Butterfly milkweed is essential for the breeding and diet of monarch butterflies, yet this formerly common plant can now be found in only 8% of Iowa landscapes. In 2013, the monarch population had declined by 82%.
The primary threats to the monarch butterfly include the loss of milkweed—the key plant that monarch caterpillars need to survive—from agricultural and natural areas, degradation of overwintering sites, and climate change. The large-scale use of systemic insecticides such as neonicotinoids within the breeding range of the monarch may pose a considerable threat. Natural enemies such as diseases, predators, and parasites likely also influence the size of the monarch population. Loss of milkweed from the American Midwest is primarily due to the dramatic increase in the use of the herbicide Roundup™ (glyphosate), made possible by the mass-planting of Genetically Modified Herbicide Tolerant corn and soy. Illegal logging has threatened overwintering sites in Mexico, and in California, numerous sites have been logged and replaced with housing developments. Extreme weather events may be negatively impacting monarchs in the eastern U.S. and low monarch populations in California are correlated with years of intense drought. Climate change models predict that future climate scenarios will not be suitable to support overwintering monarchs or the oyamel fir trees that they use in Mexico. – From The Xerces Society