The main cause of the monarch butterfly’s decline is the loss of milkweed — its food — in its U.S. breeding grounds, a new study has found. That all but confirms that the spread of genetically modified crops is indirectly killing the monarch.
This past winter, the number of monarch butterflies wintering in Mexico fell to its lowest since 1993, when records first started being kept, the World Wildlife Fund and Mexico’s Environment Department reported in January. That report blamed the loss of milkweed owing to genetically modified crops and urban sprawl in the U.S. and illegal logging in the butterflies’ Mexican wintering ground.
Now, an analysis combining all the known data about monarch populations and the factors that influence them shows that the monarch’s biggest threat is in the U.S., not Mexico.