South American palm weevil wreaking havoc on local palm trees
The majestic palm tree is part of San Diego’s iconography — symbolizing Southern California and the beach, the feel of a tropical paradise.
Most of San Diego’s palm trees are classified as Canary Island date palm trees. But Canary Island date palms also are a favorite snack of the South American palm weevil, a type of beetle that first made its presence known in Southern California in 2011. Once a palm weevil attacks a tree, the tree begins to show signs of disease, such as brown, drooping fronds.
At this point, according to experts, it’s most likely too late to salvage the tree.
Ocean Beach and Point Loma have reported dead palm trees due to the palm weevil. In Ocean Beach, diseased palms can be seen along Santa Monica Avenue, according to Mark Winkie, president of the Ocean Beach Town Council. And in Point Loma, the trees that line Catalina Boulevard are showing signs of infestation, according to Mike McCurdy, chairman of the Point Loma Association.
The weevil infestation issue is multifaceted, said Mark Hoddle, an extension specialist in biological control in the department of entomology at UC Riverside. The weevil has been slowly working its way north from the Baja peninsula, where it was first discovered around 2000.
Several of the Canary Island date palm trees that typically stand tall, full and lush along the La Jolla coastline may be falling prey to the South American palm weevil, causing them to droop, turn brown and die.
“It looks like San Diego County’s pretty much infested from the border all the way up to San Marcos out to probably El Cajon,” he said. “The weevil is actually spreading fairly slowly, given the fact that we’ve pretty much determined that they can fly a long way. We think the reason for its relatively slow spread throughout San Diego County is because there are just so many palm trees to feed on. And I suspect that the intensity of the attacks is now becoming much more apparent to people, because I’m just getting overwhelmed now with email inquiries about what to do for palm trees to try and protect them from the weevil. It’s really hitting a lot of people’s radars now.”
The weevil is about 2 inches long and can fly surprisingly great lengths.
“We think they could easily fly probably 15, 20 miles in a day if they really wanted to do that,” Hoddle said. “In nature, we don’t know if they can fly like that, but the lab studies suggest that if they wanted to, they have the capacity to fly long distances.”
With so many palm trees, San Diego’s weevils don’t need to fly far to the next tree, so there’s no motivation for them to spread quickly, Hoddle said.
Here’s how infestation works: “Adult weevils buzz around and when they find a palm tree that they like, they’ll land at the top of it, in the crown. And the adults are very good at pushing their way down between the palm fronds. They’ll try to get as close to the trunk of the tree as possible,” Hoddle said.
The female then drills a hole into the top of the palm with her rostrum (an elephant-like nose or beak) and lays eggs in the hole. The eggs hatch and the weevil larvae boar their way into the crown area of the palm tree.
Once the larvae hatch, they begin to feed on the palm heart, the type you could buy in a grocery store to eat.
“Weevils love eating that meristematic tissue, or the palm heart. It’s so rich and juicy, and that feeding damage basically results in the death of the palm tree. It can’t grow any more new palm fronds, so the tree dies,” Hoddle said.
Dead trees look like “giant brown umbrellas” or “big brown mushrooms” because the dead fronds turn brown and hang from the top of the tree trunk.
“And that’s it, the palm is pretty much dead by that stage,” Hoddle said.
Ron Matranga, consulting arborist at Atlas Tree Service, said it wouldn’t take many larvae to kill a palm tree.
One female can lay 100 to 700 eggs. “And it can take as few as 30 larvae to completely damage a palm,” Matranga said.
Saving a palm tree that is visibly infested is next to impossible. “I have on rare occasions seen people treat their tree at the last minute out of extreme desperation, and there must have been just a little bit of living palm material at the very top of the palm, because about six to eight months later, it just started sprouting back,” Hoddle said. “I was absolutely amazed that they had saved that tree. But it had got to the point for them where it was like, ‘We’ve got nothing to lose. We can pay $150 and treat the tree and maybe it will come back to life, if we’re lucky. If not ... it’s going to be removed anyway.’”
Controlling the infestation requires affected palms to be removed immediately to reduce the risk of the weevils finding another host tree. Treating healthy surrounding trees with insecticide is another way to control the spread.
“The treatment is a dual-pronged treatment: There’s a crown spray, which goes directly up into the top of the palm,” Matranga said. “And then there’s a soil treatment. This chemical is put into the soil, the tree roots pull it in and is what’s called systemic, where it’s ingested into the system of the plant and moves up inside into the vascular part of the palm.”
The treatment must be repeated three or four times a year to remain effective.
“It can cost upward of $400 to $500 per treatment for the first palm, and then sometimes only $100 to $200 per palm after that if you have multiple palms on a site,” Matranga said. “But it does take a commitment on the palm owner’s part. You can’t just treat once and you’re good.”
Removal of a dead palm could cost $1,000, depending on the location of the tree, Matranga said.
Brian Widener, forester for the city of San Diego, said the city is well aware of the problem but a lack of resources limits its ability to control the spread of the infestation.
“It’s a challenging issue for us, as we’re juggling multiple issues at all times,” Widener said. “It’s very limited on what we’re able to do at this point in time in regard to these trees. We have been removing trees as one way to keep the beetle from spreading from tree to tree.
“And obviously we do tree removals, too, for public safety. Once the tree gets infected with the weevil, the fronds start to die from the top down at the top of the tree and then basically they fall off and it can be a public safety issue just from the fronds, but it’s even possible that the top of the tree could fall off, too. We always encourage people to call or use the Get It Done app as soon as possible and report the problem.”
The city is monitoring the issue while treating some palms with insecticide. The palms currently being treated are part of a pilot program to test the effectiveness of such treatments.
“With these kind of issues, monitoring the situation is always one of the key components,” Widener said.
So far, the city has treated palm trees with insecticide in two areas: The trees lining Catalina Boulevard in Point Loma and other affected trees in University Heights.
Hoddle said managing the infestation is possible. He has worked on studies with similar bugs in the Middle East. In Saudi Arabia, where palm trees flourish due to the intense heat, the infestation of the red palm weevil, a close relative of the South American date palm weevil, has been contained through a combination of insecticides, removal of dead trees and using pheromone traps to lure the weevils into buckets, where they are captured and killed.
“There is a pheromone for the South American palm weevil and it’s very good at drawing the weevils into traps. We can catch lots and lots of them. But the problem is right now there’s no systematic trapping plan for South American palm weevils in San Diego County,” Hoddle said. “It would require a big input of money, either from the state or the federal government through the USDA to lay out a big response to this weevil through using these traps. And the work on other weevils and the South American palm weevil suggests that you can trap it down to very low numbers if you’re willing to put the resources into financing that.”
Widener said it’s important to keep dialogue open with local politicians about options and treatment plans.
“Date palms, in particular Canary Island date palms, can have a lot of maintenance work over the years,” he said. “Even though we all appreciate them as being a very iconic type of palm in our city, it’s a balancing act on how we keep some of them, at least if we’re going to do future treatments, and which ones we have to let go.”
For now, Widener said, concerned residents can reach out to a tree care company if the tree is on private property, or notify the city through the Get It Done app if the tree is on public property.