Knockdown of dorsal/relish increases larvae susceptibility to microbes in Spodoptera litura

The tobacco cutworm, Spodoptera litura has developed high levels of resistance to frequently-used insecticides, and failures of chemical control has frequently been reported in China. Moreover, the control efficiency of biological control agents was not satisfactory by farmers, and thus the development of novel or combined techniques for such pest control becomes an imperative requirement. In this study the potential application of innate immune-related genes in insect control was investigated. Sldorsal and Slrelish were identified from S. litura. Sldorsals were mainly expressed in epidermal and fatbody of larvae, and Slrelish highly expressed in fatbody and haemolymph of larvae. Their expression characteristic suggests potential roles in defense against fungi or bacteria. The expression of Sldorsal was up-regulated by the challenge of entomogenous fungi Isaria javanica and Beauveria bassiana, however, the expression of Slrelish could only be induced by Escherichia coli challenge. Injection of Sldorsal or Slrelish dsRNA down-regulated the expression level of the corresponding gene and broke specific signaling transduction pathway of innate immunity in S. litura. Knockdown of Sldorsal by RNAi significantly increased the mortality of S. litura larvae caused by I. javanica and B. bassiana, and Slrelish RNAi highly increased the mortality caused by E. coli. Sldorsal and Slrelish are essential genes for innate immunity, their knockdown by RNAi increases S. litura susceptibility to insect pathogens, therefore enhances the mortalities caused by I. javanica, B. bassiana and E. coli respectively.