Bay Checkerspot Butterfly

The federally threatened Bay checkerspot butterfly once thrived in Edgewood’s native grasslands, but by 2002 it had disappeared. Learn about our efforts to bring this iconic species back!

A Mysterious Disappearance

The Bay checkerspot butterfly, Euphydryas editha bayensis, is one of Edgewood Park’s most beloved species. These little guys are extremely picky about their environment and depend on a handful of California native plants that have adapted to the austere conditions of the serpentine grasslands found at Edgewood Park.

The Bay checkerspot butterfly was once found throughout thousands of acres in the San Francisco Bay area. During the late 1990s and early 2000s, the checkerspot’s population at Edgewood and elsewhere began to drop precipitously. In 2002, conservation biologist Stuart Weiss documented the last caterpillar at Edgewood. Although large numbers of the butterfly could still be found farther south in Santa Clara County, locally the butterfly had gone extinct.

California plantain
California plantain (Plantago erecta)
Owls' Clover
Owl’s clover (Castilleja densiflora)

Mystery Solved

What could have caused this population crash? Scientists point to a likely cause: urban smog and automobile emissions. Each day, more than 100,000 vehicles speed past Edgewood on Highway 280, spewing nitrogen oxides (NOx) and ammonia (NH3), both potent slow-release fertilizers. This air pollution fertilizer is enriching our nitrogen-poor serpentine soils, allowing non-native annual grasses to crowd out the native plants.

The California plantain, Plantago erecta, is a native annual and an important host plant for the checkerspots. By 2002, non-native Italian ryegrass had overtaken much of the butterflies’ favorite habitat and choked out the California plantain that they needed to survive.

Collecting Bay Checkerspot larvae
Collecting Bay Checkerspot larvae
Bay checkerspot caterpillar closeup
Bay checkerspot caterpillar closeup

Life Cycle of the Checkerspot

Checkerspots are a short-lived creature; their entire life-span is completed in a year. In March and April, female butterflies lay clusters of eggs on California plantain. Tiny black caterpillars hatch about ten days later. The caterpillars feast on the plantain and grow quickly. Some caterpillars move to other food plants, including purple owl’s clover. The caterpillars must race to get large enough to enter summer diapause (dormant state) before their host plants die off and they starve.

When the plantain germinates after fall rains, the caterpillars resume eating. Once they are large enough, the caterpillars form a chrysalis in early spring. A few weeks later, adult butterflies emerge. In the few days they live as adults, the checkerspots fly about the serpentine grasslands, sipping nectar from native wildflowers. They mate, lay eggs, and die, completing their year-long life cycle.

Caterpillars Ready to be Placed
Caterpillars Ready to be Placed
Careful Introduction of Bay Checkerspot Caterpillars
Careful Introduction of Bay Checkerspot Caterpillars

Our Restoration Efforts

Dr. Stuart Weiss of Creekside Science has been studying the Bay checkerspot butterflies at Edgewood since the early 1980’s. He realized that there was a good chance that the Bay checkerspot could be restored, if its habitat could be brought to a less weedy state.

In 2005, thanks to generous grants from PG&E, Dr. Weiss began mowing plots of affected grasslands where he had counted large butterfly populations in years past. He found that the native plants — tidy tips, goldfields, California plantain, and others — staged comebacks when competitive invaders were reduced.

In February and March 2007, Dr. Weiss and his assistants started the first of several attempts to re-populate the butterflies by introducing a thousand caterpillars collected from Coyote Ridge in Santa Clara County, where checkerspots can still be found in large numbers. This was done under permit from the U.S. Fish and Wildlife Service, which protects this federally threatened animal.

 First Recorded Butterfly to Emerge in 2008, Dubbed Edward The First
First Recorded Butterfly to Emerge in 2008, Dubbed Edward The First

Unfortunately, 2007 was the fourth driest spring since 1895 and the relocation efforts failed. In all probability the host plants dried up before the new caterpillars were large enough to enter diapause, the dormant stage in which they spend the hot, dry summer.

Dr. Weiss and his team did not give up! They obtained a 5-year permit from the U.S. Fish and Wildlife Service and beginning in February of 2011 they transferred about 4,000 caterpillars from Coyote Ridge, where the checkerspot continues to thrive. An additional 4,000 to 5,000 caterpillars per year were introduced between 2012 and 2016, and another 2,400 in 2017 under a renewed permit.

This phased introduction increases the chances of hitting a good weather year that is conducive to population establishment. Mowing and dethatching continue to take place rotationally throughout the butterfly habitat, creating dense fields of California plantain, gold fields, tidy tips, and other nectar sources.

Checkerspot Monitoring

Each spring our volunteer Checkerspot Monitors walk a permanent course off trail, counting the number of checkerspots they see in 1.5 minutes on each of 36 50-meter segments. These dedicated citizen scientists help us track the number of adult checkerspots, calculate their reproduction rates and note environmental factors that could be having an impact on their survival.

Re-establishing a self-sustaining colony of checkerspots is an on-going effort. Our monitors sighted only 9 butterflies back in 2007, but they saw numbers steadily increase each year up to a high of 800 in 2014. Unfortunately, numbers have again declined over the last several years with less than 50 butterflies counted in 2018. Climate change, the density of host plants and nectar sources, and the small size and topographic variability of the area suitable for butterfly habitat at Edgewood may all play a role in the year to year success of the restoration. As part of our new habitat management program, Project 467, the Friends of Edgewood will continue to work hard to bring checkerspots back home to Edgewood forever.

Counting Bay checkerspot butterflies
Counting Bay checkerspot butterflies

Here is a more detailed history of the Bay checkerspot restoration efforts.

About Project 467

Restoration work at Edgewood does more than recreate a home for the threatened butterfly. Brighter spring wildflower displays offer evidence that thriving native grasslands also encourage other native species. This work also demonstrates the feasibility of preserving habitats by dynamically correcting for environmental change.

The Friends of Edgewood launched Project 467 in 2018. Project 467 is a comprehensive habitat management program that includes the ongoing efforts to restore the Bay checkerspot butterfly, as well as the San Mateo thornmint and white-rayed pentachaeta. It supports the existing Weed Warriors program and the new Green Grass project, which aim to bring Edgewood’s grasslands back to their native state. Read more about Project 467.

Help us restore and protect all 467 acres at Edgewood Park and Natural Preserve.

Donate Now

Other Ways to Help

Unfortunately, the Bay checkerspot butterfly is just one of many victims of nitrogen pollution. Nitrogen is saturating ecosystems worldwide at an ever-increasing rate, largely due to use of fossil fuels. Scientists say the threat to biological diversity is comparable to that from global climate change.
We each have a role to play to help decrease our collective nitrogen emissions, including:

  • Drive less, and slow down. Ammonia and NOx emissions increase exponentially with speed–so driving the speed limit makes environmental sense.
  • Support local organizations working to protect these natural resources, including Friends of Edgewood and the San Mateo County Parks and Recreation Foundation.
  • Stand strong for environmental protections, including the Endangered Species Act.
  • Support efforts to regulate nitrogen pollution.