Are monoterpenoids and phenylpropanoids efficient inhibitors of acetylcholinesterase from stored product insect strains?

In previous papers the toxicity of the monoterpenoids linalool and carvone and the phenylpropanoids trans ‐anethole and estragole, as well as the capacity to induce resistance to Sitophilus oryzae and Cryptolestes pusillus trough acetylcholinesterase (AChE) inhibition from either susceptible or tolerant (up to the tenth generation) insect populations, was tested at different concentrations of substrate. In this study, the activity of AChE inhibited by monoterpenoids and phenylpropanoids at lower concentrations (from 0.02 to 10 mM) was measured by Ellman's method at the same concentration of substrate (1 mM). Mainly, all of the monoterpenoids and phenylpropanoids applied turned out to be weak inhibitors at low concentrations, except for linalool and estragole, which were relatively the most powerful inhibitors for S. oryzae susceptible and tolerant strains, and for carvone, but just for the tolerant strain. However, in all these cases, high concentrations of inhibitors were necessary. The compounds studied on C. pusillus turned out to be extremely weak inhibitors, with inhibition not exceeding 40%. Higher values at half‐maximal inhibitory concentration (IC 50 ) were found for tolerant than susceptible strains in S. oryzae and C. pusillus by using linalool , indicating that esterases, or specifically AChE, could be involved in the mode of action. In addition, in relation to the types of inhibition, linalool and estragole were competitive inhibitors, whereas carvone presented a non‐competitive inhibition. This combined information should be taken into account when essential oils, monoterpenoids or phenylpropanoids are used in organic farming, because these compounds can be used as effective insecticides, although they were deficient AChE inhibitors, should this situation be acceptable for ecological agriculture. Copyright © 2014 John Wiley & Sons, Ltd.