Red mites of the species Tetranychus merganser are little arthropods and close relatives to spiders which spin silk webs for their colonies. In China, the U.S., Australia and Mexico they feed on crops such as papaya, beans, prickly pear and ornamental plants causing economic losses. But now, a group of scientists has found a potential natural insecticide from a plant native to India and Pakistan: Moringa oleifera.
The use of moringa as a bioinsectide is not only effective against mites; it also offers a less polluting and safer alternative for human health than chemical insecticides, explain researchers from several Mexican institutions who published their work in the journal Plants last May.
“[With the moringa extract] we also consider the human aspect. People working with agrochemicals are directly exposed and that is obviously something dangerous compared to the use of moringa,” explains agroecologist Rapucel Heinz-Castro of the Autonomous University of San Luis Potosí and first author of the study.
In bean plants, for example, damage from these mites appears as white spots near the veins of the affected plant. Despite living only about eight days, these small arachnids reproduce fast causing great problems in crops. If plants are left unattended, they eventually die.
Methods to control mite pests include chemical insecticides, but such compounds have an increasingly bad reputation not only because they are highly polluting and leave residues in the place of application, but because they are harmful to people who handle them. They also lose effectiveness over time when mites become immune to this type of products, as it happens in other species of the genus Tetranychus.
Moringa oleifera is a fast-growing tree living in tropical and subtropical environments which has has been reported to be drought tolerant, making it ideal for growing in areas where water is scarce.
“[The moringa tree is] easily adaptable to semi-arid and arid areas due to its resistance to water stress,” says Agustín Hernández-Juárez, from the Universidad Autónoma Agraria Antonio Narro and co-author of the study. “Having a crop or product that can be produced at low water costs is a great advantage.”
Researchers from five universities in northern Mexico found that the extract of moringa leaves causes sterility in female mites, meaning fewer eggs.
Researchers collected clean, mature leaves from a two-year-old moringa tree, from which they obtained an ethanolic extract that they later analysed to determine its chemical composition. They sprayed different concentrations of the extract on female mites feeding of bean leaves placed in Petri dishes.
The group of scientists analysed the effect of the moringa extract only on female mites as they lay the eggs and therefore need to feed constantly causing the most damage to the plant, explains Julio César Chacón-Hernández, from the Applied Ecology Institute at the Autonomous University of Tamaulipas and coauthor of the study.
Of the seven dilutions analysed, the extract with the highest concentration was the most efficient in causing mortality in more than 90% of the sprayed females compared to the control.
Additionally, the application of the bioinsecticide also decreased the number of hatched eggs by preventing gas exchange of the embryos. Secondary metabolites, like alkaloids, present in the leaves of Moringa oleifera are responsible for this mortality. These chemical compounds also act on the nervous system, affecting behavior and slowing growth.
Another advantage of using moringa as a bioinsecticide is that the trees constantly produce leaves and that collecting them does not put the tree at risk, according to the researchers.
Although the road to the commercial use of a moringa-based bioinsecticide is long, the group of researchers is not ruling it out.
“There is a lot of work ahead,” says Hernández-Juárez. “We are working towards integrated pest management. We do not seek to eliminate pests or insects, [but] rather reduce their populations so that they do not cause economic damage.”
The group of scientists also mentions that it will be important to know the effects of the moringa extract on the red mite’s natural enemies and on other beneficial insects. It will also be necessary to take the experiment from the lab and into the field to determine its effectiveness in natural conditions, which include interactions with abiotic and biotic factors.
Heinz-Castro, Rapucel T.Q., Roberto Arredondo-Valdés, Salvador Ordaz-Silva, Heriberto Méndez-Cortés, Agustín Hernández-Juárez, and Julio C. Chacón-Hernández 2021. “Bioacaricidal Potential of Moringa oleifera Ethanol Extract for Tetranychus merganser Boudreaux (Acari: Tetranychidae) Control” Plants 10, no. 6: 1034. https://doi.org/10.3390/plants10061034
Carolina Hurtado-Torres is a student of Environmental Horticulture at the Autonomous University of Querétaro. She’s passionate about writing and draws her inspiration from the desert landscapes of her home state. Through her studies and her field observations, Carolina has become an advocate for the protection of the environment.
English translation by Lorena Villanueva Almanza