Marenda is an assistant professor of biology, co-director of the Cell Imaging Center and director of the biology graduate program in the College of Arts and Sciences.
O’Donnell is a professor and associate department head for the Department of Biodiversity, Earth and Environmental Science in the College of Arts and Sciences.
Six days. It only took six days for all of them to die.
It was a shocker for Assistant Professor of Biology Daniel Marenda — he was certain the fruit flies in his son’s sixth-grade science fair experiment would last a few weeks, a month even. Something in the artificial sweetener the flies were eating was killing them, and fast. But what?
Inspired by the questions raised by his son’s project, Marenda, who uses fruit flies in his research to study the human brain, sought out Drexel’s head “bug guy,” Sean O’Donnell, to get some answers.
“I only use insects to study the brain, so I needed someone who knew something about insects,” says Marenda. “So, I bring the story to Sean and he looks at me and his eyes keep getting bigger and bigger, and he said, ‘Let’s try and test this for real.’”
Together, the pair performed a number of simple lab experiments with Drosophila melanogaster fruit flies and a variety of natural sugars, as well as artificial sweeteners including the brands Truvia and PureVia.
“I assumed that it was going to be the stevia extract causing all of the death,” confesses O’Donnell, who is the associate department head for the Biodiversity, Earth and Environmental Science Department in the College of Arts and Sciences.
The experiments showed that Truvia, a top-selling sugar substitute developed by The Coca-Cola Co. and Cargill, was especially fatal to the flies. The sweetener is marketed as a derivative of the stevia plant; however, a large concentration of it (89 percent) is erythritol, an FDA-approved, non-nutritive sugar alcohol that humans have been consuming for over a decade. It’s safe for humans, even in large quantities, but lethal to fruit flies even in small doses, according to O’Donnell and Marenda’s experiments.
In short, the toxic agent in Truvia is erythritol, and even a little goes a long way when it comes to offing fruit flies.
“By days 3 and 4, the flies’ nervous systems were shot,” says Marenda, “and by days 5 and 6, all of the flies in the Truvia vials were dead.”
O’Donnell and Marenda wrote a paper on their findings, which was recently accepted for publication in the online journal PLOS ONE.
They propose in the paper the use of erythritol as an environmentally sustainable, human-safe pesticide. In fact, Drexel University and the researchers are in the process of pursuing a patent on this specific application of erythritol.
“I feel like this is the simplest, most straightforward work I’ve ever done, but it’s potentially the most important thing I’ve ever worked on,” O’Donnell says.
Marenda goes on to say that around the time the two were performing their experiments in the lab, a tragedy occurred in Bihar, India — 23 children died after eating rice that had been contaminated with traces of organophosphorus, which is used in pesticides.
“It really underscored our need to have human-safe, yet highly effective, pesticides,” Marenda says.
Because erythritol is sweet, flies are drawn to it. Things like fungus in rotting fruit are most palatable to fruit flies, Marenda says, but sugars and sugar alcohols are a close second. “Insecticides are not going to be useful if insects don’t want to eat them,” he says.
Even when given access to alternative food sources, the flies were attracted to erythritol like…flies. And they dropped like them, too.
“Erythritol baits could function as an effective insecticide delivery mechanism in naturalistic situations where insects have access to other foods,” O’Donnell says.
It was clear to Marenda and O’Donnell that erythritol is lethal to fruit flies. But how exactly are they dying? The researchers aren’t quite sure.
“We have speculation about how the erythritol is killing the flies,” says O’Donnell. One possibility, he explains, is that the erythritol is blocking sugars in the flies’ digestive systems and “literally starving them to death.”
The next step in their research is to learn more about what’s happening to the flies physiologically, O’Donnell says. The researchers plan to perform experiments on other insects such as termites, cockroaches, bed bugs and ants. They also plan to conduct experiments with praying mantids, which eat fruit flies, to test erythritol’s upward strength in the food chain.
More important, though, is further examination of erythritol’s applications as a pesticide.
“We are not going to see the planet sprayed with erythritol and the chances for widespread crop application are slim,” O’Donnell says. “But on a small scale, in places were insects will come to a bait, consume it and die, this could be huge.”
Note: Marenda’s son, Simon, didn’t win the science fair, but he was able to present his findings to his class. And, he’s credited as both a co-author on the paper and as the inspiration and intellectual curiosity behind this research.