Late summer western monarch survival is affected by shade environment and milkweed species

Background Periods of extreme heat are increasing, impacting many plants and animals. In the western United States, temperatures often exceed growth and survival thresholds for monarch butterflies ( Danaus plexippus) , necessitating that conservation plans for this iconic species develop new alternatives to augment larval performance in the planning process. Microrefugia, small scale habitats that buffer organisms from extreme conditions, may be a useful strategy when restoring or conserving monarch habitat. Methods To understand the role microclimate as a feature of monarch habitat, we conducted an observational study across five western states to determine use of shaded habitat relative open sunny habitat when shade is available. We then conducted an experimental field study in which we placed second instar caterpillars in experimental enclosures in two microhabitats (sun or shade) with two hostplant species (showy or narrow-leaved milkweed) and quantified responses in the last generation before fall migration. Results In the observational study, monarchs used shaded habitat at a higher rate than full sun habitat when shade was available. The experimental study indicated that survival to adults on showy milkweed was higher in shade while survival on narrow-leaved milkweed was similar across environments. Similarly, adults which foraged on showy milkweed in sunny habitat were smaller than those in the shade, while those which foraged on narrowed-leaved milkweed were not impacted by the microhabitat. Implications for Insect Conservation Together the findings suggest that monarch conservation plans would benefit from including shade microrefugia as well as narrow-leaved milkweed, when appropriate, to support late-season monarch breeding.