Swamp milkweed and monarch butterfly by Victor Quintanilla
A monarch butterfly feeds on swamp milkweed in Connecticut. Photo by Victor Quintanilla.
On a recent trip to California, I stopped by Natural Bridges State Beach, a lovely seaside protected area in Santa Cruz that’s best known for the monarch butterflies that overwinter there. Unlike monarchs east of the Rocky Mountains, which fly up to 3,000 miles to mountainous central Mexico for the winter, butterflies west of the Rockies spend the cooler months in about 200 smaller sites scattered along the California coast. Of these, Natural Bridges is the only state preserve specifically set aside to protect the insects. As a graduate student at the University of California-Santa Cruz many years ago, I lived close enough to the preserve that I could walk there regularly and observe the spectacular monarch gatherings—among the most memorable experiences of my time living in California.

This visit, however, turned out to be disappointing. Though the boardwalk leading down to the protected monarch grove looked just as I remembered it, I spotted only a handful of butterflies flitting among the eucalyptus and Monterey pines that once teemed with them. A reserve employee suggested I drive about a mile south to Lighthouse Field State Beach, a more open area bordered by roads and houses and visited by large numbers of bikers, dog walkers, beach goers and surfers. Some monarchs, in fact, were there—at least several hundred clustered in a small, roped-off grove of pine and eucalyptus. Why had the insects moved down the road to this apparently less hospitable habitat?

John Dayton, a San Jose State University biologist conducting a survey of the colony at Lighthouse Field, provided an answer.  During the 1990s, Dayton told me, a deadly fungal disease, pine pitch canker, killed off most of the large Monterey pines that had sheltered the monarch grove at Natural Bridges from strong winds. Without this windbreak, many of the large eucalyptus trees at the northern portion of the grove blew down during winter storms. The loss of these trees has degraded the roost area, forcing monarchs to seek shelter elsewhere beginning about mid-December.

But Dayton had even more troubling news: Since the mid-1990s, he said, the number of overwintering monarchs up and down the California coast has declined by nearly 90 percent. According to the Xerces Society, butterfly populations at Natural Bridges alone have dropped from about 120,000 in 1997 to just over 1,000 (when the insects are there at all).

Monarch Caterpillars by Larry Lynch
Two monarch caterpillars nibble on a milkweed stem. Photo by Larry Lynch.
Equally sad news is coming out of Mexico, where the majority of North American monarchs spend the winter. Last week, scientists who conduct annual surveys of the overwintering colonies reported that the total area occupied by the butterflies (a proxy for their numbers) was just 2.94 acres this season, a 59 percent decrease from 2011-2012’s results—and the lowest figure tallied in two decades. Though monarch numbers can fluctuate from year to year due to weather and other variables, Chip Taylor, director of Monarch Watch, says populations have been trending downward for years.

Dayton, Taylor and other scientists point to several culprits. In California, coastal development and habitat degradation have reduced the area suitable for overwintering monarchs. In Mexico, illegal logging, poorly regulated tourism and water withdrawals near the butterfly colonies are taking a toll. Last year, when monarchs headed north to feed and breed beginning in March, they encountered extreme drought and heat waves that persisted in some areas from spring through fall.

Throughout the butterflies’ North American range, declines in milkweed plants—which monarchs need in order to reproduce—also are knocking down the insect’s numbers. The problem is particularly acute in the U.S. Midwest, where genetically engineered, herbicide-resistant corn and soybeans now allow farmers to apply the chemicals broadly, wiping out milkweed that once thrived between crop rows and in fallow fields on millions of acres of agricultural land.

According to an article in the April/May 2013 issue of National Wildlife magazine, climate change may worsen the monarchs’ situation. In her story, “Catering to Butterfly Royalty,” writer Doreen Cubie reports results of experiments suggesting that as atmospheric levels of carbon dioxide continue to rise, milkweeds are likely to produce less of the toxic compounds that protect both butterflies and caterpillars from predation. Milkweed leaves also may get tougher. “Caterpillars would have a harder time chewing the leaves,” says ecologist and study author Rachel Vannette of Stanford University.

Cubie concludes her text with a hopeful message: While there’s not much an individual can do about effects climate change may have on butterflies in the future, she writes, “there is a simple way to help today’s monarchs. You can grow more milkweeds in your garden.”

Find out about five native milkweeds that are easy to cultivate, then help butterflies and other backyard wildlife by turning your property into a Certified Wildlife Habitat® site.