Monarch population numbers have fallen by 90οΏ½percentοΏ½in less than 20 years. This year’s population was the second lowest since careful surveys began two decades ago. The critical driver of monarch decline is the loss of larval host plants in their main breeding habitat, the Midwestern Corn Belt. Monarchs lay eggs exclusively on plants in the milkweed family, the only food their larvae will eat.
Monarch butterflies have long coexisted with agriculture, but the proliferation of herbicide-resistant GE crops is threatening that balance. Monsanto’s glyphosate-resistant Roundup Ready corn and soybeans have radically altered farming practices, sharply increasing the extent, frequency and intensity of glyphosate use on farm land. Glyphosate—one of the very few herbicides that kills common milkweed—was little used two decades ago, but has become by far the most heavily used herbicide in America thanks to GE Roundup Ready crops. As a result, corn and soybean fields in the Corn Belt have lost 99οΏ½percentοΏ½of their milkweed since just 1999.
Milkweed is a perennial that often bears blossoms and fruit at the same time. The plant may be 0.5-1.0 meters high. Greenish-white flowers are borne in umbrella-like clusters. Leaves may be narrow or broad. Leaves or other above-ground parts of the plant are poisonous. They contain several glucosidic substances called cardenolides that are toxic. Milkweed may cause losses at any time, but it is most dangerous during the active growing season.
Several species of milkweed are poisonous to range animals. Labriform milkweed (Asclepias labriformis) is the most toxic. Other species in order of toxicity include western whorled milkweed (A.subverticillata), woollypod milkweed (A. eriocarpa), and Mexican whorled milkweed (A. fascicularis).
Milkweed poisoning occurs frequently in sheep and cattle and occasionally in horses. Most livestock losses are a result of hungry animals being concentrated around milkweed-infested corrals, bed grounds, and driveways. Poisoning also may occur if animals are fed hay containing large amounts of milkweed.
The primary toxic principle, galitoxin, is of the resinoid class. Galitoxin is found in all vegetative parts of the plant. In addition, a group of toxicants known as cardenolides may be responsible for digitalis-like signs that cause or contribute to death. In general, it appears that the broad-leaved species produce cardiotoxic and GI effects while the narrow-leaved species are more commonly neurotoxic. Dosages of whorled milkweed as low as 0.1 % - 0.5% of the animal's body weight may cause toxicosis and, possibly, death. Cattle, sheep and horses are most susceptible. Toxicity is not lost when the plant is dried. Therefore, contaminated hay is potentially toxic.
In the 1990s and early 2000s, Jenkins said, he would tag hundreds of monarchs in a week. Now, like many others across the continent, he sees fewer monarchs each year, though there are occasional influxes. Some years, the number he tags drops to single digits.
Jenkins is witnessing the North American monarchs’ rapid decline firsthand. Eastern monarch populations have decreased by at least 80 percent in the past two decades, according to Xerces Society for Invertebrate Conservation. The Western monarch, which migrates only on the coast of California, has declined by 99 percent. The entire North American population is currently under review to be placed under the Endangered Species Act.
It was an unusual sight Tuesday in Riverside Park. A group of University at Buffalo students armed with white, mesh butterfly nets fanned out over the Olmsted park in the northwest corner of Buffalo in search of common milkweed.
The unadorned, bushlike plant with the oblong leaves might be considered an unattractive nuisance in most gardens.
However, it is the primary source of sustenance for the monarch butterfly. It was purposely planted in various spots in the park by the Olmsted Parks Conservancy to attract the butterfly.
That makes Riverside Park an ideal place for capturing and tagging the colorfully winged critters before they continue their migration south, said Dr. Nicholas Henshue, a professor in the Department of Environment and Sustainability at UB.