Increasing nitrogen fertilizer rates lead to greater <i>Diatraea saccharalis</i> (Lepidoptera: Crambidae) injury in select Louisiana sugarcane cultivars

Levels of plant nutrients, particularly nitrogen, can drastically alter insect herbivory. In agricultural systems, nutrients are intentionally altered via fertilization to increase plant productivity. Prior studies found that greater nitrogen fertilizer rates can increase crop injury by Lepidopteran stem borers. However, little has been done to evaluate how nitrogen rates impact herbivory in local sugarcane by the primary insect pest in Louisiana, the sugarcane borer, Diatraea saccharalis (F.) (Lepidoptera: Crambidae). To assess this, D. saccharalis injury and relative survival were evaluated at different nitrogen fertilization rates in 3 field trials conducted from 2017 to 2022, with 1 trial directly comparing yield metrics for 2 cultivars differing in D. saccharalis resistance. The influence of nitrogen rates on larval development was assessed in a single cultivar laboratory assay. Higher nitrogen rates resulted in greater D. saccharalis injury in all 3 cultivars tested and increased larval survival in 2 of the 3 cultivars. Greater nitrogen application corresponded with an increase in foliar nitrogen, phosphorous, and potassium, potentially increasing the nutritional benefit to D. saccharalis. The laboratory evaluation indicated larval weights were greatest and development time shortest in moderate nitrogen-addition treatments. Despite the increased injury and developmental success with greater nitrogen, stalk weights and yield increased, compensating for damage-related losses. The present study aligns with findings in other sugarcane growing regions that nitrogen increases stem borer injury but not necessarily yield loss. Therefore, the adverse effects of D. saccharalis associated with higher nitrogen were minimal on the overall productivity of the sugarcane cultivars evaluated.