Translocation Project Brings Mission Blue Butterflies Back to Sweeney Ridge

National Park Service staff capturing a butterfly on San Bruno Mountain. This year, staff  translocated 57 Mission blue butterflies from San Bruno Mountain to Sweeney Ridge. On the first day of translocations, they captured six butterflies. NPS / Eric Wrubel

May 2022 – Sweeney Ridge was once one of the few remaining locations supporting federally endangered Mission blue butterflies on the San Francisco Peninsula. But the quarter-sized, iridescent insects disappeared from the site after a fungal outbreak decimated their silver lupine host plants in the late 1980’s. Recently, a multi-agency collaborative formed to bring mission blues back to Sweeney Ridge. Thanks to its efforts, Mission blue butterflies are now flying there again for the first time in 35 years!

The collaborative's work at Sweeney Ridge builds on the success of recent translocations from the more robust San Bruno Mountain mission blue population to Milagra Ridge and Twin Peaks. In preparation, National Park Service staff have been busy enhancing grassland habitats and planting lupine host plants at Sweeney since 2019. The site is now in good shape. So this year, staff translocated 57 butterflies from San Bruno Mountain.

After transporting the butterflies to Sweeney Ridge, staff fed them a sugar water solution and then released them onto their lupine host plants. NPS / Eric Wrubel

All told, project staff successfully translocated 38 female and 19 male Mission blues. After capturing the butterflies at San Bruno Mountain and transporting them to Sweeney Ridge, they feed the butterflies a sugar water solution. Then, they released the Mission blues onto their lupine host plants. All the butterflies appeared healthy and active, with several females seen laying eggs on host plants on their release dates. Staff were also excited to find eggs in the vicinity of release sites during post-release egg monitoring. Most recently, they've found hatched eggs and newly emerged larvae (tiny light green caterpillars) as well! That's an indication that the butterflies are exploring the habitat, finding it to their liking, and successfully reproducing at their new home.

This translocation project will continue for at least another two years, and maybe more depending on annual population monitoring results. It is an important step towards recovery for this beautiful endangered butterfly that should help bolster the population overall. Crucially, having another geographically distinct Mission blue sub-population should also help boost the species' resilience to local threats long into the future.

For more information

• Check out the Pacific Coast Science and Learning Center's Mission Blue Butterfly page
• Contact Golden Gate National Recreation Area Biologist Eric Wrubel

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• Golden Gate National Recreation Area

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