Alfalfa weevil, Hypera postica, insecticide resistance on the Canadian prairies: revealing mechanisms through transcriptomics

The alfalfa weevil, Hypera postica (Coleoptera: Curculionidae), is one of the most important insect pests of alfalfa in North America and is especially destructive in alfalfa seed production regions in Canada. Currently, in western Canada insecticides are the most common control strategy used to combat alfalfa weevil in seed production fields and recent control failures have questioned whether insecticide resistance occurs in populations in the province of Alberta. Insecticide resistance vial tests on adult weevils with deltamethrin (synthetic pyrethroid) found populations of weevils with high resistance (survival at >100X commercial rates). Adults pre- and post-insecticide exposure were collected and RNA sequenced (RNA-Seq). A comprehensive adult transcriptome was created and differential expression analysis conducted. There was an upregulation in detoxification genes (e.g. cytochrome P450s, glutathione-S transferases, senecionine N-oxygenase) in resistant compared to naïve weevils. Surprisingly, the common knockdown resistance (kdr) allele often associated with pyrethroid resistance does not appear to be present. These results suggest that insecticide resistance is conferred through metabolic resistance rather than target site resistance. This study is the first evaluate insecticide resistance and its mechanisms in alfalfa weevils in Canada.