2020 San Bruno Elfin Butterfly Survey: Milagra Ridge

Shaw, Grace; Crooker, Christina

The San Bruno elfin butterfly (Callophyrys mossii bayensis), in the family Lycaenidae, was listed as a federally endangered species in 1976. Along the San Francisco Peninsula, habitat destruction and fragmentation due to urban development has forced butterfly populations into isolated metapopulations. In the mid-1980s, Robert Langston discovered a population of the butterfly at Milagra Ridge, part of the Golden Gate National Recreation Area. Following this discovery, observations and surveys of the butterfly at Milagra Ridge continued through 1996 when we did not detect any adults or larvae (Newby 1999). In 1999, monitoring resumed under the direction of the Golden Gate National Parks Conservancy’s Park Stewardship Program. The goal was to establish a long-term monitoring protocol to determine the presence and abundance of the San Bruno elfin at Milagra Ridge. This report summarizes the findings of the 2020 monitoring season and examines patterns in San Bruno elfin butterfly larval abundance, distribution, and phenology. It also analyzes the relationship between abiotic and biotic factors and butterfly abundance. SUMMARY PHENOLOGY The 2020 San Bruno elfin larva monitoring season at Milagra Ridge began on May 7th and ended on June 10th. We observed larvae during each survey between May 7th and May 27th. On-transect observations peaked on May 14th and 21st, while off-transect observations (larvae seen anywhere other than on an open Sedum inflorescence) peaked on May 14th. Over the last six years, we have seen San Bruno elfin larvae earlier in the monitoring season, which is true this year as well. Each year, we initiate San Bruno elfin larval monitoring when Sedum spathulifolium (Sedum) inflorescences open. This timing focusses on 3rd instar larvae which feed on Sedum flowers. Observations from Milagra Ridge over the past three years suggest that larvae do not need to feed on open Sedum inflorescences. In fact, in most recent years, we have seen the peak number of larvae (including off-transect) within the first two weeks of monitoring, before open Sedum inflorescences reach their peak. However, in 2020 open Sedum numbers remained consistent (545 to 630) from May 14th through the end of the monitoring season. Depending on the instar of these larvae, the protocol should be revised so that monitoring begins earlier in the year to capture the peak in 3rd instar larvae. Another option is to expand the protocol to include all larvae regardless of substrate. ABUNDANCE This year we observed 13 larvae on open Sedum flowers (on-transect) and 70 larvae on closed Sedum flowers, Sedum rosettes, or the ground within the four 25-foot radius monitoring areas (off-transect). 2020 has shown an increase in on-transect observations from 2019 (5 observations), but fewer off-transect observations (117 observations in 2019). Neither 2019 nor 2020 reached the high number of on-transect observations we saw in 2018 (53). Since 2017, we have actively searched for off-transect larvae. Off-transect observations remained higher than on-transect observations this year, reinforcing our belief that it is important to continue this method. DISTRIBUTION We observed San Bruno elfin larvae at all four monitoring points both on- and off-transect in 2020. Point 2 had the highest combined on- and off-transect observations. Based on the high proportion of off-transect observations, it may be worth monitoring at historic Points 5 and 6 in 2021 to see if populations exist there that were not detected when searching only on-transect. SEDUM Sedum flowers are used by 3rd instar San Bruno elfin larvae. In 2020, we observed the highest number of open Sedum inflorescences ever recorded across the current four monitoring point with 3219 open inflorescences. The number of Sedum with open inflorescences per survey was also the highest it has ever been (537 open Sedum/survey day. Over the course of monitoring since 2009, open Sedum inflorescence have been as low as 41 (2016).