Listening to Larvae: How Acoustics Can Measure Efficacy of Palm Weevil Management

red palm weevil larva

The larva of the red palm weevil (Rhynchophorus ferrugineus) is a destructive pest of palm trees in subtropical regions. During larval stages, R. ferrugineus tunnels deep within the palm trunk, making detection and pest management difficult. However, researchers have found that they can measure R. ferrugineus larval activity acoustically, and a new study shows that the success rate of a leading management method for the pest can be evaluated by noting changes in acoustic signals from within an affected palm tree. (Photo credit: Katja Schulz/Flickr CC BY 2.0)

By Andrew Porterfield

The red palm weevil (Rhynchophorus ferrugineus) is a common pest of palm trees in subtropical regions of the globe (and, for a brief time, in the United States). Working unseen and deep inside the palm trunk, the weevil larvae tunnel and feed, eventually killing the palm.

Andrew Porterfield

Working around the weevil’s penchant for feeding and burrowing out of sight, pest management researchers and managers have for years tried to exploit the insect’s unique sounds emitted when active.

By capturing the characteristic sounds of the weevil, pest managers can determine how effective insecticides are working against the insect pest. In fact, for the weevil, acoustic monitoring has been about the only successful way to determine the impact of pest control. Adult weevils can be monitored with pheromones, but larvae, which do the most damage, can’t be so easily detected.

To make matters more challenging, weevils have developed a resistance to some chemical pesticides, while other chemicals often kill nontarget organisms and natural enemies of the weevil as well as the weevil itself. Therefore, early detection is key to target biological controls that halt proliferation of R. ferrugineus.

One of these biological controls is Beauveria bassiana, an insect-killing fungus that has been studied for management of aphids, caterpillars, whiteflies, grasshoppers, and a number of other leaf-eating insects. However, it has not been entirely clear how well B. bassiana works against the elusive weevil larvae once it sets about its subsurface activities.

Richard Mankin, a research entomologist at the U.S. Department of Agriculture’s Agricultural Research Service in Gainesville, Florida, and his colleagues at the University of Alicante, Spain, and Universiti Kebangsaan Malaysia, found that acoustic measuring technology could detect the actions of red palm weevil larvae during various instars, the key stages in their lifecycles between moltings. The sound “bursts” emanating during these instars could help detect the effectiveness of biological pesticide treatments. Their study, “Acoustic activity cycles of Rhynchphorus ferrugineus early instars after Beauveria bassiana treatments,” was published last week in the Annals of the Entomological Society of America.

The researchers collected 24 two-week-old weevil larvae and dipped them in suspensions containing B. bassiana. Two different concentrations of the B. bassiana suspension were used. The treated larvae were then implanted into the trunks of 12 palm trees.

Sounds of the larvae were recorded for three minutes on multiple days using a sensor/amplifier system, for about 30 days. To overcome the problem of capturing short-duration cycles of feeding and movement among young larvae, the researchers devised a way to subdivide long-term recording periods into activity cycles.

Based on previous research showing the acoustic profiles of specific sound bursts emitted by the weevils, Mankin and his team could classify the sound impulses recorded from the palm trees. The researchers discovered three main activity cycles from the sound recordings—at 18-20 days, at 25-29 days, and at 35-46 days. Measuring these bursts could help them more realistically measure the effects of pesticides on the weevil’s natural active and quiet periods. In fact, treatments that were applied had a significant effect on these sound activity cycles as the experiment continued. Bursts of sound and numbers of impulses were reduced in treated versus control (untreated) larvae.

After 55 days, the palms were dissected. Among the highest dosage of B. bassiana, all larvae were killed. In the lower dose of B. bassiana, about half the larvae were dead and the other half had transformed into pupae. In the untreated control group, 75 percent of those larvae had pupatd, and just 20 percent had died. The treated dead larvae all showed signs of B. bassiana infection.

“These experiments suggest that aspects of R. ferrugineus larval development, including feeding and molting of instars, and the effects of management treatments can be acoustically monitored,” Mankin and his colleagues wrote. Acoustic technology is particularly important to measure pesticide effectiveness with weevils and other similarly unseen insects, “because the alternative method for assessment of treatment effectiveness in the field is palm dissection, which is time-consuming, expensive, and destructive.”

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.

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