Warming‐Mediated Decreases in Nectar Quality Translate Into Lower Energy Reserves of the Monarch Butterfly ( <i>Danaus plexippus</i> )

ABSTRACT Warming can alter plant–pollinator interactions through indirect, plant‐mediated pathways, yet the consequence for pollinators remains poorly quantified. We tested whether modest warming (0.57°C) negatively impacts nectar plants and whether these changes carry over to affect the body condition of migratory monarch butterflies ( Danaus plexippus ). We conducted a field warming experiment at the northern edge of the eastern monarch breeding range (Ottawa, Canada) using open‐top chambers deployed on three frequently used late‐season nectar species: Monarda fistulosa , Solidago canadensis , and Symphyotrichum novae‐angliae . Our design allowed adults to feed on warmed plants while spending nonfeeding time outside chambers, isolating plant‐mediated effects from direct warming effects. Across most species, warming reduced floral abundance (12.9% fewer open flowers per stem) and decreased nectar sucrose concentration by ~24%, while nectar volume in M. fistulosa did not differ between treatments. We then conducted 5‐day feeding trials in which adult monarchs foraged on warmed versus control plants and quantified body composition (fat, lean, and water mass). Monarchs feeding on warmed plants had ~26% lower fat mass than controls. Our findings provide experimental evidence that even modest warming can depress nectar quality and floral display, with measurable carry‐over effects on pollinator energy stores. Because fat reserves fuel migration and overwinter survival, warming‐induced declines in nectar quality may impair monarch performance during the critical late‐season fueling period. These results highlight the need to prioritize late‐season floral resources that maintain nectar quality under warming and to integrate plant thermal responses into habitat restoration and climate‐adaptation strategies for migratory pollinators.