Efficacy Evaluation of Oregano Essential Oil Mixed with Bacillus thuringiensis israelensis and Diflubenzuron against Culex pipiens and Aedes albopictus in Road Drains of Italy

SIMPLE SUMMARY: The continuous use of synthetic insecticides against mosquitoes has led to resistance problems, while the use of bio-insecticides may face limitations in their actual use in the field. Alternatively, binary mixtures of botanicals with conventional synthetic or bio-insecticides have been proven, primarily on the laboratory scale, to be potential eco-friendly mosquito larvicides. In the current study, we investigated in the field the joint action of a carvacrol rich oregano Essential Oil (EO) with two conventional insecticides, namely, the insect growth regulator diflubenzuron and the bio-insecticide Bacillus thuringiensis israelensis (B.t.i.), in road drains of Crevalcore city, Italy, against Culex pipiens and Aedes albopictus. The results showed that the application of mixtures of EO with diflubenzuron or B.t.i. provided sufficient control of immature mosquito population for a 2–3 week period. These findings suggest the potential of mixing carvacrol-rich EO with diflubenzuron and B.t.i. as an efficient eco-friendly alternative to single insecticide applications in road drains against Cx. pipiens and Ae. albopictus larvae. ABSTRACT: Mosquito management programs in the urban environment of Italian cities mainly rely on larval control with conventional insecticides, primarily targeting the road drains that constitute the principal mosquito breeding sites encountered in public. The repeated utilization of synthetic insecticides may have adverse effects on non-targets and lead to resistance development issues, while the performance of biopesticides encounters limitations in field use. Botanical insecticides as single larval control agents or in binary mixtures with conventional insecticides have been extensively studied in the laboratory as an effective and eco-friendly alternative mosquito control method with promising results. The study herein concerns the investigation, for the first time under realistic conditions in the field, of the joint action of a carvacrol-rich oregano Essential Oil (EO) with two conventional insecticides, namely, the insect growth regulator diflubenzuron and the bio-insecticide Bacillus thuringiensis israelensis (B.t.i.), in road drains in Crevalcore city, Italy, against Culex pipiens and Aedes albopictus. According to the obtained results, the application of both plain EO and its mixtures with diflubenzuron and B.t.i. exerted very high efficacy in terms of immature mosquito population reduction over a two-week period. Three weeks after treatment, the performance of the oil and its mixtures diminished but remained high, while the addition of diflubenzuron potentiated the persistent action of the oil against Cx. pipiens. These findings are indicative of the potential of mixing carvacrol-rich EO with diflubenzuron and B.t.i. as an efficient eco-friendly alternative to mono-insecticide applications in road drains against Cx. pipiens and Ae. albopictus larvae.