A growth-based screening method for entomopathogenic bacteria against Spodoptera frugiperda

Abstract

Efficient screening methods are essential for rapidly identifying active microbial insecticidal resources, whereas current methods are mainly based on bioassay of alive pests, and it is time-consuming. For developing new screening methods, we first collected field fall armyworm, Spodoptera frugiperda, and then we isolated 34 pathogenic bacteria isolates from dead larvae. After detecting six isolates of Serratia and Pediococcus which were the abundant bacteria species, we tested their proteinase and chitinase activities, along with their colony diameter on milk and chitin medium, and their biological activity against S. frugiperda. Based on a correlation analysis, we found a significant relationship between virulence and colony diameter in the six isolates. And then we verified the relationship by testing the other 28 isolates from fall armyworm. Therefore, we established a method for preliminary screening of potential pathogenic bacteria based on colony diameter of the isolate on milk medium, only isolates with colony diameter higher than 2.15 cm (cultured at 28°C for 72 h) were selected for further exploitation. Using the method, three bacteria isolates including JSJN2102 (Serratia nematodiphila), JSJN2103 (Providencia rettgeri) and JSJN2209 (Bacillus thuringiensis) were successfully obtained for their potential against S. frugiperda. The method will enable researchers to screen entomopathogenic bacteria more efficiently.