New evidence reported on Wednesday shows that the number of monarch butterflies completing their annual migration to their winter home in a Mexican forest has dropped to a twenty year low. Experts believe the drop is due to a one-two punch: extreme weather and changing farming practices in the United States.
It is impossible to count the exact number of monarchs during the migration, so scientists measure the area of forest the butterflies occupy. Once, the butterflies occupied up to 50 acres of forest, but the annual census conducted last December showed the monarchs only occupied 2.94 acres, a decrease of 59 percent from the previous year.
The drastic decline was brought on in part by the drought and intense heat last year. The heat led the insects to arrive earlier last spring and travel further north than usual, disrupting their breeding cycle. In addition, higher temperatures dried out monarch eggs and lowered the nectar content of milkweed, their primary food source.
Increased planting of genetically modified soybeans and corn also impacted butterfly numbers. These modified crops are designed to tolerate herbicides. In previous years, monarchs could feed on plentiful milkweed growing in the American Midwest between rows of soybean and corn, but now with the modified crops, farmers have eradicated most of the milkweed using herbicides.
So far, monarch butterflies still have a robust enough population to survive, but if their population drops much more, coming back from decline could be difficult. It will take concerted effort between Mexico and the United States to preserve this specie for future generations.