Efficiency of Niang shell Archidendron pauciflorum (Benth.) extracts in the control of adult Aedes aegypti (L.) (Diptera: Culicidae)

Mosquitoes are more than just a summer nuisance; they are the world's most dangerous animals, responsible for transmitting diseases like dengue fever, Zika, and yellow fever that threaten millions of lives. The primary culprit is the Aedes aegypti mosquito, which thrives near human settlements and breeds in clean water containers. For decades, the standard defense against these insects has been synthetic chemical sprays. However, these chemicals have a significant downside: mosquitoes are evolving resistance to them, and the chemicals themselves can harm the environment and human health. This has pushed scientists to look for alternatives that are both effective and safe. One promising avenue is the use of botanical extracts—poisons derived from plants that target insects but break down naturally. The challenge lies in finding the right plant, extracting its active ingredients, and figuring out the best way to deliver the poison so the mosquito actually consumes or touches it.

In a recent study, researchers in Thailand explored a solution that turns a local agricultural problem into a public health weapon. They focused on the shells of the Niang tree, known scientifically as Archidendron pauciflorum. In southern Thailand, people harvest the seeds of this tree to eat as a vegetable or dessert, but the hard outer shells are usually discarded as waste. Instead of letting this biomass rot and pollute the environment, the research team applied a "zero waste" concept to transform these discarded shells into a biopesticide. They wanted to see if the extract from these shells could kill adult Aedes aegypti mosquitoes and, if so, how it compared to standard testing methods. The team tested two different ways of exposing the mosquitoes to the plant extract: one method involved mixing the extract with sugar water so the mosquitoes would drink it, and the other involved coating the inside of a glass bottle with the extract so the mosquitoes would touch it as they flew around.

The researchers began by collecting the Niang shells, washing them, and drying them in the sun until they were completely dehydrated. They ground the dried shells into a coarse powder and soaked it in ethanol, a type of alcohol, for a week to pull out the active chemical compounds. After filtering out the solid plant matter, they evaporated the liquid to create a concentrated, dark brown extract. To understand what was inside this mixture, they used two types of light-based analysis. One method, called infrared spectroscopy, identified specific chemical bonds that suggested the presence of phenolic compounds and a group of natural toxins known as rotenoids. Another method, using ultraviolet light, showed absorption patterns that closely matched those of rotenone, a well-known natural insecticide found in other plants. These chemical clues suggested the extract contained substances capable of disrupting the energy production inside an insect's cells.

To test how well this extract worked, the team set up two different experiments using laboratory-reared female mosquitoes. In the first experiment, they used a technique called Attractive Toxic Sugar Baits. Since mosquitoes need sugar from flower nectar to survive, the researchers mixed the plant extract with a sweet sugar solution. They offered this mixture to the mosquitoes on cotton swabs. The mosquitoes, attracted by the sugar, drank the solution and ingested the poison. In the second experiment, they used a standard method known as the CDC bottle bioassay. Here, they coated the inside of glass bottles with the extract and placed the mosquitoes inside. This method relies on the mosquitoes touching the poison with their legs and body as they fly, rather than eating it.

The results showed a clear and dramatic difference between the two methods. When the mosquitoes drank the sugar mixture containing the extract, the results were highly effective. The death rate rose steadily as the concentration of the extract increased. At the highest concentrations tested, every single mosquito died. The researchers calculated that a very low concentration of just over two percent was enough to kill half of the mosquito population, and a concentration of about six percent was sufficient to kill nearly all of them. The relationship between the amount of poison and the number of deaths was consistent and predictable.

In contrast, the method where mosquitoes simply touched the poison in the glass bottles did not work as well. The results were erratic and did not follow a clear pattern. Sometimes a higher concentration of the extract killed fewer mosquitoes than a lower one. The researchers found that the plant extract was thick and sticky, making it difficult to coat the inside of the bottles evenly. Because the coating was uneven, some mosquitoes likely did not come into contact with enough of the poison to die, while others did. This inconsistency meant that the researchers could not calculate a reliable measure of how much poison was needed to kill the mosquitoes using this contact method.

The study concludes that the Niang shell extract is a potent weapon against Aedes aegypti, but only when delivered through ingestion. The sugar bait method proved far superior because it bypassed the physical barriers of the mosquito's body and delivered the toxin directly into its digestive system. Once inside, the rotenoid-like compounds appear to shut down the mitochondria, the energy powerhouses of the cell, causing the insect to die from internal failure. This mode of action is different from synthetic sprays, which typically attack the nervous system, suggesting that this natural extract could be useful even against mosquitoes that have become resistant to common chemicals.

By turning a waste product into a highly effective tool for disease control, this research offers a sustainable path forward. It demonstrates that agricultural byproducts, often seen as trash, can be upcycled into valuable resources that protect human health without relying on harsh synthetic chemicals. While the study confirms the potential of the Niang shell extract, the researchers note that future work is needed to refine the product, perhaps by mixing it with other substances to make it easier to apply or to improve its stability. For now, the findings provide a strong foundation for developing a new, eco-friendly way to fight the mosquitoes that carry some of the world's most dangerous diseases.