Chitin synthase (CHS) is an important enzymatic component, which is required for chitin formation in the cuticles and cuticular linings of other tissues in insects. CHSs have been divided into two classes, class A and B, based on their amino acid sequence similarities and functions. Class A CHS (CHS-A) is specifically expressed in the epidermis and related ectodermal cells such as tracheal cells, while expression of class B CHS (CHS-B) is expressed in gut epithelial cells that produce peritrophic matrices. In this study, we cloned the CHS-A gene from the beet armyworm, Spodoptera exigua. The SeCHS-A mRNA was expressed in all developmental stages and specifically in the epidermis and tracheae tissue by RT-PCR analysis. Expression of SeCHS-A gene was suppressed by feeding double-stranded RNA (dsRNASeCHS-A, 150 ng/larva) in the fifth instar of S. exigua. The suppression of SeCHS-A gene expression significantly induced mortality on pupal stage. Also, larvae fed with dsRNASeCHS-A significantly enhanced pathogenicity of an entomopathogenic fungus, Beauveria bassiana ANU1. These results suggest that the SeCHS-A gene plays an important role in development of S. exigua and dsRNA, which is a specific to SeCHS-A gene, may be applied to effective pest control with B. bassiana.

Entomopathogenic fungi have been studied to develop for biological control agents as an alternative to chemical control agents in insect pest management. Two Lepidopteran insects, Spodoptera exigua and Plutella xylostella, are serious insect pests infest various crops, but not effectively controlled by commercial chemical pesticides due to its high insecticide resistance. A fungal isolate was isolated from S. exigua larvae collected from green onion field in Andong, Korea. To identify the fungal isolate, 18srRNA sequence for internal transcribed spacer (ITS) and β-tubulin regions were sequenced. The ITS and β-tubulin sequence were highly matched to Beauveria bassiana and morphological characteristics also was fit to known B. bassiana. Finally, isolated fungus has identified as B. bassiana and named B. bassiana ANU1. The result of bioassay, median lethal concentrations were 2.7×103 and 0.9×103 conidia/ml and medial lethal times were 65.6 and 60.8 h to S. exigua and P. xylostella, respectively. B. bassiana ANU1 showed high pathogenicity to two insect pests from 20℃ to 30℃ at 50% relative humidity (RH) and more than 40% RH at 25℃ with 107 conidia/ml of concentration.