Spatial and temporal effects of Eurosta solidaginis-induced stem galls on leaf physiology in Solidago canadensis

Insect-induced galls act as resource sinks, but their effects on ungalled plant tissues are poorly understood. We examined how stem galls induced by Eurosta solidaginis affected Solidago canadensis leaf physiology, measuring multiple parameters in leaves close to and far from galls (and similar locations in ungalled plants) throughout late summer and autumn. While chlorophyll concentration was unaffected, stomatal density was higher in leaves close to galls, suggesting localized effects on stomatal development. Starch levels were lower in close and far leaves from galled plants, consistent with galls acting as powerful carbon sinks. Catalase activity was elevated only in leaves close to galls, yet these leaves showed no increased resistance to oxidative damage, suggesting catalase upregulation may instead protect larvae from photosynthetic reactive oxygen species (ROS). Salicylic acid, a defense hormone, was higher in leaves close to galls but only in some years, indicating spatially localized, environmentally-dependent defense responses. Early-season increases in leaf moisture content in leaves from galled plants may minimize larval freezing risk, while lack of increased transpiration rate over the season in galled plants may represent improved leaf cuticle maintenance to prevent desiccation. These spatially and temporally dynamic responses reveal that Eurosta manipulates host physiology in ways that could maximize its energy acquisition while minimizing freezing risk and oxidative stress. Our findings may inform biological control strategies for aggressive and invasive Solidago populations.