The five major forces responsible for Earth’s current species extinction crisis can be described by the acronym “HIPPO”, a useful tool devised by Harvard biologist E. O. Wilson. Each of the letters in HIPPO stands for one of the major forces responsible for species loss, the “H” for habitat destruction, including the impact of climate change on ecosystems, the “I” for invasive species, the first “P” for pollution of all types, the second “P” for human population expansion, and the “O” for over-harvesting of Earth’s resources. Each of these forces results from human activity.
Notwithstanding our awareness of its destructive capacity, the HIPPO juggernaut rolls on unabated and largely ignored by most people. Gardeners, however, particularly those who garden in tune with nature, are well aware of the dependence of their success on the biodiversity of their gardens, both in the soil and above ground. Every organism, every creature, counts.
Gardeners across North America suddenly find themselves on the front line in efforts to preserve habitat essential to the monarch butterfly’s annual migration from overwintering sites in the Sierra Madre of Mexico to points east, west, and north. This migration differs from those of birds in that the monarchs that arrive in New England’s summer gardens are not the same butterflies that left the overwintering site. The butterflies that leave Mexico stop in Texas where they seek out milkweeds (plant species in the genus Asclepias) on which to lay their eggs. Over the next two generations, monarchs spread eastward toward the Atlantic Ocean, westward to the Rocky Mountains, and northward as far as Toronto, Canada, continuing to lay eggs on milkweeds, the obligate food source for monarch caterpillars. Thus the monarchs laying eggs in New England gardens are the great grandchildren of those that left Mexico. The migration back to the forests of Mexico is completed by a single generation of monarchs.
The sight of hundreds of thousands of black-and-orange butterflies migrating together inspired Annie Dillard, in her 1974 book Pilgrim at Tinker Creek, to write: “It looked as though the leaves of the autumn forest had taken flight, and were pouring down the valley like a waterfall, like a tidal wave, all the leaves of hardwoods from here to Hudson’s Bay.” Countless others have also been inspired by such a sight, a vision now at risk of never being seen again.
Monarch populations have decreased dramatically throughout their range in recent years. Much of their Mexican overwintering habit has been cut for timber and while the Mexican government has stepped in to protect what is left, illegal logging and tourism continue to be problems. Climate change along the migration routes is also a factor with several consecutive spring droughts in Texas that have dried out monarch eggs, reducing the numbers that hatch. Also, major storms, rain, and cold temperatures in the overwintering area have all contributed to the loss of adult butterflies before migration starts.
The major cause of monarch decline, however, is the loss of milkweed populations throughout the monarch migration range, the result of extensive use of herbicides on herbicide-resistant crops, primarily corn and soybeans. Since 90% of all milkweed habitats used by monarchs occur within agricultural landscapes, farm practices strongly influence monarch populations. It is estimated that wide-spread use of herbicides in production of these crops has resulted in the destruction of 80 million acres of monarch habitat. Other causes for loss of milkweed habitat include development (subdivisions, shopping areas, etc.) and the use of herbicides and frequent mowing in roadside management.
Monarch caterpillars must feed exclusively on the leaves of milkweed in order for the insect to complete its life cycle. From these leaves the larvae ingest chemicals called glycosides, concentrating these compounds in their bodies and subsequently in the bodies of the adult monarchs. The bitter-tasting glycosides are toxic to many birds and mammals that would otherwise prey on the monarch. These would-be predators have learned to recognize the monarch’s orange and black markings as a warning to stay away.
Gardeners across the migration paths of the monarch can create monarch habitat in their gardens. In addition, they can encourage and assist with the development of monarch habitats at schools, businesses, parks, zoos, nature centers, along roadsides, wherever there are unused plots of land. Remember that adult monarchs are nectar feeders, so monarch habitat in your garden should include milkweed plants for caterpillars and nectar-producing plants for the adults.
The organization Monarch Watch (www.monarchwatch.org) has a program called Monarch Waystations. Here are some guidelines that they recommend for creating monarch habitat in your garden:
Size: A space or at least 100 square feet.
Exposure: At least six hours of sun a day.
Drainage and Soil Type: Milkweeds and most nectar plants grow best in low-clay soils with good drainage.
Milkweed Plants: Plant at least 10 individuals, preferably of different species to increase the length of time nectar will be available.
Nectar Plants: Plant at least four species with bloom times distributed throughout the summer.
Management: Water during prolonged drought, weed regularly, and mulch with compost or shredded leaves each spring.
Which of the over 100 native milkweed species in North American should New England gardeners plant for monarch habitat? For the answer to this question, I contacted Ann Judd, veteran Master Gardener Volunteer and leader of the monarch project at the Charlotte Rhoades Park and Butterfly Garden in Southwest Harbor, Maine (http://www.rhoadesbutterflygarden.org/index.html). This garden is a Monarch Waystation as well as a tagging station where adult monarchs are tagged in a effort to learn more about their migration patterns, life span, effects of weather on migration, and the differences in migration from year-to-year.
Ann suggests planting Asclepias syriaca, common milkweed, in an open field or other area where its rhizomatous habit can be tolerated. It is the best species from the monarch’s point of view but too aggressive for the home garden.
A. incarnata, the swamp milkweed, works well in the perennial border where it will be a magnet for bumblebees as well as butterflies. A. tuberosa, with the apt common name of butterfly weed, works will in a sandy, well-drained, hot and dry site. At the Charlotte Rhoades Butterfly Garden, they are also trying A. exaltata, poke milkweed, in a shady damp area, as well as A. purpurascens, purple milkweed, and A. speciosa, showy milkweed, both with very pretty blossoms. All are native to New England except A. speciosa which is native to the western half of the U.S.
“Even a tub of the South American species A. curassavica, tropical milkweed, a continuous bloomer, can get a family interested in the monarch cycle if they don’t have a back yard for a garden,” says Ann. “My goal is to have schools and senior centers, libraries, golf courses – everyone really, getting something started. It would make a difference in any community if everyone tried.”
Ann Judd remains optimistic, even though last year there were very few monarchs in New England. In one field of common milkweed near the Charlotte Rhoades Butterfly Garden, only two monarchs were spotted during the entire year. To maintain a presence of monarchs for visitors to the Garden, Ann had to purchase young caterpillars from Monarch Watch, releasing them to feed on the Garden’s various milkweed species.
Those of us who treasure the presence of monarch butterflies in our gardens and lives join Ann in hoping that this year there will be an abundance of these beautiful creatures everywhere that milkweeds are allowed to flourish.