Toxicity and repellency of essential oils of Lippia alba chemotypes and their major monoterpenes against stored grain insects

The insects Sitophilus zeamais and Tribolium castaneum cause losses in stored grains and are considered pests of wide distribution and global significance. In the present study, we evaluated the toxicity and repellency of essential oils of different Lippia alba genotypes (carvone chemotypes LA-13 and LA-57 and citral chemotypes LA-10 and LA-44) and their major monoterpenes, carvone and citral, on S. zeamais and T. castaneum. Toxicity bioassays by exposure of the insects on treated filter paper were performed to determine the concentration and lethal time. Repellency tests were performed using the most toxic compounds according to the toxicity bioassays. The carvone chemotypes were more toxic than the citral chemotypes for both species: for S. zeamais, the LC50 values were 15.2 mu L/mL, (LA-13) and 16.7 mu L/mL (LA-57) and for T. castaneum, the LC50 values were 28.7 mu L/mL (LA-13) and 19.7 mu L/mL (LA-57). Isolated carvone (LC50 = 8.8 mu L/mL) was more toxic than citral. For S. zeamais, the monoterpene citral had the lowest lethal time (LT50 = 6 h), whereas for T. castaneum, the monoterpenes carvone and citral showed a more rapid toxicity (LT50 = 7.3 h). The compounds tested were highly repellent to T. castaneum; however, no repellency's effect was observed against S. zeamais, except for LA-13 chemotype. The essential oils from the carvone chemotype and the monoterpene carvone have potential for the development of natural insecticides against stored grain insects S. zeamais and T. castaneum. (C) 2015 Elsevier B.V. All rights reserved.