Food Sweetener Safely Slays Insects

CERTAIN SUGARS CONSIDERED SAFE as sweeteners in the human food supply can double as environmentally-friendly pest remedies, and even make biological control of insects by beneficial fungi more practical for households, farms and gardens. Considering that caffeine from coffee grounds can be used against deadly dengue mosquitoes and that a variety of traditional herbs can blast away bed bugs, insecticidal sugar compounds should come as no surprise. Perhaps the only remedy more surprising is that rain water or simulated rain sprays from hoses or irrigation equipment can safely wash away pests with no toxic pesticide residues to worry about in the environment.

Using sugars directly to slay insects is somewhat unusual. However, sugars are commonly used as attractants, for instance to lure fruit flies, moths or ants to baits and traps both for population control and as a survey method or monitoring tool. California citrus growers have a long history of using sugar sprays as an IPM (integrated pest management) strategy to lure fruit-scarring citrus thrips to organic or botanical formulations of ryania (“from woody stem and root materials of plants of the genus Ryania”) or sabadilla (alkaloids from seeds of a lily bulb, Schoenocaulon officinale). “INTEGRATED PEST MANAGEMENT implies that techniques used to manage one pest species should not disrupt techniques used to manage other pests of the same crop,” wrote J.D. Hare and Joseph Morse in the Journal of Economic Entomology. “In citrus pest management in California, this situation is well illustrated in the choice of pesticides for the management of one major pest, citrus thrips, Scirtothrips citri (Moulton), without disruption of several effective biological control agents of the other major pest, California red scale, Aonidiella aurantii (Maskell).”

That sugars can be lethal to pests and be a source of environmentally-friendly pesticides is not exactly intuitive. “Potential of the non-nutritive sweet alcohol erythritol as a human-safe insecticide” was the strangely intriguing title of Drexel University’s Sean O’Donnell’s presentation at the Entomological Society of America (ESA) annual meeting. Many of the details were previously published in PLoS ONE, an open access journal, and in part because of the origins of the research in a grade school science project by one of the researcher’s sons, aspects of the story have been widely reported in various media. “Erythritol is a zero-calorie sweetener found in fruits and fermented foods,” summarized Lauren Wolf in Chemical & Engineering News, and “is Generally Recognized As Safe by the Food & Drug Administration and has been approved as a food additive around the globe.”

“Many pesticides in current use are synthetic molecules such as organochlorine and organophosphate compounds,” and “suffer drawbacks including high production costs, concern over environmental sustainability, harmful effects on human health, targeting non-intended insect species, and the evolution of resistance among insect populations,” write the researchers in PLoS ONE. “Erythritol, a non-nutritive sugar alcohol, was toxic to the fruit fly Drosophila melanogaster. Ingested erythritol decreased fruit fly longevity in a dose-dependent manner, and erythritol was ingested by flies that had free access to control (sucrose) foods in choice and CAFE (capillary feeding assays) studies…

“We initially compared the effects of adding five different non-nutritive sugar substitutes (Truvia, Equal, Splenda, Sweet’N Low, and PureVia,” wrote the researchers in PLoS ONE. “Adult flies raised on food containing Truvia showed a significant reduction in longevity…We noted that adult flies raised on food containing Truvia displayed aberrant motor control prior to death. We therefore assayed motor reflex behavior through climbing assays…Taken together with our longevity studies, these data suggested some component of the non-nutritive sweetener Truvia was toxic to adult Drosophila melanogster, affecting both motor function and longevity of this insect…

“Our initial analysis of sweeteners included two sweeteners that contained extracts from the stevia plant, Truvia and Purevia. While adult flies raised on food containing Truvia showed a significant decrease in longevity compared to controls, this was not the case for flies raised on Purevia. These data suggest stevia plant extract was not the toxic element in these sweeteners. Purevia contains dextrose as a bulk component, while Truvia contains erythritol as a bulk component…To determine if erythritol was the toxic component of Truvia, we repeated our longevity studies on food containing equal weight/volume (0.0952 g/ml) of nutritive sugar control sucrose, and non-nutritive sweeteners Truvia, Purevia, and erythritol. We assured the flies were successfully eating the foods containing these sweeteners through dye labelling the food with a non-absorbed blue dye (blue food), and visual confirmation of blue food present in fly abdomens and proboscises daily…The average percentage of blue abdomens throughout the study were 97.46%.”

“These data confirm all treatment foods (including Truvia and erythritol treatments) were consumed by adult flies, and suggest mortality was not due to food avoidance and starvation…A large body of literature has shown that erythritol consumption by humans is very well tolerated, and, indeed, large amounts of both erythritol and Truvia are being consumed by humans every day throughout the world. Taken together, our data set the stage for investigating this compound as a novel, effective, and human safe approach for insect pest control. We suggest targeted bait presentations to fruit crop and urban insect pests are particularly promising.”

Interestingly, a few decades ago UK researchers found that the sweeteners (sugar alcohols; polyols) erythritol, glycerol and trehalose rendered more effective several insect biocontrol fungi, Beauveria bassiana, Metarhizium anisopliae and Paecilomyces farinosus. These insect-killing fungi need a relative humidity (RH) near 100% for germination of their conidia (seed-like propagules). “Conidia with higher intracellular concentrations of glycerol and erythritol germinated both more quickly and at lower water activity,” wrote UK researchers J.E. Hallsworth and N. Magan in the journal Microbiology. “This study shows for the first time that manipulating polyol content can extend the range of water availability over which fungal propagules can germinate. Physiological manipulation of conidia may improve biological control of insect pests in the field…Although fungal pathogens have been used to control insect pests for more than 100 years, pest control has been inadequate because high water availability is required for fungal germination.”

Curiously, erythritol and glycerol, besides being sweetening substances, also function as antifreeze compounds. Certain Antarctic midges, known as extremeophiles for living in an ultra-cold habitat, ingest and sequester erythritol from their food plants; and as antifreeze it protects the adult flies from freezing. Indeed, many mysteries remain. Besides being found in green plants like stevia and in lower amounts in fruits, erythritol is found in certain mushrooms, lichens and algae. Human and animal blood and tissues apparently have low endogenous levels of erythritol; and erythritol is a yeast fermentation product (hence in sake, beer, wine). In human medicine, erythritol has been used for coronary vasodilation and treating hypertension; and according to Japanese microbiologists, erythritol ingestion may mean fewer dental cavities (caries) than sucrose sugar.

Drones, Bug-Bombs & Future Weed Control

FUTURE WEED CONTROL, looking out several decades, will inevitably by necessity gradually start shifting towards weed-eating insects for biological control, with a lesser mix of herbicides and tillage. Drones delivering “Bug-Bombs” with payloads of beneficial weed-eating insects may not be the fastest or deadliest means of killing weeds, but it is an ecological strategy with many benefits for fighting weeds in remote terrain, rangelands and large, hard-to-reach areas in general.

Yong-Lak Park, a West Virginia University (Morgantown) entomologist, calls it “Shooting insects from the sky: Aerial delivery of natural enemies using aerospace engineering.” At a late-night session of the KYE (Korean Young Entomologists) in Austin, Texas as part of the Entomological Society of America (ESA) annual meeting, Park’s informative slide show (now posted on the Internet) depicted a range of Unmanned Aerial Vehicle (UAV) designs and even a California vineyard in the agricultural vanguard with its own drones (not unlike flying model airplanes). Indeed, it is not hard to imagine a New Feudalism, where behind moated walls with locked gates and barking dogs, in an entertainment room with big screens and small monitors, sit modern medieval lords with joy sticks in hand commanding drone armies and air forces trying to rule universes, suburban lots and whatever.

Like model airplanes, UAVs are lightweight, inexpensive and relatively safe and easy to control, Park told the room full of Korean entomologists and a lone non-Korean writer in attendance. Equipped with sensor modules, GPS, digital cameras and video image analysis capabilities, UAVs can monitor weeds and detect weed biocontrol weevils on the ground with a resolution of up to 3 inches (8 cm). UAVs similar in design to the infamous drones used by certain governments for extrajudicial killings, and small helicopter-like octarotors are among the aerospace vehicles capable of delivering beneficial “Bug-Bombs” (bug pods) to large, hard-to-reach areas for biological control of weeds such as morning glory and mile-a-minute weed (Polygonum perfoliatum).

Galileo’s legendary sixteenth century scientific experiment dropping objects from the Leaning Tower of Pisa to see how fast they fell came to mind when Park described his rooftop tests dropping Bug-Bombs filled with weed-eating weevils from different heights. Only rather than challenging Aristotle’s ancient teachings, Park wanted to see if the bug pods, which are cannisters shaped like the bombs you see dropping from Allied planes in World War II film footage, would cushion the weevils when they hit the ground from different heights. Indeed, 80-90% of the beneficial weed-eating weevils inside the bomb-shaped pods survived being dropped 0, 10, 20 and 30 meters (0, 33, 66, 98 ft). The idea being that the pods pop open when they hit the ground in some remote weed-infested area, and the weevils hop out and go about their everyday life of eating their favorite weed and reproducing new generations of weevils.

Basically, you get an army of weevils on the ground doing weed control, as opposed to aerial bombardment with herbicides and all their environmental side effects. This is known as classical biological control of weeds, and it has a long track record. On the downside, it is expensive to find the right insects, as they must be collected, reared and tested to make sure that they stick to the weeds (so you don’t inadvertently introduce a crop pest, for instance). Then you need permits. It might be millions of dollars and decades later before all the hurdles are leaped and a successful program is out the gate. But it has worked against several dozen weeds, and often a successful program can then be easily replicated in a new location.

Beneficial Bugs Challenge Theoretical Physics

INSECTS, MICROBES, PLANTS and other organisms form complex ecological systems with all sorts of synergisms, antagonisms and cooperative interactions, leading oftentimes to beneficial insects controlling what we consider pests. Whether it be forest, desert, farm field or garden, intricate and nuanced ecological communities can be nurtured to provide a measure of “natural” biological pest control.

The nuanced complexity of biological and ecological systems has at times intrigued theoretical physicists usually more attuned to quarks, neutrinos, chaos theory and quantum phenomena. Murray Gell-Mann, winner of the 1969 physics Nobel Prize as a Caltech (Pasadena, California) professor “for his contributions and discoveries concerning the classification of elementary particles and their interactions,” created the Santa Fe Institute (New Mexico) to better focus on “the theory of complex adaptive systems.”

Humans, plants and animals are individually and collectively at the ecosystem level examples of complex adaptive systems. Which is one reason creating sustainable agriculture is such a challenge, and companies such as Rincon-Vitova Insectaries end up with catalogs of 55 pages of beneficial insects, microbes, seeds, traps and other inputs for creating sustainable garden and farm systems. And even then, it is not always easy and can take longer than expected to force changes in even the smallest complex adaptive system that is a backyard garden.

“Unfortunately, it will be a long time before human knowledge, understanding, and ingenuity can match–if ever they do–the “cleverness” of several billion years of biological evolution,” wrote Gell-Mann in his book, The Quark and the Jaguar. “Not only have individual organisms evolved their own special, intricate patterns and ways of life, but the interactions of huge numbers of species in ecological communities have undergone delicate mutual adjustments over long periods of time.”