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<p>Periods of hot weather currently impact many plants and animals. In the western United States, temperatures often exceed growth and survival thresholds for monarch butterflies (<i>Danaus plexippus</i>), necessitating that conservation plans for this iconic species develop alternatives to augment larval performance in the planning process. Microrefugia, small scale habitats that buffer organisms from hot weather, may be a useful strategy when restoring or conserving monarch habitat. To understand the role of microhabitat as a feature of monarch habitat, we conducted an observational study across five western states to determine use of shaded habitat relative to 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 host plant species (showy or narrow-leaved milkweed) and quantified survival, development time, and adult size in the last generation before fall migration. 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 and higher than on showy milkweed. Similarly, adults which foraged on showy milkweed as larvae in sunny habitat were smaller than those in the shade, while adult size of those which foraged on narrowed-leaved milkweed were not impacted by the microhabitat and were larger than caterpillars foraging on showy milkweed. Together the findings suggest that, to support late-season breeding, monarch conservation plans would benefit from providing shaded microrefugia and planting or promoting narrow-leaved milkweed where both narrow-leaved and showy milkweed species co-occur. More broadly, these findings suggest the importance of including a diversity of milkweed species, as appropriate to the region, across a range of microhabitats when planning monarch conservation efforts.</p>

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

  • 期刊论文

  • 发表日期：

    2025年9月13日

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  • Michelle L. Boone,
  • Stephanie McKnight,
  • Kelsey C. King,
  • Erica H. Henry,
  • Cheryl B. Schultz

摘要

Periods of hot weather currently impact 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 alternatives to augment larval performance in the planning process. Microrefugia, small scale habitats that buffer organisms from hot weather, may be a useful strategy when restoring or conserving monarch habitat. To understand the role of microhabitat as a feature of monarch habitat, we conducted an observational study across five western states to determine use of shaded habitat relative to 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 host plant species (showy or narrow-leaved milkweed) and quantified survival, development time, and adult size in the last generation before fall migration. 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 and higher than on showy milkweed. Similarly, adults which foraged on showy milkweed as larvae in sunny habitat were smaller than those in the shade, while adult size of those which foraged on narrowed-leaved milkweed were not impacted by the microhabitat and were larger than caterpillars foraging on showy milkweed. Together the findings suggest that, to support late-season breeding, monarch conservation plans would benefit from providing shaded microrefugia and planting or promoting narrow-leaved milkweed where both narrow-leaved and showy milkweed species co-occur. More broadly, these findings suggest the importance of including a diversity of milkweed species, as appropriate to the region, across a range of microhabitats when planning monarch conservation efforts.