Efficacy of different formulations of essential oils against the cotton mealybug, Phenacoccus solenopsis (Hemiptera: Pseudococcidae) under laboratory conditions

In recent decades, one of the most important methods of replacing synthetic pesticides is the use of new formulations based on plant essential oils (EOs) that can increase the quality and effectiveness of them. Due to application restrictions of EOs in their pure form, preparation of commercial formulations of these compounds is essential. In this research, At first, the contact toxicity of fifteen different plant EOs was evaluated on reared Phenacoccus solenopsis 1st instar nymphs at 25±1°C and 65±5% RH, with a 16:8-h L:D photoperiod. Three EOs of Mentha longifolia (L.), Mentha piperita (L.), and Oliveria decumbens (Vent.), had the highest contact toxicity and were selected for the rest of the experiments. According to GC and GC/MS analysis, pulegone (51.49%), menthone (22.75%), and 1,8-cineole (11.69%) were the principal components of M. longifolia, menthone (36.51%), menthene (28.51%), menthol (8.12%), and 1, 8-cineole (7.66%) were the principal components of M. piperita and the main components of O. decumbens EO were thymol (43.99%), γ-terpinene (13.96%), and p-cymene (12.62%). After making formulation, contact toxicity of the EOs before and after formulation, was evaluated on P. solenopsis 1st instar nymphs under laboratory conditions. Based on lethal concentration trials, LC50 values of pure and formulated EOs of M. longifolia, M. piperita, and O. decumbens on 1st instar nymphs after 48 hours were 113.49, 129.74, 149.93, and 48.22, 55.55, 61.68 ppm, respectively. Therefore, the contact toxicity of formulated EOs of M. longifolia, M. piperita, and O. decumbens were 2.34, 2.36, and 2.43-fold higher than the pure EOs.