Type: report

Article abstract: This report summarizes Harbison’s dun skipper (Euphyes vestris harbisoni) larval surveys during the winter of 2024/2025 and adult surveys during the summer of 2025. The Harbison’s dun skipper has a very restricted distribution in southern California and northern Mexico and entomologists have expressed concern that threats will lead to the extirpation of populations. The larvae of this skipper feed only on San Diego sedge (Carex spissa) and are generally associated with riparian oak woodlands. During the winter of 2024 – 2025, surveys for Harbison’s dun skipper larvae were conducted to assess the annual larval population size. Transects were established in the drainages/canyons as a first attempt to standardize these surveys, with one larval survey conducted per site. Adult surveys occurred during the summer of 2025, starting in a relatively small geographic area where skippers were observed in past years, but expanded to surrounding areas if skippers were not detected. Due to the relatively consistent numbers of adults through the month of June, each site was surveys two times. A total of 91 larvae and 37 adults (21 males, 16 females) were observed. The number of adults in 2025 was similar to 2024 and higher than 2023. In addition to these efforts, there were Harbison’s dun skipper observations at two locations near Potrero, an area that had not been surveyed for the skipper in 44 years. Adult local populations remain relatively small, but larger than in recent years with extreme drought. The known distribution has remained relatively unchanged, but occupancy was confirmed in the Tecate Peak/Potrero area. One possible change in occupancy is on the western side of Otay Mountain due to the Border 2 Fire, which likely impacted habitat with recent observations. Since we have only one year of larval data, comparisons across time are not possible yet. To facilitate a shift to using larval surveys to monitor annual population sizes, larval surveys will be conducted during the winter of 2025 – 2026 and adult surveys will be conducted during the summer of 2026. This will provide two years of data to compare adult and larval counts.