Fear No Weevil: Entomologists Say Cone Traps Better Protect Palm Trees
By Melissa Mayer
Imagine staring at screens for hours and hours for months on end, waiting to catch a 1.5-inch insect in action. That’s exactly what researchers from the University of California, Riverside’s Department of Entomology did to analyze more than 20,000 hours of video in a new study on the effectiveness of traps for the invasive palm weevil.
The weevil Rhynchophorus palmarum, also known as the American or black palm weevil, has wreaked havoc on San Diego County’s Canary Island date palms (Phoenix canariensis) over the past decade. Palm weevil larvae emerge from eggs laid in the trees and then burrow to the crown and feed on the palm tissue there. The wound ferments, and that odor—combined with pheromones released by adult weevils—brings even more weevils to the infected palm tree, ultimately killing the tree within just a few months. It’s so striking that entomologists call it Palmageddon.
“Of the 600 palms we mapped in the Sweetwater Reserve where this study was conducted, nearly 30 percent of them have been destroyed by the weevils over the course of this study alone,” notes Ivan Milosavljević, Ph.D., assistant project scientist at UCR’s Hoddle lab and one of the study authors.
It’s a Trap!
It’s important to detect palm weevils early, and that means using traps. The team’s previous research showed that bucket traps performed far worse than cone traps, so they set out to determine why. The results of their new study were published in September in the Journal of Economic Entomology.
The team loaded traps with weevil pheromone, bait made from Medjool dates and baker’s yeast, and a solution of propylene glycol and water to drown and preserve trapped weevils. They also set up 10 infrared digital video recorders to capture weevil activity at the traps for a total of 7 months.
Then, Milosavljević and lab techs Kelly Girodano and Mike Lewis sat down to process all that video data. “It was challenging, to say the least,” says Milosavljević. “Hours and hours of tenacious staring at the videos! We would all get excited when we see a mouse chasing a weevil or a racoon messing with the ground trap.”
Fortunately, technology gave them an assist on the gargantuan job of analyzing the 20,211 hours of digital data: they could use two monitors to simultaneously watch four frames at eight times the normal speed. When weevils showed up, the researchers meticulously logged their behaviors as they interacted with the traps. Then the team compared escape and capture frequencies for the two trap designs, as well as duration of weevil encounters with the traps.
So Many Weevils, So Little Time
The results confirmed that cone traps work better than buckets. A total of 25 weevils visited the bucket traps—but they only captured four weevils. The cone traps had 19 weevil visitors and trapped 16 of them. That’s a retention rate of 18 percent for bucket traps, versus 89 percent for cone traps.
In the video, the researchers saw those weevils walking over the bucket traps’ four entrances or going inside but popping back out again—and weevils inside the bucket could walk around the bucket’s walls. The cone trap is funnel-shaped, with just one entrance, and the weevils moved around inside the trap much less. In fact, weevils interacting with bucket traps walked around the trap for 12 to 27 minutes, while those dealing with cone traps only did that for 90 to 376 seconds.
Based on their findings, the research team recommends trapping programs switch to cone traps whenever possible.
Milosavljević says the stakes are high. “We are in a race against an invasive creature on a path of destruction,” he explained. Palm weevils have killed thousands of Canary Island date palms, and they’re also a problem for edible date palms (Phoenix dactylifera).
This is bad for the economy because California’s ornamental palm industry brings in $70 million each year, and date growers produce $100 million worth of fruit—and provide jobs for 6,000 people. Tree loss also affects property values, as well as recreational areas and urban wildlife habitats, and it’s expensive to remove and replace infected trees.
It’s not a stationary problem, either. “The weevil is not going to stay in San Diego County,” warns Milosavljević. “It is now as far north as La Jolla, and there is really no reason to think it will stop there. It will work its way into Orange and Los Angeles counties for sure.”
In addition to trapping, the researchers are exploring a range of pesticides to protect the palms. They’re even looking at importing a natural predator as an ally in the battle against the palm weevil: parasitic tachinid flies (Billaea spp.). “Current observations from the native range show that the fly can kill up to 50 percent of the weevil population over the course of a year,” explains Milosavljević. “We are eager to travel to Brazil and bring the fly back to our quarantine facility at UCR for observation and study before releasing it into the wild.”
Journal of Economic Entomology
Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: firstname.lastname@example.org.