Inhibitory effect of imidacloprid pretreatment on rice resistance to gall midge and its association with total phenolic content dynamics

【Objective】To investigate the inhibitory effect of imidacloprid pretreatment on rice against the rice gall midge（Orseolia oryzae） and its impact on the total phenolic content of secondary metabolites in rice, which could provide new theoretical reference for elucidating the population decline mechanism of the rice gall midge in Guangxi ricegrowing regions. 【Method】The international standard seedling screening method for insect virulence, conventional pest biology observation techniques and spectrophotometry were employed. These methods systematically evaluated the inhibitory effects on the galled seedling rate, pest oviposition, hatching rate, and larval development of rice gall midge after its introduction to pretreated rice at various intervals. Additionally, temporal changes in the total phenolic content of rice secondary metabolites and its correlation with the galled seedling rate were analyzed. 【Result】Both imidacloprid seed dressing（exposed to rice gall midge 5-25 d after treatment） and foliar spray（exposed to rice gall midge 1-20 d after treatment） significantly inhibited rice gall midge infestation（P＜0.01, the same below）. The silver shoot rate of rice was reduced to 0-3.95% and 0-4.51% respectively. When insects were introduced 13 d after seed dressing, oviposition was significantly reduced（P＜0.05, the same below） to 42.33 egg/10 insects. However, the hatch rate reached 69.32%, significantly higher than the water control. When introduced 18-23 d after treatment, both oviposition and hatch rates of rice gall midge showed no significant difference from the control（P＞0.05, the same below）. For foliar spray, oviposition（21.00 egg/10 insects） was extremely significantly lower than the control when insects were introduced 5 d after treatment, but showed no significant difference compared to control when introduced 10-15 d after treatment. Larval development was markedly inhibited at 13 d after seed dressing and 5 d after foliar spray, the larval body length was 0.43-0.59 mm for seed dressing group, 0.43-0.48 mm for foliar spray group and 0.82-2.47 mm for water control group. By day 22 after insect introduction, larvae in treated groups remained at early instar stages, while the control group had completed pupation, indicating extremely significant differences in developmental progress. Further analysis revealed that both seed dressing and foliar spray application methods induced an increase in total phenol content in rice plants. However, TP levels gradually declined over time, and their dynamic changes showed a significant negative correlation with the incidence of rice silver shoot rate. 【Conclusion】Imidacloprid pretreatment establishes an early insect resistance defense mechanism in rice by inducing the accumulation of total phenols, thereby achieving indirect regulation of rice gall midge population dynamics.