Resistance to pyrethroids in the maize weevil, Sitophilus zeamais

The maize weevil is a pest of stored crop products, predominantly maize across humid areas of the world. The Sitophilus zeamais were controlled with pyrethroids until resistance emerged in areas across Brazil. This resistance was found to be conveyed by a super-kdr mutation in the voltage gated sodium channel (VGSC). A change from threonine to isoleucine at the 929 position in domain II, segment 5 of the VGSC was found to be the cause of resistance. This mutation also conveyed cross resistance to DDT. The project aimed to assess the level of resistance conveyed by the T929I mutation in lab populations of the maize weevil, to test for the presence of this mutation in the field, and finally to assess mutations at the T929 site using electrophysiology. Bioassays showed resistant lab populations of the maize weevil to be 800-1500-fold less sensitive to deltamethrin and 80-105-fold less sensitive to DDT. The bioassays also allowed the calculation of a concentration of insecticide that could be used to select for resistance in populations of unknown resistance. 12 field populations of the maize weevil were selected for resistance using this discriminating dose before being tested for the T929I allele using a taqman diagnostic assay. No individuals were found to carry the T929I allele, therefore suggesting it is no longer a problem in the field and this assay would not be a useful field tool. Two electrode voltage clamping (TEVC) on Xenopus oocytes expressing the T929V para VGSC mutation, found in the cat flea, showed that 1µM permethrin was unable to illicit a tail current. Therefore this mutation conveys very high levels of resistance to pyrethroids, similar to that of the T929I mutation