“Scientists originally suspected the problem was illegal logging in local forests, which are protected as part of the Monarch Butterfly Biosphere Reserve. But as they later discovered, the decline has a lot more to do with what’s happening thousands of miles away in the US. Over the last few decades, industrial farms have decimated grasslands rich in milkweed, the only plants that monarch caterpillars can eat. Less food in the US means fewer butterflies thousands of miles away in Mexico.”
https://www.vox.com/down-to-earth/22823993/monarch-butterflies-mexico-milkweed