Cuticular protein genes are involved with insecticide resistance mechanism in red flour beetles, Tribolium castaneum (Coleoptera: Tenebrionidae)

The insect integument, which consists mainly of chitin microfibrils and cuticular proteins, is an extracellular matrix that covers the entire external and some internal surfaces of the insect body and serves as a primary barrier against several environmental stresses. Using RNAi-mediated gene knockdown procedure, we performed functional analyses of three adult-specific cuticular protein genes, CPR4, CPR18, and CPR27, regarding their involvement in repelling chemical insecticides in the red flour beetles, Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae). The depletion of these CPRs produced adult phenotypes with malformed (wrinkled/dimpled) elytra as well as thinner and disorganized cuticle of abdominal sternite. After knocking down the CPR genes, the beetles were exposed to six types of neonicotinoid-class insecticides, and the short-term neurotoxic effects were evaluated. The knockdown of any of CPR genes, either singly or in combination, increased the beetles' susceptibility to neonicotinoids compared to negative control beetles. The results suggested that CPR4, CPR18, and CPR27 have an indispensable role in organizing the insect integument structure that can regulate the penetration rate of xenobiotics such as neonicotinoid insecticides, presumably by helping built proper structural features of cuticular layer.