Entomopathogenic Potential of a Species of <i>Serratia marcescens</i> Originating from the Gut of Melon Fly: Isolation, Characterization and Effect of Oral Administration

The melon fly, Bactrocera cucurbitae (Coquillett), represents a devastating agricultural pest species. Conventional chemical control strategies are increasingly compromised by the development of insecticide resistance. Given the pivotal role of the gut microbiota in insect physiology and host fitness, it is necessary to isolate gut-derived bacterial strains from adult melon flies and assess their insecticidal efficacy against adult flies. Isolation of culturable gut microbiota from adult melon flies that died of natural causes yielded 122 bacterial strains. Among the 10 virulent bacterial isolates, Serratia marcescens strain SMLY-1 was identified as a highly pathogenic strain against adult melon flies. In addition, oral administration of SMLY-1 significantly reduced survival rates of adult melon flies in a dose-dependent manner. Histopathological analysis suggested that SMLY-1 administration resulted in progressive midgut damage, which was manifested by epithelial cell desquamation and vacuolization. Moreover, the gut microbiota homeostasis was disrupted, and microbial richness was reduced. Notably, the genus-level Wilcoxon signed-rank test showed a significant increase in Serratia spp. abundance in the SMLY-1 treatment group compared to the control group, indicating abnormal proliferation of Serratia spp. in intestines. Enzymatic assays showed that key digestive enzymes (AMS, LPS, trypsin) and detoxification enzymes (CarE, GST) activities were significantly reduced as well as antioxidant enzymes exhibited significant alterations. qRT-PCR results suggested that immune-related genes in adult melon flies including Attacin, JNK, and PPO were sustained upregulated whereas Cecropin, PGRP, IMD and GST were significantly downregulated. These findings establish SMLY-1 as a promising candidate for the biocontrol of melon flies in the future.