The effect of temperature and tropical milkweed on monarch butterfly migration disruption

Abstract The monarch butterfly (Danaus plexippus) has become a well-established model for studying migration disruption. In recent years, the number of individuals in reproductive diapause forming roosts along the migration route has declined, coinciding with reduced overwintering populations and rising winter-breeding populations throughout the United States, suggesting some migrants break diapause to drop out and breed. Elevated temperatures and tropical milkweed may each trigger reproductive development during migration, potentially driving this dropout. Understanding which factor is of greater influence is critical for conservation, as the corresponding management strategies may be at odds. To assess their relative influence, we experimentally exposed wild-caught migrants to warm or cool simulated migration temperatures and either tropical milkweed or a non-milkweed nectar plant, followed by warm or cool simulated overwintering temperatures without plants. Warm temperatures initiated mature oocyte (MO) development and increased MO production, mating frequency and mortality risk in the absence of milkweed during both the migration and overwintering phases. Milkweed exposure during migration, however, only initiated MO development during overwintering. While both factors influenced reproductive development, temperature appeared to be a more robust reproductive cue. This raises important conservation questions and suggests current strategies should be approached with caution until further data are available.