Under Concurrent Drought and Herbivory, Drought Dominates Herbivory on Morpho-Physiological Responses in Soybean

Drought and insect herbivory constantly threaten yield and productivity of crops like soybean (Glycine max). Recent advances in crop science have examined these stressors either individually or sequentially, but concurrent interactions have not been well understood. Therefore, using two soybean cultivars (Blackhawk- drought susceptible and Magellan- drought tolerant), we investigated how concurrent drought and herbivory by the fall armyworm (Spodoptera frugiperda, FAW) affect soybean and FAW traits. Four treatments drought-D, herbivory-H, drought × herbivory-DH, and well-watered-WW were imposed at the third-trifoliate stage (V3) for a week. During the treatment period, daily measurements of net photosynthesis rate, stomatal conductance, transpiration rate, and soil moisture were taken, whereas plant height and chlorophyll content were recorded during alternate days throughout the treatment period. In addition, FAW mass gain was also measured daily. Leaf trichomes, a major physical defense system in plants, were estimated immediately post treatment and six days after. Our results showed that concurrent DH significantly impaired physiological traits, reduced the soil water content, and affected plant growth. Trichomes were significantly higher under DH compared to WW and the effect persisted after treatment. Although FAW performed similarly in both drought-stressed and well-watered plants, strong cultivar effects were observed for larval mass gain and FAW seemed to perform better on the drought susceptible cultivar. This study established a clear trend showing drought is the dominant stressor on soybeans, compared to herbivory alone and hence informs the growers for prioritization of stress management. Overall, this study presents novel insights into the effects of concurrent drought and herbivory, with implications for resistance breeding using tolerant cultivars against stressors like herbivory and drought.