By Susan Milius
A fuss over trends in monarch butterfly populations has flared up with a flurry of new research papers, all based on records from volunteer butterfly watchers.
There’s no dispute that numbers of monarch butterflies are dwindling at winter refuges in central Mexico (SN: 4/23/11, p. 18). But eight papers published online August 5 in Annals of the Entomological Society of America split over the reason why.
In one view, the monarch population that summers east of the Rockies is shrinking year-round, not just in winter. In the butterfly breeding heartland of the U.S. Midwest, the decline may come from loss of the milkweeds that caterpillars need for food, explains Karen Oberhauser of the University of Minnesota in St. Paul. It takes about 29 milkweed plants, Oberhauser’s team has found, to produce a caterpillar that grows up to fly off toward Mexico. But the once-common plants are growing scarcer as farmers intensify weed control for genetically modified corn and soybeans engineered to tolerate herbicides.