Evaluating the Response of Native and Introduced Nectar Plants to Warming and the Associated Impacts on the Body Composition of the Monarch Butterfly (Danaus plexippus)

Abstract

Climate change is altering plant-insect interactions through changes in plant growth, flowering phenology, and nectar resources. These effects may be especially important for migratory insects such as monarch butterflies (Danaus plexippus), which rely on late-season floral nectar to accumulate lipid reserves required for migration and overwintering. Introduced plants are frequently used by monarchs as nectar sources, but their quality and response to warming remain poorly understood. I used a field warming experiment in an old-field habitat near Ottawa, Ontario, to examine how passive warming affected the growth and floral production of two native nectar plants, Solidago canadensis and Symphyotrichum novae-angliae, and two introduced nectar plants, Lythrum salicaria and Trifolium pratense. I also conducted controlled feeding trails, in which adult monarchs forage on one of the four species in warmed and controlled chambers to determine whether warming has an impact on monarch body composition. The experiment occurred during an unusually dry season, with soil moisture declining to a predicted minimum of 6.74% in mid-August. Warming did not consistently affect plant growth, chlorophyll content, flower abundance, or flowering onset. Instead, warming altered some seasonal plant trajectories, including species-specific stem growth patterns and seasonal flowering distributions, while drought conditions likely constrained floral availability across species. Native species exhibited higher percent stem growth than introduced species overall, but introduced and native species did not differ consistently in their response to warming. Warming also did not significantly affect monarch wet mass, dry fat mass, dry lean mass, or water mass in S. canadensis feeding trials. In nectar-source comparisons, monarchs feeding on introduced L. salicaria had lower dry lean mass than those feeding on native S. canadensis, although wet mass, dry fat mass, and water mass did not differ between species. These findings suggest that L. salicaria did not reduce monarch lipid accumulation relative to S. canadensis, though differences in non-fat body components may reflect variation in nectar composition or feeding conditions. Overall, my results suggest that short-term warming alone had limited effects on plant condition and monarch body composition, while drought and seasonal dynamics played a larger role in shaping floral availability and monarch responses. Restoration efforts should therefore prioritize diverse, drought-tolerant, late-season floral communities capable of maintaining nectar availability under increasingly variable climatic conditions.