Abstract: Western monarch butterflies (Danaus plexippus) and their milkweed host plants (Asclepias spp.) may be uniquely prone to phenological mismatch because monarchs migrate through multiple different climate zones during their life history. Additionally, monarchs and milkweed may time their life history in response to different phenological cues. Although phenological drivers of monarch life history events have been investigated in the past, drivers of milkweed phenology are less well-studied and may be equally important to understanding mismatch. We investigated the contribution of moisture and temperature as phenological drivers predicting the emergence and reproductive phenology of five common California native milkweed species. We viewed photo observations from the citizen science platform iNaturalist and scored plants as newly emerged vs. established, and scored their reproductive phenology. We correlated these observations with local, year-specific climate data. We found that some milkweed species’ phenology correlated only with temperature, some only with moisture, and some with both temperature and moisture. In some cases, these effects were life stage specific. Climate variation in the last 8 year has correlated with variations of up to 14 days in the average date of observation of some milkweed life stages.
