But animal models are not fans of the "natural" sweetener Truvia either. A study tested survival rates of house flies (Musca domestica), on erythritol--the main ingredient in Truvia--compared to natural sugar, agar, and water alone. They found that the flies both strongly preferred high-calorie natural sugar when presented with the choice and, when fed only erythritol, they died no faster than when fed only water or agar.
So the flies didn't get harmed by it, they just don't want to consume it.
Scientists also tested mannitol, xylitol, and sorbitol, and found only the sorbitol to have any nutritive value for flies. They identify several possible reasons for the flies' inability to survive on artificial sweeteners, including an inability to digest them, a negative interaction with flies' gut microbes, or digestive blockage caused by the sweeteners. But, while the sweeteners led to high mortality in a lab setting, real-world applications would face practical challenges.
"Based on our results, erythritol does not appear to have insecticidal properties on house flies in the traditional sense, and would not be a candidate for adult control," says Michael Fisher, BCE, lead author of the study and a graduate student at NC State.
Citation: Michael L. Fisher, Fallon E. Fowler, Steven S. Denning, and David W. Watson, 'Survival of the House Fly (Diptera: Muscidae) on Truvia and Other Sweeteners,' Journal of Medical Entomology, April 5th 2017 http://dx.doi.org/10.1093/jme/tjw241