Artificial light at night (ALAN) can be an anthropogenic stressor, yet its effects on wildlife, especially diurnal insects, remain poorly understood. We test how ALAN influences larval growth, development, and performance of monarch butterflies ( Danaus plexippus ) reared on two host milkweed species ( Asclepias syriaca and A. incarnata ). Field experiments with four cohorts over 2 years revealed that exposure to ALAN from whi...