Evaluation of biological effects of essential oils of 14 plants species against cotton mealybug, Phenacoccus solenopsis (Hem.: Pseudococcidae) under laboratory conditions

Background and Objectives Phenacoccus solenopsis (Tinsley) (Hemiptera: Pseudococcidae) is an invasive polyphagous and rapid spread pest attacking cotton, vegetables, ornamentals and other plants. Plant essential oils may be a secure alternative approach than synthetic pesticides for P. solenopsis control because of their effectiveness and versatility. This study was directed to assess the biological effects of essential oils of 14 plants species including peppermint, poleigamander, rosemary, catmint, wild mint, sweet basil, spearmint, moldavian balm, marzeh khuzestani, oregano, myrtle, river redgum, weeping bottle-brush and ribbed cachrys against P. solenopsis under laboratory conditions. Materials and Methods Plants were collected from different areas. Their essential oils were extracted via steam distillation using a Clevenger apparatus. P. solenopsis adult females were fed on potato Solanum tuberosum (L.) buds. The toxicity of essential oils for first instar nymphs and adults of P. solenopsis were assayed by the leaf-dip method. Bioassay tests were conducted under laboratory conditions in petri dishes. Leaves of Chinese hibiscus of approximately the same size were dipped in desired concentrations for 15s and air-dried for 30 min. Control leaves were dipped only in Methanol (70%). Control and treated leaves were placed on a layer of agar in the petri dishes, then ten P. solenopsis first instar nymphs and ten adults were released at the center of leaf discs in the petri dishes. For each treatment, we applied four different concentrations in three replicates. After 48 h, mortality percentages of nymphs and adults were recorded. Repellency experiments were carried out with two treatments; the leaf discs were cut to two equal parts. Half of the leaf was dipped in desired concentrations and the other half was dipped only in Methanol. The dried treated leaf discs were laid on a layer of agar in the center of petri dishes and twenty adults were released in the petri dishes. Each treatment was repeated three times. Mortality was evaluated after 24 h. For persistency experiments, leaf discs of Chinese hibiscus of approximately the same size were dipped in desired concentrations and after drying the treated leaf discs were placed on a layer of agar in the petri dishes. Three days later, ten P. solenopsis first instar nymphs and ten adults were released at the center of leaf discs in the petri dishes. Mortality was evaluated after 24 h. The same experiment continued for 5, 7 and 9 days later. Results The most efficacious essential oils against first instar nymphs came from Mentha longifolia, Mentha piperita, Mentha spicata, and Nepeta cataria, with LC50s of 113.49, 129.74, 163.04 and 193.05 ppm, respectively.  Essential oils from Mentha longifolia, Mentha piperita, Prangos ferulacea, and Satureja khuzistanica, with LC50s of 370.12, 400.58, 413.87 and 430.81 ppm, respectively, were the most efficacious against adult mealybugs. The essential oil of M. longifolia showed the highest persistence and repellency on P. solenopsis. Discussion The results clearly indicated that essential oils possessed many useful properties to control insect pests. Also, the essential oil of Mentha longifolia might be used as either a safe compound or a model in construction of new synthetic compounds against P. solenopsis.