Impacts of Common Milkweed (Asclepias syriaca) Leaf Age on Larval Monarch (Danaus plexippus) Survival, Growth, Development, and Feeding Behavior

Milkweed (Asclepias spp.) is the sole host plant for monarch butterfly (Danaus plexippus) larvae, yet its quality varies within and among individual plants. This study examined how leaf position, used as a proxy for leaf age, on Asclepias syriaca influences monarch larval performance and feeding behavior. We found that younger leaves from the tops of milkweed stems had higher nitrogen levels and were easier to consume than older, lower leaves. Larvae that fed on top leaves grew larger, developed faster, and exhibited consistently higher survival rates, although the magnitude of these effects was modest. In choice bioassays, both neonates and third instars showed preference for younger leaf tissue, suggesting larvae can differentiate leaf types. These findings indicate that within-plant variation in milkweed quality can affect monarch fitness. Because milkweed quality declines as plants mature, periodic disturbance (e.g., mowing or burning) that promotes regrowth may improve leaf quality and larval performance. However, such practices involve trade-offs, including potential impacts on other species and potential disruption or diapause cues and migration timing. Selective or rotational mowing may offer a more sustainable approach by balancing regrowth benefits with preservation of natural phenological cues.