Bay Checkerspot Butterfly

The federally threatened Bay checkerspot butterfly used to thrive in Edgewood’s native grasslands but had disappeared by 2002. Learn more about our efforts to bring this species back!

The Good Ol’ Days

For years and years, Bay checkerspot butterflies enjoyed a comfortable, if short, life throughout thousands of acres in the Bay area. Although fairly picky about their environment, these butterflies thrived in serpentine grasslands. Serpentine, our state rock, breaks down into soil poor in the nutrients that all plants need. But native plants have adapted to these austere conditions, and, in turn, the checkerspot has evolved to depend on a handful of these native plants.


In March and April, female butterflies lay clusters of eggs on California plantain.

Bay checkerspot caterpillar

Tiny black caterpillars hatch ten days later. The caterpillars feast on the plantain and grow quickly. Some caterpillars move to other food plants, including purple owl’s clover. As the plants die off in the summer heat, the caterpillars stop eating and enter diapause, a resting state.

When plantain germinates after fall rains, the caterpillars resume eating. Once they are large enough, the caterpillars form pupae in early spring. A few weeks later, adult butterflies emerge. In the mere ten 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.

An Unexpected Turn

During the late 1990s and early 2000s, the checkerspot’s population at Edgewood began to drop precipitously. Conservation biologist Stuart Weiss had been studying the insect for years, and in 2002, Dr. Weiss counted the last caterpillar at Edgewood. Although large numbers of the butterfly could still be found farther south in Santa Clara County, the butterfly had gone locally extinct. What could have caused this population crash?

Mystery Solved

By 2003, the Bay checkerspot butterfly could no longer be found at Edgewood Natural Preserve. Elsewhere on the peninsula, local populations had gone extinct as well. Scientists discovered a common thread: urban smog and automobile emissions.

This air pollution is a slow-release nitrogen fertilizer: it enriches nitrogen-poor serpentine soils, allowing European grasses to crowd out the native plants. The prime suspect at Edgewood: Highway 280.

Each day, more than 100,000 vehicles speed past, spewing nitrogen oxides (NOx). Catalytic converters introduced in the 1990s inadvertently introduced a more potent fertilizer, ammonia (NH3) — worsening the problem.

The butterfly’s favorite habitat — serpentine grasslands — had been converted to fields of non-native annual grasses, thanks to the enrichment of the soil. These grasses were crowding out the native plants that the butterflies and caterpillars required. Italian ryegrass seemed to be one of the worst offenders.

Scientists confirmed that the closer you got to the freeway, where Edgewood’s nitrogen pollution was measured at the highest levels, the thicker the ryegrass. The farther from the freeway, the more Bay checkerspot host and nectar plants.

Preparing for Restoration

Dr. Weiss realized that there was a good chance that the Bay checkerspot could be restored, if its habitat could be returned to something close to its original state. Thanks to generous grants from PG&E, Weiss began mowing plots of affected grasslands in 2005 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 these competitive invaders were removed.

They’re Baaack

In February and March, 2007, Dr. Weiss and his assistants collected hundreds of caterpillars from Coyote Ridge in Santa Clara County, where they still can be found in large numbers. This was done with special permission from the U.S. Fish and Wildlife Service, which protects this federally threatened animal.

Collecting caterpillars from Coyote Ridge, February 2007

Then, with great care, the scientists transported the caterpillars to Edgewood and placed them in the mowed areas, where the California plantain had already begun to emerge.

Placing caterpillars in new home at Edgewood, February 2007

After pupating, the caterpillars began emerging as adult butterflies.

Edward I
The first recorded butterfly to emerge, dubbed Edward The First

The Disappointment

2008 brought disappointing news. Only one caterpillar was counted. In 2009 there were none.

What could have accounted for this? 2007 was the fourth driest spring since 1895! 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.

The biologists re-thought their approach, and came up with an improved plan.

Plan B

After the disappointing results from the 2007 re-introduction, the biologists knew they had to find a way to mitigate nature’s unpredictable weather. Plan B consisted of acquiring permission from the US Fish and Wildlife Service for a 5-year permit to transfer about 4,000 caterpillars from Coyote Ridge, where the population is booming.

This phased introduction will further increase 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 dwarf plantain, gold fields, tidy tips, and other nectar sources.

So in February 2011, about 4,000 butterflies were collected and moved to their new home at Edgewood.

The larger number of caterpillars will increase the critical density of adult butterflies. Butterflies judge habitat quality by the presence of others; they need a critical mass to settle down and develop a sedentary tendency. Introducing too few butterflies, as may have been the case in 2007, may increase the likelihood that they will fly away looking for other individuals.

2013: An Excellent Year for the Bay Checkerspot Butterfly at Edgewood

Things keep getting better for the Bay checkerspot population at Edgewood.

Checkerspot monitors saw 699 checkerspots this season. This is up from 333 checkerspots in 2012, 129 in 2011, and only 13 in 2007. Our volunteer monitors walk a permanent course off trail, counting the number of checkerspots they see in 1.5 minutes on each of 35 50-meter segments. This year’s higher numbers are likely the result of both cool weather last spring and additional translocations.

Weather patterns are always important to the survival of our spotted friends. The most critical point of their lives is whether they can grow large enough as larvae (caterpillars) in late spring to enter diapause before their host plants dry out. Reaching this dormant phase is critical for survival during the long, hot summer when their food sources are no longer present. You can think of it as a race between larvae and the plants.

Last year we had a cool spring, with high numbers of larvae likely making it to diapause. Our volunteer monitors observed peak flight in the first week of April, and host plants dwarf plantain and owl’s clover stayed fresh until the second week in May. This gave the butterflies plenty of time to mate, lay eggs, and hatch tiny pre-diapause larvae. We think high numbers of those larvae won the race to reach diapause before their host plants dried up.

Although overall precipitation was low this year, the early rains caused dwarf plantain to germinate early as well. Early germination is a positive sign because it allows the larvae to emerge from diapause and begin feeding. Starting the race early increases the likelihood the larvae can move through their life cycle in time. Heavy rains in the early winter soaked the soils to capacity, allowing for a decent, if brief, wildflower year. Once the new year began, rain and even clouds were sparse, and we had long periods of low temperatures. The larvae grow quickly in sunny conditions, while plants tend to grow slowly in cool temperatures. More good news in the race!

Pupating butterflies can be vulnerable to rain damage. The sunny weather later in the season probably kept pupal mortality low, allowing lots of adult butterflies to emerge. The flight season began early this year. The first checkerspot was seen February 25. Last year they were first noted on March 4, and in previous years they were not observed until mid-March. The early flight season will be critical this year because things did start to heat up and dry out very early. Peak flight this year was March 18 (50 butterflies seen in a 2-hour period), with some host plants hanging on until the last week of March. While we had high numbers of butterflies and abundant host plants, we hope that plenty of eggs were laid on cool slopes, where host plants remain fresh longer.

We released an additional 5000 larvae this year, translocated from very high-density locations on Coyote Ridge in south San Jose. The populations there were also on the increase this year. In fact, our collections may even help those larvae from overshooting their carrying capacity. By thinning the ranks, we may reduce the likelihood that those source larvae will eat all their host plants before they can reproduce. So, too many checkerspots can be a problem. On the other hand, butterflies judge habitat quality by the presence of others. They need a critical mass to settle down and develop a sedentary tendency. Introducing too few butterflies could increase the likelihood that they will fly away looking for other individuals.

Many of the larvae we translocated were included in two 2013 Year of Edgewood Adopt-a-Caterpillar events. We allowed a limited number of people to sign up in advance to sponsor and release a caterpillar into the restored habitat. This event was hugely successful, with adults and children savoring the chance to be part of a threatened species comeback at the preserve they love.

An effort in 2007 was not successful due to very dry weather conditions and a small number of transferred caterpillars. Through the setback, mowing continued to keep the site ready for the eventual butterflies’ homecoming. In February 2011, more than 4,000 caterpillars were introduced. To further the odds of firmly reestablishing the species, another 4,852 caterpillars were introduced February 2012. Translocations may continue in 2014 and 2015.

Partners of the San Mateo County Parks include: Creekside Center for Earth Observation, San Mateo County Parks Foundation, United States Fish and Wildlife Service, Pacific Gas & Electric Company, the Jiji Foundation, Microsoft, the California Native Plant Society, and the Friends of Edgewood.

This video shows the recent translocation efforts.

How Many Checkerspots Can Checkerspotters Spot?

In 2014, a lot! What started as a shaky year due to extreme drought has turned into an amazing show of Bay checkerspot butterfly activity at Edgewood, with our checkerspotting team noting record numbers since the reintroduction began.

While Edgewood generally averages about 21 inches of rainfall a year, the 2013 calendar year saw only 4.2 inches. October 2013 to January 2014 had only 1 inch, with literally zero measurable rainfall in January (Western Regional Climate Center, Pulgas Ridge station). By the end of January, the normally green grasslands at Edgewood were still eerily brown. Few annuals had germinated, and many that did died from moisture stress. Those that survived were usually shaded by rocks or small divots in the ground. Perennials were heavily browsed.

Bay checkerspots are always in a race against their host plants, so an early start to the growing season (think November rather than December), is often key to their survival. With essentially no Plantago erecta for them to eat by late January, we questioned whether they would emerge at all, survive a second year of diapause, or emerge late only to not have enough time to complete their life cycle. Prospects seemed grim both at Edgewood and at the source population in Coyote Ridge, which was booming in 2013.

Finally our first larvae were spotted following germinating rains in early February. We estimated close to 4000 larvae, which was a good number but still below the replacement rate of the 5000 we introduced in 2013. Because numbers were even lower at Coyote Ridge, we decided to cancel transfers for the year.

By mid-March, however, conditions had changed dramatically. We found large pockets of record-high checkerspot densities on Coyote Ridge. Nearly 4.5 inches of rain in February recharged the soils, and host plants developed with little nonnative grass competition. (It appeared that many of the nonnative grasses germinated and then died during the dry January.) While the larvae got a very late start, they developed quickly in the long, sunny days. We brought 4105 larvae to Edgewood by March 12, with hopes that cool weather would prevent host plants from drying out too soon.

Our team of checkerspot monitors began surveying for adults in early March. As of late April, the checkerspotters have seen a record number of butterflies since the reintroduction began. In 2007, volunteers saw only 9 adults over two weeks. In 2011, there were 120 over four weeks. They have increased every year, with 2014 our new record.

As of this writing in late April, it looks like we’ll have an 8-week flight season with 799 adults spotted. The long flight season means more opportunities for mating and egg-laying. The butterflies are spreading out. At least one checkerspot has been found on the Clarkia trail, and other trails had unconfirmed sightings as well.

The high numbers and the long flight season are very exciting, but perhaps even more so is the timing between the butterflies and the host plants. As a rule of thumb, we want to see at least three weeks between peak flight and the complete drying out of host plants. We consider this a minimal amount of time for butterflies to mate, lay eggs, and for their small larvae to grow large enough to go into diapause. This is the dormant stage the larvae must enter before the long, hot summer where there is nothing for them to eat. The longer the plants stay fresh after peak flight, the better. Peak flight was probably early in the second week of April, and by the end of the third week of April, none of the host plants had dried out. While there were a few hot days in April, overall temperatures were cool. A little bit of refreshing rain late in the month adds to the positive outlook.

The habitat continues to look amazing, thanks to a rotational mowing program completed by County Park staff. Plots mowed in 2012 and 2013 had an average of 34% Plantago erecta and 5% nonnative annual grass this year, while paired unmowed plots only had an average of 10% Plantago and 30% nonnative annual grass. Host plants are clearly responding positively to the management treatment, and weeds declining. Different areas are planned for mow treatment in 2014.

We were so pleased to see the year shift from gloomy to glittery. It was interesting to watch the larvae develop so quickly with the longer days when they started late in the season, and to see that host plants starting so late still needed a certain amount of time to complete their lifecycle. It appears that the larvae will win the race this year. While numbers will always vary dramatically, it is encouraging to finally feel that the Bay checkerspots are making a solid home at Edgewood.

The Hope

Unfortunately, the 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. Effects include grasses invading California deserts and coastal scrublands, dwindling floral diversity in European grasslands, and ocean dead zones. Scientists say the threat to biological diversity is comparable to that from global climate change.

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.

How Can you Help?

Scientists are using existing technologies to help decrease our collective nitrogen emissions, but we each have a role to play:

  • Reduce your emissions: 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.