Monarch caterpillars are crawling across native milkweed on a Napa ridge that burned in the 2017 Atlas Fire, a small but telling sign for conservationists who have spent three years trying to coax the threatened butterfly back to a 190-acre restoration site at Sutro Preserve. Land Trust of Napa County staff documented about 20 monarch caterpillars and several adult butterflies using heartleaf milkweed there this season, after the insects arrived at the preserve in June.
The discovery matters because western monarch numbers remain near historic lows. As reported by The Press Democrat, the 2025 Western Monarch Count ranked as the third-lowest overwintering population since counting began in 1997, recording a peak of just 12,260 butterflies. Tosha Comendant, senior stewardship program manager at the land trust, said finding monarchs in the restoration area suggests the habitat-expansion effort may be working, and that local work can influence monarch recovery across California and beyond.
Sutro Preserve stretches about 4 miles along a ridge near Atlas Peak Road and covers nearly 1,400 acres, all managed by the Land Trust of Napa County. Heartleaf milkweed, scientifically known as Asclepias cordifolia, survived in portions of the preserve even after the Atlas Fire tore through in 2017 — a blaze that burned roughly 51,000 acres, destroyed more than 780 structures, and killed six people. The land trust began planting at the site in 2023, starting with 226 milkweed plants according to the same account.
Building a Pipeline From Seed to Seedling
The restoration effort grew from there. Crews added about 800 more milkweed and native nectar plants in 2024, and the project has now added more than 1,000 plants total, including roughly 600 milkweeds, per the Press Democrat's reporting. Restoration managers harvested wild heartleaf milkweed seeds directly from surviving plants at Sutro Preserve and sent them to the Laguna de Santa Rosa Foundation Native Plant Nursery in Santa Rosa, where staff germinated seedlings adapted to the preserve's high-elevation, rocky terrain.
By mid-May, heartleaf milkweed was blooming across portions of the preserve — the plant generally flowers from May through July. Crews and volunteers have kept young plants alive by mulching and watering them, while also removing invasive yellow starthistle and goatgrass that compete for the same ground. The project is supported by the U.S. Fish and Wildlife Service's Western Monarch Habitat Improvement Project and a local donor, the article notes.
Why Native Milkweed Beats the Tropical Kind
Monarchs depend entirely on milkweed to reproduce: adults lay eggs on the plants, and caterpillars feed exclusively on the leaves, retaining toxic compounds called cardenolides in their bodies as they grow. But not all milkweed is equal. Non-native tropical milkweed stays evergreen through mild coastal winters, letting the protozoan parasite Ophryocystis elektroscirrha build up on leaves across generations and infect caterpillars, according to the Xerces Society. Native California milkweeds like heartleaf die back naturally each fall, cleansing habitat of the pathogen — which is why conservation groups ask residents to plant heartleaf rather than tropical varieties.
Heartleaf milkweed carries history beyond its ecological role. Indigenous Miwok communities historically made deer nets, including nets up to about 40 feet long.
A Species Facing Long Odds Despite the Good News
The stakes for this kind of restoration are steep. A U.S. Fish and Wildlife Service assessment reports that western monarchs had an extinction probability of more than 99% by 2080, while another source reports a probability greater than 95% by 2080. The agency has proposed listing the monarch as threatened under the Endangered Species Act, attributing the decline to habitat loss and degradation, pesticides, disease, and climate change. According to the U.S. Fish and Wildlife Service, loss and degradation of habitat are among the primary drivers affecting the health of North American migratory monarch populations. Federal action, however, remains stalled: the monarch butterfly rulemaking stage was listed as long-term actions, according to Monarch Watch. That leaves local nonprofits filling the gap through seed collection, nursery propagation, and volunteer plantings while federal rulemaking sits in limbo.
Western monarchs migrate between inland and higher-elevation habitat and coastal overwintering sites, using roughly 400 protected locations stretching from the Mendocino Coast to Santa Cruz and Baja California. A California State Parks Foundation analysis found that in some years, more than half of all recorded overwintering monarchs were counted in state parks, with California State Parks hosting at least 20% of monarchs recorded at overwintering locations over the past 26 years. The annual Western Monarch Count monitors butterflies at these coastal sites and begins again in October.
Part of a Larger Napa Conservation Push
Sutro Preserve isn't an isolated pocket of habitat. Sutro Preserve, Walt Ranch, and Mead Ranch total about 4,998 acres and are described as nearby conservation blocks rather than a completed contiguous 7,500-acre corridor, according to Space Daily. The Land Trust of Napa County, which marked its 50th anniversary this year, a footprint built amid a stretch of years in which wildfires burned over 65% of the county.
The land trust also conducts monarch and milkweed monitoring at Sutro. Beyond its own crews, the organization asks anyone who spots a monarch to submit the observation through iNaturalist. The nonprofit will host a planting day at the Sutro restoration area on October 15 for native milkweed planting.
The Sutro work is part of a broader state effort. The California Department of Fish and Wildlife has identified monarch priorities in its conservation planning. Since 2021, the California Department of Fish and Wildlife has co-facilitated work focused on developing sources of early-season milkweed for restoration projects, according to the agency.
The timing lines up with California Biodiversity Week, running September 5 through 13. The state reported in September 2024 that California had conserved 25.2% of its land area toward the goal of protecting 30% of state lands and coastal waters by 2030, adding 861,000 acres of conserved land that year alone, according to the Office of the Governor of California. Whether volunteer-driven efforts like the one at Sutro can scale fast enough to support migratory monarch populations before federal rulemakers issue a final listing decision remains an open question.
