Insecticidal activity of essential oils: octopaminergic sites of action

A study was conducted to determine the insecticidal activity and mechanism of action of three essential oils (eugenol, α-terpineol and cinnamic alcohol) and an equal part mixture (3-blend) against American cockroaches ( Periplaneta americana). To address species differences in response to treatment with the test oils, Carpenter ants ( Camponotus pennsylvanicus De Geer), and German cockroaches ( Blattella germanica) were included in this study. Exposed American cockroaches demonstrated hyperactivity followed by hyperextension of the legs and abdomen, then fast knockdown or quick immobilization followed by death. Ants and German cockroaches showed fast immobilization/knockdown followed by mortality. The 1:1:1 mixture (3-blend) was substantially effective against all test insects. One of the most remarkable observations was the increased frequency of heartbeats of American cockroaches in response to topical application of test oils. The changes in the pattern of cAMP level was biphasic. A significant increase in the cAMP level was found in response to 1 nmol/ml of eugenol, or 3-blend or 10 nmol/ml of α-terpineol. At higher concentrations a significant decrease in cAMP level was found. Blockage of octopamine receptors binding sites was also illustrated at lower concentrations of the test chemicals as judged by the decreased binding activity of [ 3H]octopamine to its receptors. In conclusion: (1) test oils are neuro-insecticides and their insecticidal activity is species-dependent; (2) a synergistic effect of the three oils was found when they were equally mixed (3-blend); and (3) the octpaminergic system mediates the insecticidal activity of eugenol, α-terpienol and the 3-blend.