Silicon Enhanced Italian Ryegrass (Lolium multiflorum) Production and Induced Defense Responses Against Fall Armyworm (Spodoptera frugiperda)

The spread of invasive pests exacerbates the direct damage to host plants and the potential threat to the environment. Silicon has the potential to enhance host plant resistance to insects while also increasing plant yield. This study evaluated changes in Italian ryegrass biological yield and resistance to fall armyworm (Spodoptera frugiperda) larvae after silicon supplementation (sodium silicate and potassium silicate at 6 mmol·L−1 were denoted as groups T1 and T2, respectively). Silicon supplementation significantly increased the shoot biological yield (T1 by 30.26%, T2 by 23.05%) and silicon content (T1 by 22.61% and T2 by 12.43%) of Italian ryegrass. At the same time, silicon supplementation increased the protein, soluble sugar, and vitamin contents of Italian ryegrass, while also stimulating the improvement of its physical and chemical defenses. Therefore, even though the nutrient intake of fall armyworm increased, the synergistic physical-chemical defense formed by silica deposition, flavonoid content, and increased protease inhibitor activity in the Italian ryegrass still weakened the antioxidant capacity of the larvae and inhibited larval feeding and protein accumulation. The larval body weight of the T1 and T2 groups decreased by 20.32% and 15.16%, respectively. The comprehensive scores showed that sodium silicate and potassium silicate of the same concentration had similar effects on the growth and insect resistance of Italian ryegrass. These findings suggest that both sodium and potassium silicate are effective silicon supplements for host plants. Therefore, reasonable supplementation of silicon fertilizer may become an important alternative plan for optimizing the comprehensive pest control strategy in agricultural production areas in the future, but this still needs further field research verification.