2020 Mission Blue Butterfly Survey: Oakwood Valley

Shaw, Grace; Crooker, Christina

Oakwood Valley (Marin County) supports the northernmost large populations of the endangered Mission blue butterfly (Icaricia icarioides missionensis) in the Golden Gate National Recreation Area. At Oakwood Valley, the Parks Conservancy’s Park Stewardship Program monitors butterflies on fixed transects. The surveys provide an opportunity to track butterfly population fluctuations and help inform management decisions to support species protections and potential habitat expansion. This report summarizes data collected during the 2020 monitoring season and examines Mission blue abundance, distribution, phenology, environmental conditions, and habitat management conducted to support their continued survival at Oakwood Valley in FY 2020. SUMMARY Abundance In 2020, we recorded 156 Mission blues on transect, with a peak of 44 individuals on May 7th. Despite survey limitations during the Covid19 pandemic, this is an increase from the 68 Mission blues seen on transect in 2019. Overall, mission blue butterfly populations have declined at Oakwood Valley. Distribution We recorded on- transect observations of adult Mission blues at 11 out of the 15 transects and feeding damage or eggs were observed at the remaining 4 transects (Transects 1, 4, 4a, and 13), resulting in 100% occupancy on monitored transects. Transect 14 had the most (34) on-transect observations. Ninety-five percent of Mission blues were found in the southern transects (Transects 6-14), which is consistent with historical trends. Overall, occupancy has been stable. Phenology We recorded a 43-day flight season over seven weeks from April 30th through June 10th. However, we were not able to survey prior to April 30th because of field work constraints during the Covid19 pandemic. Because we saw Mission blues on our first survey date of the year, the flight season likely started earlier than we were able to record. The peak on May 7th is within the normal range of peak flight season. While declining in recent years, overall, the length of flight season has been stable. Nectar plants Twenty-three known nectar sources were available to the Mission blue during the flight season. Common native nectar sources flowering during the flight season included Eschscholzia californica, Heterotheca sessiliflora ssp. bolanderi, and Wyethia angustifolia. Common non-native nectar sources included Hypochaeris radicata, and Linum bienne. Habitat management Over the past several years, we have removed invasive brooms and cotoneaster, invasive perennial grasses, and native scrub from Mission blue habitat and surrounding grasslands to maintain grasslands and re-create corridors between Mission blue habitat patches. We were able to follow up on all this work despite staffing restrictions due to the Covid19 pandemic. We also continued removing incipient species such as Vinca major, Oxalis pes-caprae, Bellardia trixago, Sparaxis tricolor, Trifolium angustifolium, Leucanthemum vulgare, and Scabiosa atropurpurea from within and near Mission blue habitats.