Identification of chemical compounds of methanolic extract of (Mentha longifolia L.) and (Thymus pubescens Boiss.) plants and their insecticidal and repellent effects on Trialeurodes vaporarium

Trialeurodes vaporarium (Westwood) is a polyphagous pest that causes economic damage to the quantity and quality of many crops around the world. In recent years, using plant compounds as low-risk insecticides to replace chemical pesticides has increased significantly. This research aimed to investigate the chemical composition of methanolic extracts of Mentha longifolia L. and Thymus pubescens Boiss. and their insecticidal and repellent effects on greenhouse whiteflies in a completely randomized design in laboratory conditions.

A contact toxicity test was performed using the two methanolic extracts, each in three concentrations (1000, 1500, and 2000 ppm) and after three durations (24, 48, and 72 hours). Filter papers impregnated with different concentrations of plant extracts were used to determine the percentage of repellency. Each filter paper was cut into equal halves, and one half was impregnated separately with concentrations of 1000, 1500, and 2000 ppm. The other half was impregnated with only methanol solvent as a control. The two paper halves were then glued together and placed in Petri dishes (with a diameter of 8 cm). In each petri dish, 20 adult whitefly (T. vaporarium) insects were placed in the middle of the filter paper, and the total number of insects on each half of the filter paper was checked after 24, 48, and 72 hours.

 The results showed a significant difference in the percentage of toxicity at different concentrations after each studied duration. Based on the contact toxicity tests, the mortality rate increased with increasing the concentration and duration of exposure to the extracts. The LC50 levels of oregano and thyme extracts were 11148 and 13980 mg/L, respectively. According to the results, the mortality rate increased with increasing concentration and time. The highest mortality rate was observed in M. longiflora extract with a concentration of 2000 ppm after 72 hours (96.25 %), and the lowest mortality in T. pubescens extract with a concentration of 1000 ppm after 24 hours (17.5 %). The repellent effect increased with the higher concentration of extracts. The repellency test showed that M. longiflora extract had a significantly high repellent power. The highest and lowest repellency effects were observed in M. longiflora extract (90 %) and T. pubescens extract (23.12 %), respectively. The chemical compounds of the methanolic extracts were identified using a GC-MS, and their diagrams were drawn. The analysis of chemical compounds found that most of the identified compounds of the two plant extracts studied were terpenoid compounds, monoterpenes, and Cesciue terpenes, and thymol (38-42 %) was the highest composition of the extracts.

The results of the present study indicated that these extracts have favorable insecticidal and repellent properties against T. vaporarium, and their use in controlling this pest is recommended.