Bay Checkerspot Butterfly

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Bay Checkerspot Butterfly

Euphydryas editha bayensis

ListedSeptember 18, 1987
FamilyNymphalidae (Brush-footed butterfly)
DescriptionMedium-sized butterfly with bright red and yellow spots on the upper forewing.
HabitatGrasslands associated with outcrops of serpentine.
FoodFeeds on plantain, owl's clover.
ReproductionComplex life cycle of egg, larvae, pupa, adult.
ThreatsUrbanization, drought, overgrazing, fire.


The bay checkerspot, Euphydryas editha bayensis, is a medium-sized butterfly with a wingspan of 1.5-2.25 in (4-5.6 cm). The upper forewing surfaces have bright red-and-yellow spots and black bands along the veins. Its appearance is more decidedly checkered than other subspecies of Euphydryas editha. It is darker than E. e. luestherae and lacks a dark red outer-wing band. It has brighter red-and-yellow coloration than E. e. insularis.


The bay checkerspot lays eggs only on certain host plants, the most important of which is the annual, dwarf plantain (Plantago erecta ); the owl's clover (Orthocarpus densiflorus ) is a secondary host. The host plants germinate from early October to late December, and senesce from early April to mid-May. Most of the active parts of the life cycle of the rare butterfly also occur during this time. The bay checkerspot reproduces once and dies in a single year. Adults emerge from pupae in early spring, and feed on the nectar of various species of plants. They mate and lay eggs during a flight season that typically lasts for four to six weeks in the period between late February to early May. Male bay checkerspots typically emerge from their pupae four to eight days before females, and find and mate with most females soon after they emerge. Males can mate multiple times, but most females are believed to mate only once. However, they are capable of re-mating four to seven days after the first copulation, and some females have been found to carry more than one spermatophore. The average life span for adults of both sexes is about 10 days, but individuals have lived for more than three weeks. Eggs are typically laid in March and April. Females lay up to five egg masses of 5 to 250 eggs each, which they deposit near the base of dwarf plantain, or less often, purple owl's clover. The lifetime production of eggs produced by female bay checkerspots is 250-1,000.

Natural recolonization of sites from which the bay checkerspot has been extirpated is rare. In 21 years of studying marked populations less than 4 mi (6 km) apart, only a single individual was observed to translocate to another colony. As the number of suitable habitat "islands" of the bay check-erspot decreases because of habitat loss, the average distance between sites increases, making recolonization an even less likely event.


The host plants only grow in sites with serpentine-influenced soil. The bay checkerspot thrives only on serpentine outcrops larger than about 800 acres (325 hectares), a size that allows the population to survive the severe population fluctuations that occur during periods of drought. Smaller out-crops support satellite populations that may thrive in years of favorable climate. However, the bay checkerspot can easily become extirpated in small habitats, which must then be recolonized from a larger, surviving habitat nearby.


The bay checkerspot ranges around and south of the San Francisco Bay in Alameda, Contra Costa, San Francisco, San Mateo, and Santa Clara Counties, California. In historical times, this area probably supported the bay checkerspot in four large serpentine habitats and dozens of smaller ones. The total historical area of suitable serpentine habitat was about 12,000 acres (4,900 hectares), but it is much less today. There are five core areas: one on the San Francisco peninsula in San Mateo County, and four on Coyote Ridge in Santa Clara County. These core areas plus smaller satellite habitats comprise two metapopulations, of which that in Santa Clara County is considered to have the greatest chance of long-term survival. At least 26 satellite colonies have been documented, but many have been lost to drought, overgrazing, or urbanization.


The bay checkerspot is threatened by habitat destruction or degradation caused by urban development, highway construction, drought, fire, livestock grazing, and invasive non-native plants (such as eucalyptus trees). When its host plants are removed or replaced by other species, the bay checkerspot cannot survive. Its largest serpentine outcrop occurs near Morgan Hill (Santa Clara County) in a narrow band extending 16 mi (26 km) from Heller Canyon to Anderson Lake. Two large outcrops in San Mateo County were fragmented by the construction of Interstate Highway 280. A population historically known from San Leandro (Alameda County) is now extirpated. A small population on San Bruno Mountain has not been observed since a 1986 wildfire.

The largest surviving population of bay check-erspot is at Morgan Hill, but the best-studied colony is at Stanford University's Jasper Ridge Biological Preserve east of Searsville Reservoir. The drought of 1976 and 1977, combined with overgrazing, greatly reduced the Jasper Ridge population. The second largest population, located in San Mateo County, is threatened by the proposed addition of a golf course and recreation area to Edgewood Park. If implemented, this construction would eliminate 65% of the remaining habitat and decimate a population that had already declined catastrophically (from 100,000 in 1981 to only about 1,000 in 1987) because of urbanization and adverse weather conditions. A larger population in Redwood City was fragmented by urbanization and reduced to satellite status. A colony near Mt. Diablo (Contra Costa County) was thought to have become extirpated during a severe drought, but bay checkerspots were found there again in 1988.

Conservation and Recovery

Much of the habitat of the bay checkerspot is on privately owned land. A conservation agreement was negotiated between the U. S. Fish and Wildlife Service (FWS) and a corporate landowner, Waste Management Inc., to protect 30% of the Morgan Hill site. The FWS believes that this conservation agreement will decrease the overall threat to the species, even though it was linked to the destruction of 10% of the critical habitat during the construction of a landfill. Waste Management Inc. agreed to support conservation activities at the site for 10 years. As a result of this conservation action, the FWS downlisted the bay checkerspot from Endangered to Threatened. The populations of the bay checkerspot should be monitored, and, if necessary, extirpated populations should be reestablished by transplantation from surviving populations. Research is needed into the biology and habitat needs of the butterfly, to provide insight into management practices to maintain or enhance its habitat. Prescribed fire, for example, appears to be useful in reducing the abundance of non-native shrubs and trees.


Regional Office of Endangered Species
U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121


Ehrlich, P. R., et al. 1975. "Checkerspot Butterflies: A Historical Perspective." Science 188:221-228.

Murphy, D. D., and P. R. Ehrlich. 1980. "Two California Checkerspot Butterfly Subspecies: One New, One on the Verge of Extinction." Journal of the Lepidopterists' Society 34:316-320.

U. S. Fish and Wildlife Service. 1987. "Determination of the Bay Checkerspot Butterfly, Euphydryas editha bayensis, to Be a Threatened Species." Federal Register 52(181): 35366-35378.