Silicon fertilization suppresses black cutworm performance and enhances nematode-induced cutworm mortality

The black cutworm ( Agrotis ipsilon ) is a destructive global pest of many plants, including turfgrasses. Silicon (Si) accumulation confers physical resistance to plants by making tissues tougher and more abrasive, reducing their digestibility and palatability for insect herbivores. However, whether Si interacts with biological control agents, such as entomopathogenic nematodes (EPNs), remains unknown. We examined the impacts of Si fertilization of creeping bentgrass ( Agrostis stolonifera ), a widely cultivated turfgrass, on black cutworm performance, larval susceptibility to the EPN Steinernema carpocapsae , and EPN reproduction. In greenhouse experiments, grasses were treated with wollastonite, a calcium silicate mineral, at rates of 1221 kg/ha, 2442 kg/ha, and 4884 kg/ha. Si fertilization increased leaf Si concentrations and decreased cutworm larval performance, with the highest application rate reducing larval weight by up to 45 % and head capsule width by 20 %. Si also enhanced turf cover and quality under herbivory. All Si rates increased EPN-induced larval mortality, and larvae fed high-Si grass clippings died faster than those fed untreated clippings. However, fewer infective juveniles (IJs) emerged from cadavers of larvae fed Si-treated grass clippings, correlating with decreased cadaver length and weight. Si treatment negatively affected IJ emergence even after controlling for cadaver length and weight, suggesting an additional effect beyond host size reduction. We provide novel evidence that Si fertilization synergistically enhances EPN-based biological control of cutworms. Although decreased IJ emergence may inhibit long-term EPN persistence, this trade-off is unlikely to limit initial biological control success, as EPNs are commonly applied via inundative release. • Silicon fertilization via wollastonite suppresses black cutworm larval performance. • Silicon enhances creeping bentgrass turf cover and quality under cutworm herbivory. • Silicon increases cutworm larval susceptibility to entomopathogenic nematodes. • Nematode reproduction declines in cadavers from larvae fed high-silicon grasses.