Area scientists step up studies of ways to control palm weevils that are killing La Jolla trees
Some techniques for trapping and killing the bugs have shown promise, but other methods, including importing a predator fly, are being considered.
Area research institutions are revving up their studies in an effort to find a way to stop a bug that has been destroying Canary Island date palm trees in La Jolla and elsewhere in San Diego County in recent years.
Though studies of possible techniques to solve the infestation of South American palm weevils are showing promise, it could be a few years yet before there are conclusive results, according to Eric Middleton, an area integrated pest management adviser for the University of California system.
Researchers are looking at the biology of the insect, how to trap and kill it without damaging the trees and even whether bringing in a predator might be an option.
“The weevil is a major pest for the public and is something people don’t know how to manage,” Middleton said. “It’s a huge problem here in San Diego and in La Jolla. ... [An answer] can’t come fast enough.”
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.
The palm weevil is a type of beetle native to parts of Mexico, Central and South America and the Caribbean.
Mark Hoddle, a biological control specialist and principal investigator at the UC Riverside Department of Entomology, previously told the La Jolla Light that the weevil will colonize a palm tree and attract other weevils by releasing a pheromone.
“Once at the palm, females lay eggs, those eggs hatch into weevil grubs or larvae that feed on the palm heart,” Hoddle said. “Once the larval stage is completed, the larvae form cocoons out of palm fibers, within which they pupate. After pupation is complete, the adult weevils emerge and the cycle repeats itself.”
The feeding by weevil larvae in the crown of a palm tree can make the tree unable to produce new fronds. The infestation causes the tree’s fronds to droop and the crown to turn brown and collapse or tilt.
“At this stage and beyond, palms now look like giant brown umbrellas or mushrooms,” Hoddle said. Eventually the trees will die.
Once the fronds start to wilt — showing symptoms of a weevil infestation — it might be too late to save the tree, Middleton added.
“Once your palm tree is infested, there is no proven cure that gets rid of the weevil and saves the tree,” he said. “The tree is essentially dead at that point.”
Trail stewards are concerned that a palm weevil infestation on a nearby tree could spread and increase the risk of using the swing.
There have been instances in which insecticides have been applied early enough in the infestation to save the tree, but “we don’t have a study that says what will work and in the majority of cases,” Middleton said.
Thus, studies are trying to determine what can be done to prevent an infestation in the first place.
“Labs are trapping the weevils [to study them] with a synthesized pheromone that mimics what the males put out,” Middleton said.
“Work is also getting started to trap the weevil by putting this pheromone next to some insecticide, so the weevil comes to us, touches the insecticide and dies. We’re looking to see if that can be an effective measure long-term.”
UC Riverside said last year that a $1.1 million grant from the California Department of Pesticide Regulations was enabling it to test the pheromone in urban areas of San Diego County, where weevils have killed tens of thousands of palms.
“If it ends up working, the goal would be for the city to do that as an areawide control method,” Middleton said. “It would be coordinated and more effective.”
As tempting as it might be for residents to try to obtain weevil pheromone to help individual trees, Middleton advises against it.
“Those that do this will attract palm weevils, which is a terrible idea because only some of the weevils you attract will die. Others will be drawn to your tree and not die,” he said. “I would discourage members of the public from trying to find and use these pheromones.”
Instead, he said, in places like La Jolla, a preventive insecticide can be applied, but it must be repeated throughout the year to be successful.
“It’s not a great situation either way, but you can wait and hope your trees don’t get hit, or you can apply insecticide two to four times a year and keep doing it forever,” he said.
In either case, he added, “we recommend working with tree care companies.”
Another option that researchers are investigating is whether a predatory fly from South America could be brought in to attack the weevil.
“Part of the reason weevils are a problem here and not in South America is we don’t have predators,” Middleton said. “Scientists will go to where these pests are native and look for the parasites and predators and bring them to the area where there are pests. But there have been many instances where people don’t think it through, so it requires a lot of thought and rigorous protocols. It takes years to be done correctly, but it is something the lab is looking at.”
The specific type of fly finds the weevils, lays its eggs and “eats them from the inside out,” Middleton said. “But that requires coordination with South American governmental and scientific agencies. Should some be brought to the United States, experiments would be done in a quarantined lab setting.
“Historically, that has been effective for other invasive pests.”
Hoddle previously said the recent infestation is part of an “invasion” that started in 2010.
“Weevils were first detected flying into San Diego in 2011, and by 2014, populations were established in San Ysidro,” he said. “Weevils are very strong fliers; they can fly 15-plus miles a day if they choose to do so — at least in the lab. We don’t know if they fly such long distances in nature. Over the course of 10 years, weevils probably flew up the Baja Peninsula to Tijuana and across the border into San Diego County.”
Middleton said the weevil is “an issue we’re going to have to keep dealing with and might be here to stay. The weevil is in San Diego, as far north as San Marcos and in areas like La Jolla. So be aware, it’s still out there.” ◆
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