Long-Distance Palm Weevil Flyers Threaten California Date Palms
By Andrew Porterfield
How long and how fast an invasive insect travels are important questions to determine the insect’s impact on plant (or animal) hosts. A new study by University of California, Riverside, researchers shows that Rhynchophorus palmarum, also known as the American or black palm weevil, can fly much further and faster than expected.
The results, published in June in the Journal of Economic Entomology, do not bode well for the southern California date palm industry.
R. palmarum is a native of South and Central America as well as islands in the Caribbean. It is a widespread and destructive palm pest in its native regions. Just 24 feeding larvae of R. cruentatus (the Palmetto weevil, a related species) could kill a Canary Island palm tree in as little as 49 days. R. palmarum has amplified this destruction by carrying a roundworm, Bursphelenchus cocophilus, as a vector for red ring disease, lethal to date palms.
Recently, R. palmarum was spotted on both sides of the border between the United States and Mexico. In 2010, dead Canary Island palms were reported in Tijuana, Baja California, Mexico, and UC Riverside scientists discovered R. palmarum was the causative insect. A year later, adult palm weevils were trapped in San Ysidro, California. In 2012 the weevils were found in Alamo, Texas, and in 2015, in Yuma, Arizona.
This behavior prompted the question: How far can R. palmarum travel, and how quickly? The UC Riverside researchers, led by entomology professor Mark Hoddle, Ph.D., tested the flight capabilities of R. palmarum, and their published findings revealed that a significant proportion of female weevils could fly more than 100 kilometers (61 miles), and a few could fly more than 140 kilometers (87 miles), in one day. These findings greatly expanded the potential for destruction by the palm weevil.
Hoddle and colleagues tested R. palmarum flight by tethering 87 weevils to a flight mill, which measured the speed and distance flown in a laboratory. The researchers had captured live adult weevils in bucket traps set up in San Diego County, California. Virgin females and males were also obtained from cocoons in Chula Vista, California.
Of the 87 weevils, 82 flew at least 1 kilometer (0.6 miles) in 24 hours and were analyzed in the study. For females, 37 percent flew 1-50 kilometers, 16 percent flew 51-100 kilometers, and five of them flew more than 100 kilometers. Of the five “super dispersive” weevils, two flew more than 140 kilometers. For males, 28 percent flew 1-50 kilometers, 13 percent flew 51-100 kilometers, and the farthest-flung male flew 95 kilometers. Data showed that more R. palmarum weevils could fly longer distances than their cousins, R. ferrugineus and R. vulneratus.
In addition, these long flights resulted in significant reductions in body weight, with an average loss of 18 percent (non-flying control weevils lost 13 percent body weight).
The findings were a surprise to Hoddle’s group. “The extreme distances female weevils could fly in 24 hours was amazing,” Hoddle says. “As is the percent body weight over a 24-hour period—it’s amazing that they survive.”
Hoddle’s study shows how the weevils could cover the territory from Mexico into San Diego rather quickly. It also indicates that California’s Coachella Valley, home to a $100 million date palm industry and just 150 kilometers (93 miles) from current infestations, could easily be overtaken by the invasive pests. And just a few “super flyers” could enable such an infestation, Hoddle says.
“A small percentage of super-flyers in a large invasive population represents a significant risk for rapid spread into new areas,” he says. “Oddly, we are not seeing this, probably because there are so many palms for weevils to attack in San Diego County. However, these weevils clearly have the capability of doing this should they elect to do so. This puts the edible date industry in the Coachella Valley at risk, and this may explain why weevils have been found in traps in Yuma, Arizona, and Alamo, Texas.”
Journal of Economic Entomology
Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.