An air of excitement and anticipation hangs over the monarch butterfly grove in Pismo Beach.
Arborists are busy trimming dead limbs and removing other safety hazards from the grove while State Parks employees and California Conservation Corps workers get the site ready for the annual influx of visitors.
This winter, thousands of tourists and local nature lovers will congregate at the park along Highway 1 at the southern edge of Pismo Beach to witness what many consider to be one of the greatest natural history spectacles on the planet — the winter aggregation of monarch butterflies.
“The grove is a fun and magical place for families to come and visit,” said Danielle Patterson, a State Parks interpreter who oversees the 85 interpretive docents who work at the site.
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Although the start of the official monarch butterfly season is still more than a month away, the colorful insects are already beginning to arrive. Patterson estimates that about 60 butterflies can already be seen flitting amongst the boughs of the grove’s eucalyptus trees. Increasing numbers of butterflies will arrive at the site through October and November. The butterfly season peaks at around Thanksgiving and lasts through early January, when it begins to taper off.
In recent years, the park has attracted between 28,000 and 34,000 butterflies, making it one of the premier monarch viewing sites in California, Patterson said. Monarch wintering sites are also found in Nipomo and Morro Bay.
“I’m really hoping there will be an increase this year,” she said.
While they are here, the monarchs mate. By the end of February, most of them are gone, as the females look for milkweed plants on which to lay their eggs.
From November through February, State Parks docents staff the Pismo grove from 10 a.m. to 4 p.m. seven days a week to talk with visitors about the monarchs. Lectures are given at 11 a.m. and 2 p.m.
Monarch butterflies from all over the United States west of the Rocky Mountains travel as far as 3,000 miles to reach overwintering sites along the central and southern California coast. Places like the Pismo grove have just the right combination of environmental characteristics that the butterflies are looking for.
The grove is close enough to the Pacific Ocean so that temperatures do not dip below freezing at night. The trees shelter the butterflies from wind, rain and extreme sunshine.
Throughout the course of a day, the butterflies will move throughout the grove, clustering in those areas that provide just the right microclimate. Nancy La Grill, a State Parks environmental scientist, recommends that people visit the grove in the afternoon when the sun has warmed the butterflies and they are most likely to be active.
The Pismo grove is almost entirely eucalyptus, but State Parks plans on adding Monterey cypress and pine to the mix in order create a more diverse habitat and more microclimates for the monarchs to choose from, LaGrill said.
“There is so much to these butterflies, and they are so fascinating,” she said
Threats to monarchs
Although the Pismo Beach grove appears to be going strong, the species as a whole is struggling. The species faces a number of threats.
The main one is the loss of milkweed. Monarchs use plants in the milkweed family as a main food source for adults and the sole host for larvae and caterpillars.
Development and pesticides have drastically reduced the number of milkweed plants available to the monarchs. Various parasites are also taking a toll. The National Wildlife Federation and other groups estimate that monarch populations in California have dropped by as much as 90 percent over historic levels.
Although Patterson is optimistic about an increase in the number of monarchs this winter, she is not willing to make any predictions.
“I have no idea,” she said “It’s always changing.”
Monarch butterflies in the United States use two different wintering sites. Monarchs west of the Rocky Mountains migrate in the fall to suitable sites along a 600-mile stretch of the California coast, with most concentrating along the Central Coast.
They form winter aggregations in groves of trees typically within a mile of the ocean. These are the monarchs we see each winter. Monarch butterflies east of the Rocky Mountains migrate to the Sierra Madre in central Mexico. They congregate by the millions in fir trees high in the mountains.
Four to six generations of monarch butterflies live and die in a year. How the butterflies find their way to the same wintering sites is not known.
According to the Monarch Butterfly Habitat Volunteer Committee in Nipomo, some scientists believe monarchs rely on the Earth's magnetic field, the position of the sun and the polarization of the sun's rays to find their way.