Study on the mosquitocidal mechanism of microencapsulated natural plant extracts based on gut microbiota regulation

This study developed three microcapsule emulsions containing thymol, D-limonene, and their 1:1 mixture as the main active ingredients, designated as THY, LIM, and T-L, respectively. Their physical stability, insecticidal activity against Aedes albopictus larvae, and sustained-release performance were evaluated. In addition, changes in the gut microbiota of mosquito larvae before and after exposure to the three formulations were analyzed to preliminarily investigate the potential mechanisms underlying their insecticidal effects. The results showed that all three microcapsule emulsions exhibited good physical stability, with T-L exhibiting the highest stability. The average particle size and PDI of T-L were 85.06 ± 3.65 nm and 0.148 ± 0.04, respectively, which were significantly lower than those of THY (109.53 ± 2.75 nm and 0.267 ± 0.03) and LIM (1192.20 ± 9.55 nm and 0.312 ± 0.06). Moreover, T-L exhibited a higher zeta potential (+ 15.30 mV) and showed no obvious phase separation after 48 h of storage. The larvicidal activity of THY, LIM, and T-L against third-instar Aedes albopictus larvae exhibited a dose-dependent manner, with LC₅₀ values of 54.22, 55.50, and 48.98 mg/L, respectively. In the sustained-release assay, T-L maintained a larval mortality rate above 50% after 120 h. Gut microbiota analysis revealed that all three treatments increased alpha diversity of the larval gut microbiota, and significant differences in operational taxonomic unit (OTU) compositions were observed among the four groups. The relative abundances of bacterial genera including Limnohabitans, Rahnella, and Serratia decreased, whereas those of potentially pathogenic genera, such as Acinetobacter, Aeromonas, and Clostridium, increased. These findings suggest that alterations in gut microbial homeostasis may be associated with the insecticidal effects of microcapsule emulsions. Overall, T-L exhibited superior stability, sustained-release performance, and insecticidal activity, providing support for the development of novel plant-derived mosquito control agents.