Ortho hydroxy-substituted molecules might be of importance for the prevention of bloodsucking by mosquitoes

Achillea millefolium extract contains several compounds, which prevent Aedes aegypti from feeding blood. Some of these compounds might interfere with each other. Caffeic acid, mandelonitrile glucoside, pyrocatechol, salicylic acid, and stachydrine were chosen for tests in paired mixtures. The effects were mainly antagonistic or additive. Exceptions were the mixtures of mandelonitrile glucoside and pyrocatechol, and possibly stachydrine and pyrocatechol where slight synergistic effects could be demonstrated. In connection with these studies the question arose if ortho substituents, especially free hydroxy groups, might be of importance for the effect on mosquitoes. It was shown that in the test series pyrocatechol (ortho-substituted), resorcinol (meta-substituted) and hydroquinone (para-substituted), pyrocatechol showed the highest activity. Furthermore, the importance of the free ortho hydroxy groups in the molecule was indicated by a decreased activity by the mono- and di-methylation of the hydroxy groups in pyrocatechol, i.e. the compounds guaiacol and veratrol.