Botanical biocides. 4. Mosquitocidal activity of certain Thymus capitatus constituents.

Abstract

Successive extraction of Thyme plant, Thymus capitatus (L.) Hoffm. and Link (Lamiaceae), by different solvents of increasing polarity, showed that potency was highly attributed to the non-polar fraction (e.g., petroleum ether) when tests were carried out against the larvae and adults of Culex pipiens (L). Of special concern to the mosquitocidal activity, the following fractions and isolates were recognized: the volatile oil, unsaponifiable portion, and certain compounds isolated from the unsaponifiable portion (e.g., Thymol, alpha-Amyrin, Carvacrol + beta-Caryophyllene). The volatile oil, Thymol, and the unsaponifiable portion proved high larvicidal potency against the tested insect (LC50 = 49.0, 58.0, and 100.0 ppm, respectively). Non-lethal concentrations of these substances synergized the toxicity of Malathion, while induced additive or antagonistic effects when mixed with Permethrin or Pirimiphos-methyl. The unsaponifiable portion and volatile oil showed the highest adulticidal potency (LC50 = 0.0070 and 0.0076 mg/cm2, respectively). The efficacy of the tested candidates as repellent agents was found in the following order: unsaponifiable portion > alpha-Amyrin > Thymol > volatile oil > Carvacrol + beta-Caryophyllene. Thymol as well as volatile oil affected egg hatchability, causing Sterility Indices accounting for 0.70 and 0.74, respectively, while the unsaponifiable portion showed lower degree of sterility (0.81). The results obtained in this study may shed light on the importance of T. capitatus as a probable source of some biologically active agents for mosquito control in the future.