Phenology, population size, and factors influencing variation in density of an endangered butterfly, the mottled duskywing Erynnis martialis

Understanding the natural history and ecology of endangered species is critical for developing effective, evidence-based conservation and management plans. The mottled duskywing Erynnis martialis is a skipper butterfly inhabiting oak savanna, oak woodlands, alvars, and tallgrass prairie habitats containing their host plants, Ceanothus spp. Listed as endangered in Canada, few populations persist in Ontario and Manitoba, but there are no formal estimates of population size or what influences spatial variation in adult density, and we have limited knowledge of phenology. To address these knowledge gaps, we conducted spatially explicit mark-re-sighting over multiple years at 2 of the largest known populations in Ontario (specific locations withheld). Population sizes at Site A were estimated to be 1159 individuals (confidence limit [CL] = 845-1598) in 2021 and at Site B, 626 individuals (CL = 466-851) in 2020 and 2227 individuals (CL = 1110-4463) in 2021. Most (81%) re-sightings occurred within 5 d of marking, suggesting that adults likely live less than 1 wk. High densities of adults were generally associated with high Ceanothus spp. abundance and open canopy, but relationships varied by site and year. Peak flight across sites and years ranged from 5-13 June. We recommend that existing and prospective habitat managers prevent succession with brush-cutting and/or prescribed burns, specifically in areas of abundant Ceanothus spp. This work provides valuable information for future management and recovery efforts, including planned translocations to both historically occupied and newly restored sites.