Evaluation of gypsum as a dispersing agent for Mentha piperita L. essential oil against Sitophilus zeamais Motschulsky, 1855 (Coleoptera: Curculionidae) for stored grain protection

The control of insect pests in stored grains carried out through insecticides that may select resistant populations and generate environmental risks. Secondary plant compounds are alternatives to control insect pests. However, the high volatility has been an obstacle in their practical use. This study develops natural dispersants for the application of Mentha piperit a essential oil for the control of Sitophilus zeamais (Motschulsky) (Coleoptera: Curculionidae). Two types of dispersants were tested: a gypsum-based dispersant prepared with a mixture of gypsum and water molded in aluminum, and a sachet dispersant composed of polyester sachets. The efficiency of the dispersants was assessed by contact, fumigation and repellency tests. For that, unsexed S. zeamais up to 15 days old were used and evaluated after 48 h of experiment assembly. The persistence effects of the essential oil in dispersants were also evaluated. The fumigation test with essential oil in the sachet and gypsum dispersants generated, respectively, LC 50 values of 23.78 and 34.32 and LC 99 values of 54.27 and 79.28 μL. For the contact test, the sachet dispersant presented sublethal and lethal concentrations of LC 50 25.84 and LC 99 48.40 μL, respectively. However, only the 26 μL concentration was repellent for both dispersants. The repellent and fumigant effects of the sachet lasted for 15 and 24 days, while These effects observed with gypsum-based dispersants lasted 21 and 30 days, respectively. Both dispersants were considered promising due to their ability to retain and gradually release the bioactive compounds present in Mentha piperita L. essential oil, thereby enhancing fumigant and repellent effects against Sitophilus zeamais Motschulsky, 1855 (Coleoptera: Curculionidae). These results indicate that gypsum is a potential carrier material for improving the residual activity and efficacy of botanical insecticides in stored grain protection systems.