Antibiotic Treatment Reduced the Gut Microbiota Diversity, Prolonged the Larval Development Period and Lessened Adult Fecundity of Grapholita molesta (Lepidoptera: Tortricidae)

SIMPLE SUMMARY: The gut microbiota, which has an important influence on insect physiology and ecology, can be affected by many factors, such as antibiotics, temperature, diet, and species. However, herein we demonstrate the effects of antibiotics on the gut microbiota and on Grapholita molesta development and fecundity. Five antibiotics, ciprofloxacin, streptomycin, chloramphenicol, ampicillin, and rifampicin, were screened by the drug-sensitive disc method. Then, the effects of antibiotics on oriental fruit moth growth, reproduction, and gut microbiota structure were investigated by comparison of biological parameters and 16S amplicon sequencing technology. The results illustrated ciprofloxacin, showing the largest inhibition zone diameter, to be the most suitable antibiotic to inhibit the gut microbiota of G. molesta a minimum inhibitory concentration of 1 μg/mL. After feeding ciprofloxacin, the relative abundance of Actinobacteria and Cyanobacteria significantly decreased, while that of Bacteroidetes and Firmicutes increased. Finally, ciprofloxacin feeding affected larval growth, development, and reproduction significantly, resulting in prolonged larval development duration, shortened adult longevity, and significantly decreased single female oviposition and egg hatchability. In addition, we isolated and purified some culturable bacteria belonging to Proteobacteria, Firmicutes, Actinobacteria, and cellulase-producing bacteria from the G. molesta midgut. ABSTRACT: Grapholita molesta, the oriental fruit moth, is a serious pest of fruit trees with host transfer characteristics worldwide. The gut microbiota, which plays a crucial part in insect physiology and ecology, can be influenced by many elements, such as antibiotics, temperature, diet, and species. However, the effects of antibiotics on G. molesta gut microbiota are still unclear. In this study, we selected five common antibiotic agents to test the inhibition of G. molesta gut microbiota, and found ciprofloxacin shown the best antibacterial activity. After feeding 1 μg/mL of ciprofloxacin, the relative abundance of Actinobacteria and Cyanobacteria decreased significantly, while that of Firmicutes and Bacteroidetes increased. PICRUSt2 analysis indicated that most functional prediction categories were enriched in the G. molesta gut, including amino acid transport and metabolism, translation, ribosomal structure and biogenesis, carbohydrate transport and metabolism, transcription, cell wall/membrane/envelope biogenesis, and energy production and conversion. Finally, ciprofloxacin feeding significantly affected larval growth, development, and reproduction, resulting in prolonged larval development duration, shortened adult longevity, and significantly decreased single female oviposition and egg hatchability. In addition, we isolated and purified some culturable bacteria belonging to Proteobacteria, Firmicutes, Actinobacteria, and cellulase-producing bacteria from the G. molesta midgut. In brief, our results demonstrate that antibiotics can have an impact on G. molesta gut bacterial communities, which is beneficial for host growth and development, as well as helping female adults produce more fertile eggs. These results will thus provide a theoretical reference for developing new green control technology for G. molesta.