Solar radiation determines host choice, larval feeding and survival throughout the life cycle of an endangered open forest butterfly

Open forest butterfly species have declined rapidly across Central Europe in recent decades, probably associated with changes in forest management that have reduced the amount of sunlight reaching the forest floor. However, little is known about the actual effects of solar radiation on butterfly demography, that is, reproduction, development, and survival. Using the Southern White Admiral (Limenitis reducta) as a model species, our study is one of the first to examine the demographic rates of a butterfly under field conditions and the role of solar radiation and other habitat characteristics on survival throughout the life cycle. We studied female host selection by comparing host plants with and without oviposition, collected life table data from egg to imago, and studied the feeding activity of post-diapause larvae in response to temperature and solar radiation with time-lapse cameras. Solar radiation played a key role at almost every life stage of the species. Females preferentially selected the most sunlit microhabitats for oviposition, and overall larval survival from egg to imago was positively associated with solar radiation, thus confirming the preference-performance hypothesis. Higher feeding activity of post-diapause larvae and accelerated development at warm temperatures or high solar radiation intensities were likely explanations for the positive relationship between solar radiation and survival, which was strongest for post-diapause larvae and pupae. Survival of post-diapause larvae decreased with conspecific density, possibly related to increased parasitism and predation. Although habitat patch size was relatively weakly correlated with survival of individual life stages, we consistently observed a negative relationship, suggesting lower overall survival in larger habitat patches. The results underpin the use of demographic field studies and clearly demonstrate the key role of solar radiation in the larval habitats of forest butterflies. We derive recommendations for establishing and maintaining a dense network of sunlit habitat patches that will support the endangered L. reducta and likely many other open forest invertebrates.