Impact of mid-season sugarcane aphid infestation from blooming to harvest on forage sorghum yield and silage nutritive value

Forage sorghum (Sorghum bicolor L.) grown for silage production is frequently infested by the sugarcane aphid (SCA; Melanaphis sacchari [Zehntner] [Hemiptera: Aphididae]). The objective of this study was to establish first-time action thresholds for SCA infestation, leaf feeding, honeydew, and/or dead leaves. In this 2-yr Texas High Plains study, we quantified the impact of varying densities of SCA and subsequent damage beginning at boot stage to yields and silage nutritive value of a susceptible forage sorghum hybrid (Pioneer 841F) by using untreated controls and different insecticide treatments (lambda cyhalothrin, chlorpyrifos, malathion, and flupyradifurone). Aphid damage occurred from boot to flowering growth stages. Damaging infestations continued until harvest in 2017, but SCA densities declined sharply after peaking at the beginning milk growth stage (BMS) in 2019. Damage levels were significantly correlated to increases in aphid densities at BMS (R-2 = .91) in 2017 and at 50% bloom (R-2 = .83) and BMS (R-2 = .86) in 2019. Yield losses increased with increasing damage at the BMS in 2017 (R-2 = .82) and 2019 (R-2 = .42). Reductions in nutritive value were correlated to yield reductions. Results indicate that SCA infestations should be controlled when >= 20% of the leaf area is infested and damaged at BMS to prevent yield and nutritive value losses in the fresh and ensiled forage. Based on this work, an action threshold for forage sorghums to minimize losses to yield, nutritive value, and economic returns is presented.