Years ago, residents of Fishers Island reported large numbers of monarch butterflies clinging to the rocks at Race Point, hitching rides on ferries, and collecting in trees in such numbers that the trees themselves would appear on fire. Today, those same residents might claim to see only a handful each year.
What happened to the monarch? The species has been in decline since the 1990s, most notably due to the loss of its food source: milkweed plants, which typically have grown in large swaths across the farm belt. Changes in agricultural practices and herbicide applications have caused a decline in milkweed plants, which in turn caused a decline in summer habitat for monarchs. Only recently, people have begun planting milkweed on private properties and parklands to encourage the population of monarchs to bounce back.
The Conservancy has developed a substantial amount of habitat to support summer monarch populations. In addition to milkweed, which provides both nectar and a safe place to lay eggs, there are fall-flowering plants, like goldenrod and fall asters, which provide good sources of nectar to feed monarchs as they migrate south into Mexico. The result has been an increase in the number of monarch butterflies on the Island, particularly in the Parade Grounds.
There is a new threat, however: the arrival of black swallow-wort, which has recently become invasive. A vine native to the Mediterranean, it was introduced into the U.S. in the 19th century as a garden plant and moves into landscapes secreting toxins that help outcompete or kill native plants. Particularly troubling is that the fact that black swallow-wort releases chemicals that mimic a “smell” similar to milkweed plants. When a monarch is searching for milkweeds to lay her eggs, she may mistake the swallow-wort for milkweed, and when her caterpillars feed on the leaves, they are poisoned and die. The black swallow-wort grows in the same habitat as milkweed, so this mistake can happen frequently.
By Adam Mitchell, Associate Wildlife Biologist®
PhD Candidate, Department of Entomology and Wildlife Ecology, University of Delaware
Learn more about the ongoing battle against swallow-wort:
- How to manage black swallow-wort.
- Island gardener fights the invader in her own garden: