Monarch butterflies are well known for their migratory behavior. In the West, this annual migration takes them between overwintering sites along the California coast inland as far as the Rocky Mountains. In recent years, however, a resident population of monarchs has established in urban gardens in Northern California, likely as a response to increased host plant availability in these landscapes. It is unclear when and how this resident population interacts with the migratory population that overwinters along the California coast. Emily Erickson, a researcher at UC Davis, is conducting monthly surveys of monarchs and milkweeds throughout the urban East Bay area to test for connectivity between the migratory and resident monarch butterflies. If overwintering monarch butterflies stop in urban gardens when leaving sites in spring, she expects a pulse of older adults in February and March. If migratory monarchs are attracted to urban gardens instead of overwintering groves, she would see more adults in October and November, as they migrate to the coast from inland breeding sites. Results to date indicate that there is a slight pulse of older monarch butterflies in early spring. Further, the number and age structure of the populations fluctuate through time, possibly in tandem with non-native milkweed (Ophryocystis elektroschirra) prevalence. This project is ongoing, and she will continue to collect data throughout the course of the year to explore the dynamics between the migratory and resident monarch populations in the West.
