Substances naturally found in garlic and spearmint could help better fight certain pest insects. Researchers have shown that their effectiveness increases when they are prepared in the form of extremely small droplets. The treatment then acts more strongly on the Chinese bruchid (Callosobruchus chinensis), a beetle whose larvae develop inside seeds.
This small insect can cause significant losses after harvests. The female lays directly on the seeds of beans and other legumes. After hatching, the larva enters the seed to feed there, progressively degrading stocks intended for food or sowing.
Callosobruchus chinensis (Linné, 1758) male.jpg
Image Wikimedia
The study focuses on two plant molecules. The first, diallyl disulfide, gives garlic its characteristic smell. The second, carvone, is found in spearmint. The researchers incorporated them into nanoemulsions, that is, liquids containing tiny droplets, here about 54 to 62 nanometers across.
This formulation clearly changes the product's effectiveness. For the garlic-derived compound, the action against adult bruchids increased by 30.5% compared with the same substance used without the nanoemulsion. The gain was more modest with carvone, at 8.19%.
The researchers also mixed the two molecules in different proportions. Their combination did not cause any unexpected additional effect: each simply continued to contribute to the insecticidal action. The garlic-derived compound remained the one whose effectiveness benefited most from the very small droplet formulation.
A useful insecticide, however, must not only kill the pest. Its effects on species that can help fight it must also be checked. The team therefore looked at a small wasp, Anisopteromalus calandrae. This insect attacks bruchid larvae and can thus naturally help control their population.
At certain doses effective against bruchids, the garlic-based formulation and one of the mixtures affected these wasps less than the targeted pest. This suggests the possibility of combined use with biological control. The trials also showed that treating bruchid larvae could promote the emergence of the wasps that parasitize them.
The treated seeds retained their ability to germinate under the experimental conditions. No measurable plant residue was detected on their surface after the studied treatments. Some formulations were even associated with longer roots and greater vigor in young seedlings.
These results, however, come from controlled laboratory trials. Before use in silos or agricultural storage facilities, the treatment's duration of effectiveness, its stability, and its effects on other organisms under real storage conditions remain to be verified.