Most of us know what constitutes a healthy diet, but that doesn’t mean we always resist when a second helping of macaroni or slice of chocolate cake is calling our name.
The same could be said of the monarch butterfly when it comes to the Asclepias curassavica, or tropical milkweed – impossible to resist, even if it means trouble for them later.
I was looking into how wildlife gardeners could best nurture monarchs, a species that is struggling due to habitat loss, and learned that gardening for this particular pollinator is a little more complicated than I had assumed.
The traditional butterfly bush, Buddleja davidii, which I know from my youth, is now considered highly invasive and of limited value because it offers only nectar for adult butterflies and no food for young caterpillars.
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Like Buddleja, another popular plant for butterfly gardens is tropical milkweed, which serves up red and yellow blossoms that stand out dramatically against dark green leaves and do a heavy traffic in butterflies, bees and other pollinators. It’s available at garden centers in the spring, pretty much nationwide. But according to recent research, it too poses problems for migrating monarchs when planted in temperate zones.
Considered the king of butterflies for its contrasting black and gold wing patterns and marathon migration, monarchs spend winters in Mexico before traveling as far as 3,000 miles north into Canada to lay eggs.
“Monarch migration is very bird-like – they spend the winter in the tropics and fly into the temperate zones when the milkweed has come up so they can lay eggs,” said Lincoln Brower, research biologist at Sweet Briar College and well-respected monarch expert. “All the native milkweed plants die back in winter, so the monarchs cannot stay in the temperate zone. It’s an evolutionary adaptation.”
During the migration, monarchs stop to eat and reproduce along the route, settling down at night in clusters known as roosts, sometimes staying put for a week or more. It takes four generations of monarchs to make the annual spring migration north and return trip back to a wintering spot some eight to 10 months later, Brower said.
Milkweed is the monarch’s sole host plant because it is the only source of nutrition for monarch caterpillars. With 100-plus milkweed species in America, the natural milkweed buffet has historically been ample. Yet, with the rise of agricultural herbicides designed to eradicate weeds among farm crops, milkweed has been practically eliminated in many areas and, as a result, monarch populations greatly reduced. The U.S. Fish and Wildlife service estimates that the population of monarchs is smaller by nearly a billion insects than a quarter-century ago. According to Monarchwatch.org, a roost of butterflies that in the mid-1990s covered 20 hectares – or about 50 acres – at overwintering sites in Mexico would cover less than a hectare – or 2.47 acres today.
As conscientious gardeners strive to offer more milkweed on their property, Brower and other biologists are concerned that the presence of tropical milkweed outside neo-tropical zones such as Mexico and Southern California may be contributing to the population loss. Some sites where tropical milkweed is prevalent attract large roosts of monarchs that interrupt their typical migration patterns to stick around the Asclepias curassavica, which “becomes a trap for the butterflies,” Brower says.
Monarchs, normally in a non-reproductive phase in the fall, also may lose the urge to migrate further south once they encounter tropical milkweed on their way. This creates a “continuously breeding population” that faces starvation when plants die back for winter. It also increases the prevalence of a parasitic disease known as Ophryocystis elektroscirrha (OE), which creates devastating health problems for monarchs – particularly for those exposed through multiple generations. Monarchs infected with OE spores are not strong enough to complete the migration and die when the weather turns cold.
Some wildlife gardeners are now cutting back their tropical milkweed to the ground in early fall to keep the monarch from interrupting their flight southward.
But Brower recommends foregoing the lovely curassavica altogether outside Mexico and Southern California and instead planting some of the many native Asclepias species, such as tuberosa (butterfly milkweed), syriaca (common milkweed), incarnate (swamp milkweed), quadrifolia (fourleaf milkweed) and others to serve as caterpillar hosts and feeding stations for the butterflies.
To learn more about the plight of the monarch, visit monarchwatch.org, a project of the University of Kansas that offers resources for supporting the monarch migration and encourages the development of Monarch Waystations – or habitats – throughout the U.S. So far, 5,000 certified habitats have been created by individual gardeners, schools, city and county governments and private companies.