Locals learn about ways to protect palms from weevil attacks

Across San Diego it is becoming more common to see the sad sight of a flattened halo of fronds atop a palm tree, the sure sign of a tree that has fallen victim to the South American Palm Weevil. Considered among the worst pests to ever hit California, the weevil has killed an estimated 20,000 ornamental and date palm trees in San Diego County, including hundreds of Canary Island date palms in Rancho Santa Fe.

In response to local residents’ growing concerns over the loss of trees, the Rancho Santa Fe Association and Forest Health and Preservation Committee held an informative presentation on Jan. 25 at the RSF Golf Club. A crowd of over 130 people showed up to learn from the “weevil whisperer” Dr. Mark Hoddle, a biological control specialist and principal investigator at the Center for Invasive Species Research at University of California Riverside.

“This weevil doesn’t know any boundaries,” said Len Gregory, the president of the Forest Health Preservation Committee. “Ninety percent of our forest is on private land. The staff can do a tremendous job mitigating the problems on Association land but really the 90% of the forest that’s on private land is where we need community engagement.”

Hoddle shared all he knows about the weevil and about a new approach proposed for Rancho Santa Fe to help protect the palms. Hoddle’s new “Attract and Kill” method uses the weevil’s biology against itself versus drenching a tree with insecticide. A dollop of insecticide is mixed with an irresistible weevil pheromone and placed in a hanging trap.

“The weevil thinks it’s found someone to mate with but it will be bitterly disappointed and then they will die,” Hoddle said. He calls this new grant project “Fatal Attraction.”

Pending approval from the RSF Association board and utilizing $1 million in grant funding from the state, Hoddle hopes to place about 1,250 hanging pheromone traps throughout Rancho Santa Fe to monitor the level of weevil activity in the community.

At UC Riverside, Hoddle has studied the South American Palm Weevil for six and a half years and has become a leading researcher of the destructive pest. The weevil was first spotted in Tijuana back in 2010 and later captured in monitoring traps in San Ysidro in southern San Diego County in 2011. When first encountering the weevil, Hoddle had never seen anything like them and he wasn’t quite sure how they got there. The U.S. does not import palm trees and scientists’ best hypothesis is that the insects were brought in from South America as the larvae is considered a popular delicacy by some in the food industry.

A large crowd gathered to hear Dr. Mark Hoddle speak on palm weevils.

(Kiersten Dyer)

The focus of much of Hoddle’s research has been in the Sweetwater Reserve in Bonita, a riparian habitat with thousands of palms that has become a “massive incubator” for the weevil since 2014.

The palm weevil is a large, black beetle with a long snout. It uses its snout to dig a hole into the tree and lay its eggs. When they hatch, hundreds of larvae in the form of big grubs begin feeding at the top of the tree, in the palm’s heart.

“Once the top of the tree is damaged severely, it will drop and it cannot recover,” Hoddle said. The tops of infected trees droop and dead fronds take on a mushroom shape, like a brown umbrella. Leaf bases can become riddled with holes that resemble Swiss cheese.

The adult weevils fly, reproduce and spread, finding more palms to attack. In his studies in his lab, Hoddle has found that the weevil can fly an average of 25 miles in a day if they want to—one super female he studied flew 142 miles in one day before dying.

“We don’t actually know if they do this in nature,” Hoddle said, but the study suggests that the weevil is a very strong flyer and more than capable of flying large distances to spreading from community to community if they can find susceptible hosts. Hoddle said it is surprising that the pest remains exclusive to San Diego County and that they have not found it further north than San Marcos.

In Bonita’s massive breeding ground, Hoddle has used his traps to study the mortality rates of the trees. Of the 521 palms monitored over six and a half years, 70% have been killed in urban areas.

“You’re likely to see something similar here in Rancho Santa Fe,” he said. “You can expect to see escalating levels of mortality due to the palm weevil activity.”

While the palm weevils prefer to feast on Canary Island date palms, they’ve also been found to lay eggs and kill a variety of Southern California palms, including fan, triangle, Senegal, Sabal, Guadalupe, Chilean wine, Bismarck, Pritchardia muroi and minor palms.

The weevil’s preferred Canary Island palms are a non-native, invasive species that originate from the Canary Islands off the coast of Africa as an ornamental palm for the horticulture trade.

“These palms invade just about anywhere where there is consistent water – irrigated properties/lands, canyons where there is irrigation runoff, river valleys and natural areas,” said Caitlin Kreutz, the Association’s environmental resource coordinator. “These palms reproduce quickly, and sap resources away from native vegetation and displace habitat for native species.”

In addition, the palms are extremely flammable and are part of the Association’s invasive species removal/ fuel reduction initiatives in the Ranch’s Arroyo Preserve and along the Lower San Dieguito River Valley.

“Humans have made this whole process possible,” Hoddle said. “We have planted palm trees in deserts and we have planted species that the weevils love to eat. None of this is natural… The weevils are tropical, who would ever have thought they could live in a desert or an arid Mediterranean environment? They obviously can and we have facilitated them with our plantings of palms.”

In his surveillance of palms in urban areas throughout San Diego, Hoddle has seen a few cases of recovery from palm weevil infestation but it does take a lot of work and care—he is hopeful about the prospects of saving more heritage palms but remains realistic about outcomes.

The best way to protect the trees is an application of insecticide at the crown or using a systemic insecticide (which can be purchased at Home Depot) to drench the soil beneath the tree. The applications have to be made frequently and it can become very expensive.

Hoddle said there are some practical management strategies that homeowners can employ.

In his research, he has found that 80% of weevils fly between April and October. Many people like to prune their palms to achieve that “pineapple top” look and Hoddle said when palms are cut it releases a volatile chemical that the weevils can smell. He said the best time to prune trees might be in November and March when weevil activity is at its lowest.

Traps are the most useful for monitoring weevil activity—the best bait for capturing weevils is made from fermenting yeast, water and dates. A bucket trap or cone-shaped trap that goes into the ground can be made with the bait and a commercially-available aggregation pheromone. Hoddle said you shouldn’t place traps directly on a palm tree or in the sun where it will get too hot and the trap will lose its potency.

Removal of dead or dying palm trees is expensive, potentially dangerous and should be done responsibly with professional arborists, Hoddle said. It is recommended that infested palm material be chipped. All transported material should be covered with a tarp and disposed of at a certified landfill that buries within 24 hours of dumping to reduce risks of spreading adult weevils into new areas.

At the Jan. 25 session, Hoddle shared the details about “Fatal Attraction”, the new trapping and monitoring program that he would like to run in Rancho Santa Fe, possibly expanding into neighboring communities of Fairbanks Ranch and Encinitas.

The traps will be loaded with a dollop of insecticide about the size of a quarter. The insecticide is toxic to fish, aquatic insects and bees but Hoddle said because it is in a dollop hanging in a trap, it’s not going to end up in waterways and he sees no reason why bees would be attracted to these traps in large numbers.

The hope is to eliminate the need to spay an entire palm with insecticide, which would kill anything that ended up on the tree.

“This would be a safer way of deploying an insecticide to kill these weevils,” Hoddle said. “We are ready to start these experiments to see whether or not this ‘Attract and Kill’ idea can actually suppress palm weevil populations in urban areas. If it does work, we have the potential to eliminate a lot of the insecticide use.”

Rancho Santa Fe Covenant residents who have questions about palms on their property and are looking for help in treatment options can reach out to Environmental Resource Coordinator Caitlin Kreutz at caitlin@rsfassociation.org. A site visit may be scheduled.

To learn more about Hoddle’s palm weevil research, visit his website at biocontrol.ucr.edu/mark-hoddle