The microbiomes associated with the gut of S. frugiperda play important and diverse roles in the growth and development of many insect species. The symbiotic associations between the insects and their gut bacteria have been studied. Firmicutes and proteobacteria were the most dominant groups in the larval gut of S. frugiperda, according to phyla-level bacterial community analysis. The present finding is similar to the proportion reported in reports of phytophagous lepidopteran insects (Xia et al., 2013, 2017; Landry et al., 2015; Ramya et al., 2016; Snyman et al., 2016; Strano et al., 2018; Chen et al., 2018). Similar observations were also reported by Priya et al. (2012), who reported that proteobacteria and firmicutes were the most dominant phyla in the gut of H. armigera larvae. On the other side, only five samples belong to the phylum firmicutes, the genus Enterococcus was mostly present in places such as Begusarai, Purnia, Katihar, Birbhum and Kaimur and showed dominance over all samples. Out of 12 samples, 7 bacterial genera, namely, Enterococcus, Klebsiella, Raoultella, Enterobacter, Citrobacter, Pantoea, and Leclercia, were recorded in this study and have previously been isolated from S. frugiperda (Almeida et al., 2017; Acevedo et al., 2017; Jones et al., 2019; Gichuhi et al., 2020; Rozadilla et al., 2020; Ugwu et al., 2020). In the same way, seven of the isolated bacterial genera namely, Enterococcus, Pseudomonas, Comamonas, Stenotrophomonas, Eshcerichia-Shigella, Acinetobacter andCarnobacterium, have been recorded using a similar methodology in the beet armyworm, S. Exigua (Gao et al., 2019). These findings revealed that some bacterial genera are often associated with lepidopteran insects, although it is difficult to define a core microbiota for such a diverse insect order.
The result was obtained despite the significant bacterial diversities in OTU composition between larvae from different sites. This was most likely caused by complex biological and environmental factors in the diverse agro-ecological zones that were sampled. Polyphagous feeding habits also play a vital role in influencing the microbiome of lepidopterans (Strano et al., 2018; Sittenfeld et al., 2002; Priya et al., 2012; Montagna et al., 2016), though in this study all samples were collected from maize plants. Therefore, the observed compositional differences are not likely to be caused solely by diet. It is interesting that many of the detected bacterial genera, such as Enterococcus, Klebsiella, Raoultella, Enterobacter, Citrobacter, Pantoea, Leclercia, Kluyvera and Yokenella were found in larval life stages, which suggests that gut bacterial community members are transmitted across developmental stages. The bacterial communities that continually pass on across developmental stages (early instar to late larval instar) may develop a mutualistic relationship with their hosts (Moran, 2006). Hence, studies should examine the effects of these microbes on host fitness and investigate the extent to which they are vertically transmitted from parents to offspring. In contrast, Citrobacter and Sphingobacterium were observed to be differentially abundant in larvae than in adults, an additional indicator that these two genera may be part of the fraction of bacterial communities that are lost during the transition of S. frugiperda into the adult stage.
On the other hand, the described taxonomic and functional profile of bacteria associated with the gut of S. frugiperda larvae plays an important role in the insect's fitness and is an invaluable tool for identifying new pest control strategies. Nevertheless, there are extremely few reports on the functionally active profile of the gut bacteria that influence different physiological aspects, especially in lepidopteran insects (Rozadilla et al., 2020; Xia et al., 2017; Kyritsis 2019; Meng et al., 2019). The phylum Firmicutes and Proteobacteria mostly play a defensive role in the lepidopteran insect gut, such as carbohydrate metabolism, nitrogen metabolism, genetic information processing, detoxification, and the development of resistance, except for the genus Pantoe, which is mostly toxic increase susceptibility to toxins by affecting midgut epithelial permeability (Broderick et al., 2009; Mason et al., 2011).