Chemical profiling of selected medicinal plants and their nanoemulsion formulation against papaya mealybug (Paracoccus marginatus Williams & Granara de Willink)

The papaya mealybug, Paracoccus marginatus (Hemiptera: Pseudococcidae), is considered a treacherous pest for many tropical and subtropical vegetables, fruits and ornamental plants. Synthetic insecticides are one of the methods available for controlling the papaya mealybugs. Nevertheless, mealybugs have developed a genetic resistance to synthetic insecticides which adversely affects the environment by polluting the air, soil, and water. The alternative approach to control the pest with minimum harmfulness is to use bioactive compounds present in plants. Many studies have revealed the possibility of utilizing essential plant oils to control insects as an eco-friendly pesticides. Therefore, the study endeavours to prepare and determine the effectiveness of nanoemulsion formulations of Artemisia herba-alba, Myrtus communis, Mentha longifolia and Salvia spinosa essential oils against papaya mealybugs, P. marginatus. The essential oils of selected medicinal plants were extracted by hydrodistillation and solvent extraction methods, and their chemical profiles were identified using GC-MS system. The insecticidal activities of the essential oils were firstly evaluated against female adults of the papaya mealybug. Then, the most effective essential oil of each plant was selected to prepare water in oil (W/O) nanoemulsion formulations. Three mineral oils as carriers and 18 types of surfactants were screened to prepare and characterize nanoemulsion formulations, and their toxicity was determined against the papaya mealybugs. The GC-MS analysis exhibited that the essential oils of the plants contain various active components such as; eucalyptol, piperitone, cineron, dehydroabietic acid, α.terpineol α-pinene, linderol and alcanfor. The quality and quantity of these components depend on the type of the plant and the method of extraction. The biological screening of the essential oil toxicity of A. herba-alba, M. longifolia, and M. communis extracted by hydrodistillation, and S. spinosa essential oil obtained by solvent extraction showed a high toxicity against papaya mealybug. After the miscibility screening, three surfactants (EW70, Termul 3540 and DB10) and two mineral oils (paraffin and methyl ester) were chosen to prepare nanoemulsion formulation of the selected plants essential oil. Three ternary phase diagrams were constructed from these materials, and three points (FN5, FN8, and FN11) were selected from each ternary phase diagrams. After miscibility and stability test, it was prepare nine formulations with 20% of A. herba-alba, M. longifolia. M. communes essential oil and three formulations with 15% of S. spinosa essential oil. The results of characterization tests confirmed that all formulations with essential oils were nanoemulsion formulations within nano-particle size at range 26.94 to 108.00mm. The formulations exhibited high stability under centrifugation and storage conditions with good physical characterizations. The toxicity of the essential oil nanoformulations against P. marginatus showed the nano formulation code FN5 (paraffin oil and DB10 surfactant) achieved the highest toxicity against papaya mealybugs and largest droplet spreading area on papaya leaves followed by nano formulation codes FN8 (methyl ester and Termul 3540 surfactant) and FN11 (methyl ester and EW70 surfactant) with all selected plants essential oils. The nanoemulsion formulation code FN5 with M. longifolia essential oil revealed the best results compared with other nanoemulsion formulations with selected plant essential oils. The present study found that nanoemulsion formulations of essential oils of A. herba-alba, M. communis, M. longifolia and S. spinosa are safe and ecofriendly insecticides. These botanical pesticides can be utilized as an alternative approach to control the papaya mealybug.

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