But this year, the numbers are starting to pick up. Biologists and volunteers across California have already counted more than 100,000 monarchs.
Richard Rachman is the coordinator for the Xerces Society's annual Thanksgiving monarch count in Los Angeles County, and has been buoyed by the numbers.
Trump administration officials are expected to say this week whether the monarch butterfly, a colorful and familiar backyard visitor now caught in a global extinction crisis, should receive federal designation as a threatened species.
Stepped-up use of farm herbicides, climate change and destruction of milkweed plants on which they depend have caused a massive decline of the orange-and-black butterflies, which long have flitted over meadows, gardens and wetlands across the U.S.
Monarch population numbers have fallen by 90�percent�in 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 99�percent�of 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.