Invasive grass negatively affects growth and survival of an imperiled butterfly

Abstract

With only ~1% of native prairie remaining in North America, populations of many prairie-obligate species, including the imperiled Dakota skipper butterfly, have drastically declined in recent decades. Unfortunately, population recovery is impeded by an insufficient understanding of Dakota skipper biology. Because larvae have never been naturally observed in the wild, even basic life history elements including preferred host plant(s) are not well understood, and potential hosts have been inferred from grasses inhabiting remnant sites rather than direct observations. To improve our understanding of Dakota skipper biology and habitat needs and inform recovery efforts, we conducted a no-choice performance experiment offering larvae 1 of 5 commonly occurring native grasses and 2 pervasive invasive grass species found across their historic range. We monitored larvae during key life history intervals and evaluated host plant quality by measuring larval and pupal mass, time to pupation, and survivorship. Larvae fed on all offered host grasses, but mass, phenology, and survivorship varied among treatments. Larvae reared on prairie dropseed and porcupine grass had the highest survival, the shortest time to adulthood, and the greatest mass, whereas larvae provided smooth brome and Kentucky bluegrass fared poorly for all observed metrics. All other grasses offered during the study were deemed ‘medium’ quality. Our results suggest that although larvae can feed on a variety of potential host plants, these hosts vary in quality. Invasive grasses across prairies in North America may pose an ecological trap to the conservation of Dakota skipper and other prairie-obligate Lepidoptera.

Endangered Oarisma poweshiek larvae vary their graminoid forage in Manitoba, Canada

“…Based on this, larval adaptations to navigate microhabitats to locate ephemerally suitable shoots and to digest multiple species would likely confer fitness advantages to O. poweshiek. This pattern may explain the polyphagy of graminoid-feeding skippers in natural habitats that have been observed globally (natural habitats : Wiklund 1984;Dana 1991;García-Barros and Fartmann 2009;lab choice: Molleman et al 2020;Nordmeyer et al 2021). The Poweshiek skipperling appears to be attracted to any species that meet its survival needs, including for ovipositing and larval feeding, possibly influenced by availability and seasonal variation in plant quality.…”

Widespread annual occurrence of pesticides within designated Critical Habitats for Endangered prairie butterflies

“…Larvae of both species likely feed on a wide range of prairie graminoids (e.g. Henault and Westwood 2022) including the little bluestem primarily sampled in this study, with Dakota skippers performing best on native prairie bunchgrasses like prairie dropseed (Sporobolus heterolepis), porcupine grass (Hesperostipa spartea), and side-oats grama (Bouteloua curtipendula) (Nordmeyer et al 2021). Unlike many other grass-feeding skippers like Dakota skipper, larvae of Poweshiek skipperling do not construct shelters and remain higher on their host grass, thus making them more exposed to a variety of environmental stressors, including possible pesticides exposure.…”

Using Plugs and Hydrogels to Establish Populations of Prairie Dropseed, a Critical Native Grass, in Restored Prairies in Minnesota

“…Prairie dropseed is a critical source of food and shelter for at least five species of native Lepidoptera including the federally endangered Poweshiek skipperling, Oarisma poweshiek, and the federally threatened Dakota skipper, Hesperia dacotae (Narem and Meyer, 2017). Prairie dropseed is not only a preferred host plant for Dakota skipper (Dana, 1991), but Nordmeyer et al (2021) found that Dakota skipper larvae on prairie dropseed plants had the highest survival rates, highest pupation weight, and shortest time to pupation compared with other native grasses. Prairie dropseed is thought to be an optimal forage for native Lepidoptera (Dana, 1991).…”