RNAi pest control and enhanced BT insecticidal efficiency achieved by dsRNA of chymotrypsin-like genes in Ostrinia furnacalis

Chymotrypsins (CTP) are a main type of proteolytic digestive enzymes in Lepidopteran insects. They can play a major role in degrading activated Bt toxin in some insect species, leading to decreased Bt toxicity and inducing insect resistance. Seven CTP-like sequences were obtained by screening transcriptome data of Asian corn borer, Ostrinia furnacalis. The expression analyses revealed that almost all of the CTP genes showed relatively higher transcript levels at the larval stage, and most showed relatively higher transcript levels in the saliva and midgut. We synthesized dsRNA from the seven CTP-like genes and fed it to larvae at the different instar stages. The dsRNAs from all seven CTPs significantly enhanced mortality of larvae from the first to the third instar larval stages, and only the dsRNA from CTP8 had higher lethality to fourth instar larvae. When larvae were fed on a diet containing 0.2 mg/ml dsRNA and Bt (5 μg/g diet), the 5-day mortalities were significantly enhanced, except for larvae treated with dsRNA from CTP5. The dsCTP16 plus Bt treatment resulted in a 5-day mortality of 100 %, and all of the dsCTPs accelerated insect death. These results indicated that members of the CTP-like gene family are effective RNAi targets for pest control. In particular, CTP8 represents an effective target for controlling the whole larval stage, and CTP16 is an effective target to enhance Bt insecticidal efficiency.