Mining novel Bacillus thuringiensis toxins and secondary metabolites for invasive fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae) control

Abstract

Bacillus thuringiensis (Bt) endotoxins are often considered environmentally friendly pest control tools. However, the development of resistance to Bt toxins and emergence of exotic pests necessitate the characterisation of new Bt isolates. This study aims to identify and characterise novel Bt toxins and bioactive compounds that may be utilised to mitigate the impact of Spodoptera frugiperda (fall armyworm, FAW), a polyphagous agricultural pest species that has recently established populations in over 80 countries including Australia. Eight Bt isolates were used in bioassays to ascertain toxicity to FAW neonates. Six Bt isolates (Bt_01-02 and Bt_05-08) exhibited potential insecticidal activities. Three isolates including Bt_01 and Bt_07-08 caused 100% mortality, while Bt_02, Bt_05, and Bt_06 induced 71.27 ± 21.17, 98.44 ± 2.21 and 92.19 ± 11.05% mortality, respectively. Genome analysis was conducted to characterise the toxins and secondary metabolite gene clusters of each isolate. Four isolates (Bt_01, Bt_06, Bt_07, Bt_08) contained the Cry1Na-partial and Cry1I toxins, while Bt_05 contained Cry2A, Cry1H and Cry1-like amino acid sequences. In addition, the gene cluster for zwittermicin A, a crystal toxin enhancer, was present in all isolates. However, no known toxins or insecticidal compounds were identified in Bt_02 despite inducing high mortality. The pathogenicity of Bt_02 was also tested against two Australian native pest species: Helicoverpa armigera conferta and H. punctigera. This includes both the susceptible and Cry1Ac-resistant (Hp9-3784) lines of H. punctigera. Bt_02 caused 74.88 ± 19.82% mortality in H. armigera, 95.65 ± 6.15% mortality in H. punctigera and 90.91 ± 12.86% mortality in Hp9-3784. This suggests that Bt_02 may possess unknown toxins or bioactive compounds responsible for its effectiveness against three species of lepidopteran pests including those that exhibited Cry1Ac resistance.