Improved activity of thyme essential oil (Thymus vulgaris)against Aedes aegypti larvae using a biodegradable controlled release system

Aedes aegypti mosquito is the major vector of urban arboviruses and the initial phase of its development occurs in containers with water. The use of chemical control agents is more effective in the larval instar than for the adult insects, due to limited mobility of the larvae in water and their higher sensitivity. Besides, acting in the initial phase is preferable because the mosquito is not yet a vector of diseases. Ae. aegypti, however, developed resistance to common synthetic larvicides that furthermore have a negative impact on both the environment and human health. It is, therefore, necessary to search for new and eco-friendly replacements. Essential oils that are used as natural larvicides are often limited in their applicability as they are poorly water-soluble, degrade easily and volatilize when applied as free oil. These limitations can, however, be overcome using microencapsulation. Thyme essential oil (TEO)contains thymol, an active compound that has a range of biological activities (antioxidant, antimicrobial, insecticidal). The objective of this work was to develop a low-cost bio-degradable larvicide system for controlled release of TEO that is to be applied in small containers in and around households. The controlled release system was prepared by thermoplastic extrusion using corn starch as wall material. The TEO was chemically determined by gas-chromatography to contain 53% w/w of thymol, and its larvicidal activity was determined. The release of the thymol from the particles is demonstrated to correlate with the swelling of the matrix when immersed in water. The particles are effective against larvae from susceptible (Rockefeller)and resistant strains, reduce the required lethal concentration by a factor 3 as compared with the non-encapsulated TEO and significantly prolonged the residual effect, maintaining 100% of larvae mortality for 10 days more than non-encapsulated TEO.