The dun skipper is found throughout much of the United States; however, the Harbison’s
dun skipper subspecies is geographically isolated in southern California and northern Mexico. It
is only known to feed on the San Diego sedge as a larva and a previous listing as a Category 2
species by the United States Fish and Wildlife Service demonstrates concern for its conservation.
I conducted surveys in 2021 and 2022 to update the status of populations, and a mark-recapture
study in 2022 to estimate population sizes and compare to visual survey estimates. Habitat
preferences were explored by collecting habitat covariates in the field and analyzing GIS
available environmental data. Skippers were detected at seven of the 17 sites visited in 2021, but
due to the small population sizes and low recapture numbers in 2022, population estimates were
able to be calculated for only two locations. Habitat analysis found no significant difference
found between used and unused portions of woodlands. Woodlands with San Diego sedge were
more likely to be historically occupied at higher elevations with warmer summer temperatures.
My results show a more restricted distribution of the skipper, and declining populations at extant
sites. Small numbers of individuals and low accuracy population estimates indicate that
maximum daily counts are a more useful monitoring method for the Harbison’s dun skipper.
Habitat analysis determined that the entirety of the woodlands should be considered important
habitat, while a large-scale approach highlighted the environmental conditions for areas that may
be colonized by the skipper. These data bring into focus areas where conservation and restoration
can focus in order to help promote the longevity of the Harbison’s dun skipper.
Recording for the August 26, 2020 SDMMP Mgmt. and Mon. Coordination Meeting. Dan Marschalek (University of Central Missouri) - "Hermes copper & Harbison's dun skipper: Where we are and where we are going?" Spring Strahm (Conservation Biology Institute) – “Status update on Laguna Mountains Skipper reintroduction effort†and “Augmenting Quino checkerspot butterfly in the San Diego National Wildlife Refugeâ€
Recording from the July 26, 2023 SDMMP Management and Monitoring Coordination Meeting. Presentations: Brief Update on Fire Coordination and Resource Avoidance Area Mapping Database (Pete Scully) and San Diego Pollinators and a Monitoring Plan (Dan Marschalek, University of Central Missouri)
The Harbison's dun skipper (Euphyes vestris harbisoni) is a rare subspecies with a restricted distribution, known only from San Diego County and southern Orange County. Larvae are host-specialists, feeding only on the San Diego sedge (Carex spissa) that is often associated with riparian oak woodlands. Entomologists have expressed concern that the Harbison's dun skipper may be threatened due to habitat loss and degradation. The skipper was once considered a Category 2 species by the United States Fish and Wildlife Service and petitioned to be listed under the federal Endangered Species Act. Currently, it is a covered species in some HCP/NCCP plans in San Diego County. Little is known about this skipper, as the few papers written are limited to describing the general life history and providing a vague distribution. This project is the first phase of a planned comprehensive monitoring program for the Harbison's dun skipper, designed to assist in the development of effective management and conservation practices. We focused on describing the distribution of the skipper, estimating population status and trend, describing habitat requirements of the larvae and adults, and identifying potential threats to the long-term persistence of the species as well as recommendations for monitoring and management. Using herbarium records, information from biologists, and historical Harbison's dun skipper locations, we were able to find San Diego sedge at 38 general locations, primarily on conserved lands. All sedge plants were found in or immediately adjacent to a riparian oak woodland except two small patches. However, both of these locations also had oak woodlands with San Diego sedge present. In many cases, oak woodlands were patchily distributed along a creek and the sedge was only found in those woodlands. We did not find San Diego sedge in or along pools of still/standing water, only in areas with moving water or a dry ravine. We were able to identify 26 historic Harbison's dun skipper localities from museum specimens, peer-reviewed literature, technical reports, and notes from local biologists. Most often, these locality data provided a general description rather than a specific point. Therefore, it can be difficult to know if we were revisiting the same location. Of the 34 locations with San Diego sedge that we surveyed for the skipper, 18 (53%) were occupied which is well below the occupancy rate described by Brown (1982). [consult report for rest of Exec. Summary]
Surveys for Harbison’s dun skipper adults were conducted to assess year to year variation in
population size. Field visits were used to document use including plants used for nectar
sources, as well as obtain non-lethal genetic samples. A rapid habitat assessment was
conducted at each site which included general woodland tree species composition, condition of
San Diego sedge plants, and recording potential threats to the Harbison’s dun skipper.
We were not able to detect Harbison’s dun skipper adults at all sites, including the canyon
south of San Pasqual Academy where it appears the skipper has been extirpated due to wildlife
and drought. At four sites (Barrett Lake, Lake Hodges, Skye Valley Road, Sycuan Peak Ecological
Reserve), the peak daily number was near or exceeded 10 individuals. At the other sites, no
more than four individuals were seen on a single day. There was not a clear relationship
between the 2014 and 2016 counts as some were higher, some lower, and some quite similar.
In 2016, we observed adults nectaring on sacapellote (Acourtia microcephala) and California
rose (Rosa californica), two plant species not previously recorded. The current list of known
nectar sources includes 20 species, nearly all of which have white, purple, or pink flowers.
Habitat assessments occurred at 23 locations with recent Harbison’s dun skipper observations.
These surveys occurred just after the flight season (27 July – 9 August 2016). Oak species
dominated the woodlands, with some sycamore and willow trees. The condition of the San
Diego sedge plants ranged from nearly all very healthy (green) to all dead. Most of the plants
had green leaves with brown tips, suggesting some water stress although this may be typical
during the late summer and early fall. The most common threat to the Harbison’s dun skipper
was the presence of the goldspotted oak borer (Agrilus auroguttatus). In addition, more than
15 non-native plant species were detected in the riparian areas.
This plan provides a summary of what is known regarding the Harbison’s dun skipper, including life
history, historic and current distribution, movement patterns, suitable habitat, and threats. A thorough
understanding of the species is necessary to make appropriate adaptive management recommendations
in an attempt to alleviate the current threats to the species. To develop this plan, we:
1. Reviewed existing data, including historic Harbison’s dun skipper locations, recent (2013-2017)
survey data, property ownership to identify conserved lands for potential surveys, management,
and acquisitions, and
2. Consulted with the wildlife agencies and other stakeholders to ensure that the most current
information regarding Harbison’s dun skipper biology, management, regulations, conserved
lands, and potential acquisitions were included.
The Harbison’s dun skipper (Euphyes vestris harbisoni) 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.
In 2023, surveys for Harbison’s dun skipper adults were conducted to collect non-lethal genetic
samples for a landscape genetics project as well as further update the status of populations in
San Diego County. Surveys started in a relatively small geographic area where skippers were
observed in past years, but expanded to surrounding areas if skippers were not detected. A
total of 9 males and 3 females were counted, with adults detected at 5 of 16 sites. Non-lethal
genetic samples were collected from all individuals except one male.
This report summarizes our survey efforts of 2021. Additional efforts for this project/task are
planned for 2022 and will include a marking study to calibrate transect counts with population
size estimates, and quantify habitat preferences. In 2021, surveys for Harbison’s dun skipper adults were conducted to assess year to year
variation in population size and update the status of each local population/site. Surveys focused
on the relatively small geographic area where skippers were observed in past years. In 2013-
2017, 14 sites had confirmed observations of Harbison’s dun skipper adults. All but one of these
sites were surveyed in 2021, with Harbison’s dun skipper adults observed at only six sites.
Population sizes at those six sites were similar to the smallest population sizes recorded during
the 2013-2017 surveys.
Overall, transect counts (visual observations) continue to describe small populations when
skippers are present. However, the large size and uneven terrain of some riparian oak
woodlands, patchy distribution of adult skippers, and shifting locations of San Diego sedge
present challenges to accurately categorizing presence/absence and relative population sizes.
(SANDAG TransNet EMP Contract #5005783)