Studying a weevil to save a tree

The root weevil, Diaprepes abbreviates, may be a small bug, but it’s a big pest to citrus growers in Florida. 

Keith Weller/USDA, Wikimedia Commons

LOS ANGELES — “My sixth grade teacher gave all his students a Florida elm tree,” recalls Evan MacKay, now 17. “I planted mine. And watered it. And it grew. But around ninth grade I noticed there were insects all over my tree.” The bugs were the root weevil called Diaprepes abbreviatus. As they swarmed over his favorite tree, the teen panicked. “My mom and I sprayed the tree down with four different pesticides and none of them had any effect.” In desperation, Evan spent hours every day finding and killing these beetles by hand. Then he tried throwing his brainpower at the problem.

What he came up with earned Evan a finalist’s spot, here, at this week’s International Science and Engineering Fair, or ISEF. Created by Society for Science & the Public and sponsored by Intel, the 64th annual Intel ISEF showcases some of the best high school science projects from around the globe. (SSP also publishes Science News for Students and this blog).

This junior at Vero Beach High School in Vero Beach, Fla.,wondered how to stop a bug that pesticides didn’t kill. The teen decided to learn more about these weevils. He hopes one day to use his discoveries about the bugs to help stop their attacks on Florida’s trees. Evan presented his findings at ISEF.

Originally found in the Caribbean, the Diaprepes weevil found its way to the United States. Today it is crawling over much of the Southeast. In Florida alone, this bug causes damage worth billions of dollars to orange and grapefruit groves. Now, the beetles have added Eucalyptus torelliana to their daily diets. This fast growing tree is being used as a biofuel, a plant-based fuel that easily can be grown over and over again.

While searching for information on the pest, Evan found that there was a U.S. Department of Agriculture laboratory doing research on weevils just 45 minutes from his home. “So I just sort of got in my car and drove there,” he says. When he asked the government scientists if he could conduct experiments there, “they said yes!”

Evan wanted to study how weevils move, where they go and what they might be attracted to. For example, if male weevils are attracted to the chemical scent emitted by a female weevil, scientists might make their own version of that scent. It could lure males into traps and away from trees.

For studies like this, you don’t need a big laboratory. You only need a plot of appealing trees, a few weevils, some paint — and a lot of sunblock. Evan spent a full day in the sun collecting around 320 weevils from an abandoned citrus grove. After carefully marking these weevils with dabs of paint, Evan released the bugs into his experimental plot. Then he came back one, two and five days later. Each time, he hunted for his painted bugs, measuring how far they had travelled. He also charted which trees they settled in and whether they were social or solitary.

The weevils proved big nomads for such little bugs. Each is about one centimeter, or 0.4 inch, long. Yet Evan showed that they could travel up to 60 meters (around 196 feet) in a single day. They also preferred taller trees, possibly because they have more foliage for food.

Another observation: To his surprise, weevils in the wild don’t behave like those in the lab. Previous studies from scientists working with the bug showed that female weevils in the lab were attracted to male weevils. The researchers even isolated a chemical scent, or pheromone, from the males that appeared to be responsible. But when they tested it in the field, the female weevils didn’t seem interested.

Evan thinks he has figured out why. “In the field, females don’t move toward males,” he found. “Instead, males move toward females.” He thinks the females might have their own mate-signaling pheromone, although he doesn’t yet know what it is.

The teen hopes to start hunting for such a pheromone. If he finds it, growers and homeowners might use it one day to lure and trap the weevils threatening their plants. His results are very different from what other scientists had predicted. But he says it’s all because he was working outside, in the weevils’ native environment. With more tests, and maybe more field work, Evan’s results might help combat this irritating pest — and maybe even keep his tree at home safe.

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Power Words

biofuels  Energy sources derived from carbon stored in living organisms. Although wood is a biofuel, most people who support “green” sources of energy consider biofuels to be liquids that can substitute for gasoline. Examples include bioethanol, an alcohol derived from crops such as corn or sugar cane. Engineers are also developing ways to make biofuels from nonfood crops, such as trees and shrubs. Renewable biofuels are an alternative to nonrenewable fossil fuels.

Diaprepes root weevil   This insect species (Diaprepes abbreviatus) is originally from the Caribbean. This beetle has moved to the United States where it has become a major pest in citrus crops, such as oranges and grapefruits.

Eucalyptus torelliana  (sometimes known asCorymbia torelliana) A tree native to Australia that has been imported to the United States. It is useful to block wind around sensitive crops and is also being used as a biofuel.

pesticide  A chemical or mix of compounds used to kill insects, rodents or other organisms harmful to cultivated plants, pet or livestock, or unwanted organisms that infest homes, offices, farm buildings and other protected structures.

pheromone  A molecule or specific mix of molecules that makes other members of the same species change their behavior or development. Pheromones drift through the air and send messages to other animals, saying such things as “danger” or “I’m looking for a mate.”

social  A characteristic behavior that is often applied to humans and other animals. It refers organisms of a species that prefer to spend time in close proximity to each other, interacting.

solitary  A characteristic behavior often applied to humans or other animals. It refers to organisms that spend much of their time alone, not interacting with other members of their species.

Bethany Brookshire was a longtime staff writer at Science News Explores and is the author of the book Pests: How Humans Create Animal Villains. She has a Ph.D. in physiology and pharmacology and likes to write about neuroscience, biology, climate and more. She thinks Porgs are an invasive species.