LAUSR

Abstract California's sage scrub habitats support a diversity of nectar and host plants for migrating and resident populations of painted lady butterflies (Vanessa cardui) throughout all seasons. North America experiences spring V. cardui migrations involving butterflies totaling in the millions in some years. These irruptive years are thought to be driven by winter weather patterns at breeding grounds near the US–Mexico border and due to their irregularity, it is difficult to study floral resource use along the migration route. Here we used the community science platform iNaturalist to quantify patterns in V. cardui nectar resource use in sage scrub over time and space during irruptive and nonirruptive years. We identified over 329 different nectaring plant species of varying functional types (72% native to California) visited by adult V. cardui, 195 of which had not been previously identified as known nectar plants for V. cardui. Vanessa cardui butterflies were observed in similar locations regardless of whether an irruptive migration occurred, indicating the presence of either sparse migrants or resident populations across California. Moreover, irruptive years were positively correlated with warmer and wetter local conditions at observation locations. Our results provide new insights into patterns of floral resource use by North American V. cardui by harnessing the power of community science data and while highlighting the factors associated with its North American migration.