To clarify community dynamics of associated microbiota in a grapevine pest Apolygus spinolae, we performed high-throughput sequencing of bacterial 16S rRNA gene. The microbiota of the plant bug was dynamically changed depending on geographic region, season, and insect developmental stage (Fig. 2, Fig. 3, and Fig. 4). Considering that A. spinolae lacks a gut symbiotic organ (Fig. 1D) and the remarkable diversity of their microbiota, it is plausible that the plant bug has no specific symbionts and acquires its microbiota from the environment every generation, although the insect looks likely to have an affinity for certain bacterial group such as Caballeronia and Serratia. Although the whole body of insects was homogenized and subjected to high-throughput sequencing in this study, most of the detected bacteria are thought to be gut bacteria, because almost no PCR amplification was detected in the insect tissues after removing the gut. However, some bacterial species such as Spiroplasma, detected only in Hakodate samples (Fig. 2 and Fig. S2), are probably associated with body tissues but not the luminal region of the gut, as reported previously in other insects [34]. There are few studies to comprehensively investigate the geography, season, and insect developmental stages that affect the gut bacterial composition of insect pests. Therefore, this report is a good example that integrated factors affect insect microbiota.
In recent years, the gut microbiota has been actively investigated in some members of the heteropteran family Miridae, including the sorghum plant bug Stenotus rubrovittatus, the small green mirid Nesidiocoris tenuis, the tropical plant bug Monalonion velezangeli, Adelphocoris suturatis, and Apolygus lucorum [14–17, 35, 36]. These studies have led to findings that the composition of the gut microbiota varies with climate, habitat, diet, and developmental stages, as well as results suggesting that some plant bugs may be vectors of phytopathogen and endophytes [15–17, 35, 37]. As shown here in A. spinolae, Proteobacteria consistently dominate the gut bacterial community in Miridae species, although the composition and dominant bacterial genus are remarkably diverse. While the feeding habits of A. spinolae are not thoroughly investigated, feeding habits of the Miridae are generally diverse from phytophagous to carnivorous through omnivorous, and change throughout their life cycle [12], which may be a factor causing their diverse gut community. Indeed, rearing experiments in Adelphocoris suturatis confirmed the diet-dependent effects on gut microbiota composition in this plant bug species [35].
In the rearing experiments on Adelphocoris suturatis, the proportion of Serratia increased when the insect was reared under carnivorous or omnivorous conditions [35], suggesting a metabolic function of this bacterial group when plant bugs feed on other insects. In addition, Serratia species are commonly detected in the gut of carnivorous and omnivorous insects, including mosquitoes (Aedes aegypti), Anopheles stephensi, fruit fly (Drosophila), and ladybugs (Harmonia axyridis) [38–41]. Indeed, Serratia species are known to have urea metabolism and secrete highly active proteases and chitinases in insect bodies, suggesting that Serratia support host digestion [42]. If so, it may be notable that Serratia shows seasonal changes in A. spinolae (Fig. 4, Table S4, and Table S5). Although speculative, the dynamics of Serratia suggest the seasonal changes in the feeding habits of A. spinolae in grapevine fields.
Alternatively, considering the Serratia OTUs detected in this study were closely related to S. marcescens, a well-known entomopathogenic bacterium [43, 44] (Fig. S12), this bacterial group is possibly a seasonal gut pathogen of A. spinolae. In A. suturatis, dietary change from herbivorous to omnivorous/carnivorous enhanced S. marcescens in the gut, which increased mortality [35]. The number of Serratia OTUs from A. spinolae appeared to decrease due to the increase of many other bacteria (Table S3). Therefore, it is hypothesized that dietary alterations in A. spinolae leads to an increase in the number of other bacteria and regulates the intestinal microflora to prevent proliferation of S. marcescens.
In addition to Serratia, members of Caballeronia were also consistently detected in A. spinolae. The genus Caballeronia is well known as the gut symbiont of several stinkbug species [10, 29, 45], wherein the symbiont plays important metabolic roles in recycling metabolic wastes and insecticide resistance [46–48]. Phylogenetic analysis showed that OTUs from Tsurunuma and Niseko were classified as SBE-β, while OTUs detected from Obihiro and Hakodate were classified as SBE-α (Fig. S11). Considering that not all but some Caballeronia strains can degrade insecticides [46], this region-dependent difference may be due not only to vegetation and diet but also to vineyard management. As discussed above, the variation may simply be related to changes in the bug’s feeding behavior. Alternatively, the bacterial groups probably inhibit each other inside the gut, and the power balance could be influenced by the insect’s habitat, environmental changes, and physiological changes. These hypotheses should be clarified in future studies.
A. spinolae collected in Hakodate had a high percentage of Spiroplasma (Fig. 2). S. citri, causing citrus stubborn disease (CSD), and S. kunkelii, causing corn stunt in maize, are known as typical phytopathogenic bacteria in the Spiroplasma family [49, 50]. Some of the Spiroplama infecting insects cause the well-known phenomenon of Drosophila male-killer [34]. Among the Spiroplasma species detected in this study, there were not only insect parasites but also OTUs classified in the same group as S. citri and S. kunkelii, which are known plant pathogens (Fig. S4). These results suggest that A. spinolae is a vector of the plant pathogen Spiroplasma. Although there are very few known cases of Spiroplasma transmission in the Miridae, the findings of this study may provide an opportunity to shed new light on how to manage phytopathogenic Spiroplasma.
In this study, we comprehensively analyzed the gut microbiota of vineyard populations of A. spinolae. To develop an integrated pest management (IPM) strategy for the pest plant bug, it is necessary to elucidate this pest's detailed developmental status and life cycle in vineyards. The clarification of the relationship and function between insects and gut symbiotic bacteria from this study not only leads to elucidating the ecological basis of the insects, but also has great potential to be developed into agricultural technology.