2020 Mission Blue Butterfly Survey: Milagra Ridge

Shaw, Grace; Crooker, Christina

Milagra Ridge (San Mateo County) supports one of the few remaining populations of the endangered Mission blue butterfly (Icaricia icarioides missionensis) in the Golden Gate National Recreation Area. In 1995, the Parks Conservancy established a long-term monitoring program to document the relative abundance of the butterfly through a system of linear transects that bisect suitable habitat. At Milagra Ridge, the Parks Conservancy’s Park Stewardship Program monitors 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 the findings of the 2020 monitoring season and examines patterns of Mission blue butterfly (Mission blue) abundance, distribution, and phenology over the last 25 years. It also briefly describes results from the lupine diversification project, lupine pathogen monitoring, and the Mission blue translocation effort occurring in 2020. SUMMARY Mission blue abundance We observed eight Mission blues on-transect and eight confirmed Mission blues off transect. A park visitor observed three potential Mission blues at Transect 10, and we later confirmed occupancy via egg observations. Overall, Mission blue relative abundance has increased since translocations, but has not recovered to pre-pathogen levels. Mission blue distribution We saw adult Mission blues at five of fifteen transects and eggs at 1 more transect. Since translocation began, distribution has steadily increased. Overall, the current distribution is on par with the occupancy seen in the 2000s but has not recovered to pre-pathogen levels. Mission blue phenology The 2020 Mission blue monitoring season started on April 11th, ended on June 2nd, and lasted seven or eight weeks, depending on whether observations at Transect 8 were of translocated or local Mission blues. Overall, flight season has increased to the lengths seen in the early 2000s but has not recovered to pre-pathogen levels. Fungal pathogen monitoring We monitored pathogen symptoms on silver leaf lupine at Transects 2, 6, 8, 9, 13, and C. This season, infection levels were lower than we have seen since 2015, with 43% of silver lupine impacted at the Quarry, and 43% impacted across all transects monitored. Of these, an average of only 5% of each plant was symptomatic. Nectar species monitoring We found a total of 27 known nectar species in flower during the flight season and peak nectar availability occurred within the Mission blue flight season. Eriogonum latifolium and Phacelia californica were the most abundant native nectar plants. We also saw abundant Lupinus species, Hypochaeris radicata, and Eschscholzia californica in flower during flight season, but we do not know how significant these species are as nectar sources. Mission blue translocation In this fourth year of the Mission blue translocation project, we translocated twenty female and ten male Mission blues from San Bruno Mountain over the course of four translocation events. We released the butterflies at Transects 2, 8, and 9. This was the first year that butterflies were translocated at Transect 8, a previously unoccupied habitat. Habitat management We treated Oxalis pes-caprae and resprouts and seedlings of native scrub across Transects 2, 4, 6, 9, 10, C, 11, and 13. We hand pulled Carduus pycnocephalus as encountered and treated a variety of encroaching invasives at Transect 13.