Abstract
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 white LED streetlights consistently increased caterpillar growth rates by nearly 16% and shortened larval development time, resulting in an 8% increase in adult fresh mass across both plant species. Nonetheless, ALAN had little effect on wing loading (fresh mass to wing surface area) or adult dry mass. Host plant interacted with ALAN to impact wing morphology: butterflies reared on A. syriaca had 7% larger wings under ALAN, while those on A. incarnata were not affected. Seasonality profoundly shaped monarch life history traits, with the migratory generation developing in late summer (August–September) exhibiting slower growth, extended developmental periods, and emerging with 44% less body mass and 30% reduced wing loading capacity compared to early summer (June–July) breeding generations. Finally, a path analysis revealed that ALAN enhanced larval growth, on par with the effects of feeding on A. syriaca compared to A. incarnata , increasing fresh mass and wing size by accelerating investment in early development. Our findings underscore that light pollution at night alters the entire developmental trajectory of these holometabolous insects, highlighting its strong capacity to reshape insect life histories in the Anthropocene.