Several factors contributed to the monarchs’ decline. Caterpillars’ sole food, milkweed, was declared a noxious weed that must be eradicated. Flowering plants along roads and unkempt lands — prime habitats — were sprayed. While herbicides killed their food, insecticides used on crops or against mosquitoes weakened and killed butterflies, bees and other pollinators.
Migration, like any great expedition, has small margins for error. En route to and from Mexico, monarchs might not only be starved, baked with climate change and poisoned with pesticides — they could even be getting lost.
Sensing the Earth’s magnetic field is essential for migration, and scientists examining mechanisms that may be disrupted by radio frequency radiation have landed on a light- and magnetic-field sensing biochemical called cryptochrome, found commonly in all species.
Monarchs brave many challenges as they migrate to overwinter in Mexico. To the rescue, human advocates protected corridors (e.g., in the central U.S.) and planted beneficial plants. This isn’t enough. Monarchs are now endangered, and insect-eating and migratory birds are declining in company with struggling insects. Pollinator declines risk fruit and vegetable crops.