With the exception of some tropical genera, most butterflies rely on nectar as the primary adult resource and feed on non-floral resources, like tree sap, opportunistically. We found that an isolated wetland population of Lycaena xanthoides (Boisdonval) (Lycaenidae) in western Oregon, USA, frequently uses both flower nectar and extra-floral resin of Grindelia integrifolia DC. X G. nana Nutt, var nana (Asteraceae) as an adult food resource. There were sex biases in nectar-versus resin-feeding preferences, with males feeding on Grindelia flower nectar more, frequently than resin, and females feeding on resin more frequently than nectar. A combination of taste tests and sucrose estimates through a handled refractometer suggested that the Grindelia resin may be a source of sugars, while a Kjeldahl analysis detected organie nitrogen at 2.6 ppm in the resin. We propose that the wetland population of L. xanthoides has either evolved or is evolving to use Grindelia resin as an adult resource because it is predictable in abundance over the landscape, unlike alternate non-floral adult resources.