a natural resources adviser and Los Angeles and Ventura Counties associate director for the University of California, Ventura County Cooperative Extension.
“One of the interesting/terrifying things about this beetle is that it has a huge host range of trees,” said Drill. “It seems like almost any tree species that doesn’t block the growth of the fungi in some way, or somehow inhibits reproduction of the beetle, can act as a host.”
The UC researchers have identified around 300 tree species that have been affected by the beetle, 35 of which can act as reproductive hosts. The avocado industry has responded to the beetle threat directly; in particular, the California Avocado Commission contributed over $2 million toward researching ways to combat the pest.
“We’ve always known it was a matter of time before it showed up here, but we were hoping we would have more time before it did,” said John Krist, chief executive officer of the Farm Bureau of Ventura County.
Krist says that an established colony was discovered in Santa Paula near an avocado farm.
“The only strategy available to us is try to minimize its spread, and as soon as it does show up in a tree, take precautionary actions to remove the affected branches and try to prevent it further from spreading,” said Krist.
The beetle rarely comes out of its home once it gets settled. A pregnant female remains in the tree, where it lays its eggs, which hatch and mate. The only beetles to leave are the new pregnant females, making traditional pheromone-based traps useless in capturing the insect. Pesticides are ineffective due to its burrowing nature.
Researchers from the University of California, led by UC Riverside entomologist Richard Stouthamer, have traveled to areas of Southeast Asia in search of natural prevention methods. A parasitoid wasp, which lays eggs on the beetle’s larvae, and bacteria that inhibit the growth of the fungus show promise.
But the beetle gets around with a little help from its human friends as well. Mulch, woodchips and firewood can transport the beetle, as was the case when it arrived in the States to begin with – via wooden pallets from Asia. In 2015, the beetle was discovered not too far from the city of Ojai’s green waste facility, assumed to have been transported in with woodchips or mulch.
At a meeting of the Ventura County Board of Supervisors earlier this year, Supervisor Steve Bennett, District 1, suggested that perhaps there should be a policy to ban green waste from being brought into the county from elsewhere.
“One of the benefits of banning green waste from coming in from outside the county is that you decrease the chances of [the beetle] coming in,” said Bennett. If wood chips and other green waste are ground down to less than an inch in length, the beetle may not survive, said Bennett, but adds that the risk is too high and is unnecessary. “We produce enough of it in VC for all of our agricultural needs.”
The Supervisors directed staff to research the possibility of enforcing such a ban. Supervisor Kathy Long, who serves District 3, which includes much of the Santa Clara River area, says that news of the beetle’s arrival was “very concerning.”
“Today you’re seeing more green waste being used for crop cover,” said Long. “We may, I expect we will see something come to the Board that will address the green waste markets.”
Drill and her colleagues have begun surveying Ventura County in search of the elusive beetle (which is no larger than a grain of rice), hoping to get a measure of how widespread the issue may be.
“What we’re starting to realize is that it’s more of a threat to riparian natural areas than it may be to avocado as an agricultural product,” said Drill, noting that sycamores, willows, cottonwoods and other trees that grow adjacent to rivers or streams are very susceptible.
Drill urges locals to visit the University of California, Agriculture and Natural Resources website at www.pshb.org to learn how to identify the beetle and the damage it causes. Suspected cases can also be submitted to the researchers via the website. Drill asks that photographs of the suspected pinhole entrance on the tree be taken using an actual pinhead for scale, as well as a photo of the entire tree for proper identification. Photographs can be submitted online.