The main cause of declines in the monarch butterfly population is loss of habitat in breeding grounds in North America, particularly the U.S. Corn Belt, says research by the University of Guelph. The study says large-scale farming contributed to a 21 percent drop in milkweed plants from 1995-2013. Monarch caterpillars feed on milkweed before becoming butterflies. Monarch populations will fall an additional 14 percent due to milkweed loss, said the study. One of the researchers suggested planting more milkweed.
In some areas, milkweed, poisonous to animals, is listed as a noxious weed, said the CBC, based in Canada. It quoted a co-author of the study, Ryan Norris, as saying “likely the biggest cause of loss of milkweed is the adoption of genetically modified crops.” The crops allow use of herbicides that kill milkweeds.