Bay checkerspot butterflies are medium-sized insects and have brilliant markings in a mosaic of white, black and reddish-orange. This butterfly was only found in Santa Clara County until recently when it was reintroduced to San Bruno Mountain and Edgewood County Park in San Mateo County, California. This species was listed as threatened in September 1987. The U.S. Fish and Wildlife Service recommended uplisting to endangered in 2022.
Historically, the subspecies occurred in the vicinity of the San Francisco Bay area from San Bruno Mountain, west of the bay, Mount Diablo, east of the bay, to Coyote Reservoir, south of the bay. The current range of the subspecies is limited to Santa Clara and San Mateo counties, California and all occurrences are on serpentine or serpentine-like grasslands except for San Bruno Mountain where it occurs on non-serpentine nonnative annual grasslands. Since 2009, Bay checkerspot butterfly reintroductions or translocations continued in Santa Clara County at Tulare Hill and in San Mateo County at Edgewood Natural Preserve and San Bruno Mountain. The Edgewood and Tulare Hill reintroductions have had limited success, while the San Bruno Mountain reintroduction has the potential for success.
At the time of listing, habitat damage resulting from urban development, highway construction, drought and overgrazing was noted as having caused the disappearance of four populations of Bay checkerspot butterfly. The threat from urban development has reduced over time as land protection has increased, and currently most of the butterfly’s habitat is protected or expected to be protected under the Santa Clara Valley Habitat Conservation Plan/Natural Communities Conservation Plan. While overgrazing may still be a threat to Bay checkerspot butterflies, there is increasing consensus that grazing is the most cost effective and flexible tool for managing California grasslands. In fact, the local extirpation of the Bay checkerspot butterfly has occurred at sites after cattle grazing was removed, for example, at Tulare Hill, which results in the displacement of larval host plants and nectar plants by non-native invasive annual grasses. The listing rule also noted, habitat degradation and pesticide use as threats to the species. Today, wildfire and small population size coupled with pesticides, non-native invasive plant species, extreme weather and climate change threaten the species. The spread of non-native invasive plants is exacerbated by atmospheric nitrogen deposition from vehicle exhaust that fertilizes the naturally nutrient-limited serpentine soils allowing nonnative plants to invade and displace the butterfly’s larval host plants and nectar plants.
The butterfly continues to be threatened by:
- Habitat degradation
- Non-native invasive plant species exacerbated by atmospheric nitrogen deposition from vehicle exhaust
- Climate change
- Fire retardant
- Small populations
The Bay checkerspot butterfly is a medium-sized butterfly and has brilliant markings in a mosaic of white, black and reddish-orange. Its forewings have black bands along the veins in the upper wing with bright reddish-orange, yellow and white spots.
The adult butterfly’s average life span is about 10 days with some individuals living over three weeks.
The Bay checkerspot butterfly reaches sexual maturity each year and generally reproduces and dies within a single year. Adults emerge from pupae in early spring from late February to early May and have an average life span of about 10 days with some individuals living over three weeks. Eggs are laid during the 4 to 6-week flight season near the base of the larval host plant and hatch within 10 days. Bay checkerspot caterpillars go through two different phases of feeding with a pause in between. The first is just after they hatch between March and May, where they will feed until they have molted three times. Larvae enter diapause and spend the summer in cracks and crevices or under rocks. Then after going dormant for the hot and dry months of summer and fall, they wake up sometime in November to February, and eat more until finally making their chrysalis in early spring. After mating, females lay 1 to 5 egg masses on the larval host plant containing anywhere from 5 to 250 eggs each. Eggs hatch in 13 to 15 days.
All occurrences are on serpentine or serpentine-like grasslands except for San Bruno Mountain where it occurs on non-serpentine nonnative annual grasslands.
Land on which the natural dominant plant forms are grasses and forbs.
The land near a shore.
The primary larval host plant for the butterfly is a small, annual, native dwarf plantain (Plantago erecta). The butterfly also frequently requires the presence of a secondary host plant, either purple owl’s-clover (Castilleja densiflora) or exserted paintbrush (Castilleja exserta), since owl’s clover and the paintbrush remain edible longer than the plantain. At San Bruno Mountain, the butterfly also utilizes the non-native English plantain (Plantago lanceolata) as a larval host plant, which is more abundant and remains edible longer than the dwarf plantain. The Bay checkerspot butterfly requires areas with topographic diversity, which are defined as having warm south and west slopes, as well as cool north and east slopes, because some slopes become unfavorable depending on annual weather conditions and time of year. The delayed senescence of host plants on cool, moist slopes allows larvae to reach their fourth instar, which is the larval development stage or molt, and enter diapause, a stage of dormancy, before host plants become inedible. Larvae that are not able to enter diapause prior to host plant senescence starve and die. Warm temperatures in the spring accelerate the senescence of the host plants resulting in fewer larvae surviving to the adult phase.
Adult Bay checkerspot butterflies feed on the nectar of several plants found in association with serpentine grasslands, including California goldfields, tidy-tips, desert parsley, scytheleaf onion, sea muilla, false babystars, intermediate fiddleneck and other species. The fecundity of the female butterflies is significantly affected by the availability of nectar.
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