Photo credit: William Warby

A study published this summer by researchers at the University of Guelph in the Journal of Animal Ecology reports that the use of herbicide-resistant genetically modified (GM) crops are largely responsible for declining monarch butterfly populations. Monarch butterflies rely exclusively on the milkweed plant to lay their eggs and as a food for monarch caterpillars.  Increased use of GM corn and soybean crops across the Midwest means that herbicides are applied more liberally, killing milkweed plants and leaving monarchs with few egg-laying sites and food sources. The study found that almost 70 percent of milkweed plants used by monarch butterflies are found in agricultural landscapes, and that the increased use of GM crops over the past two decades has led to a 21 percent decline in the number of milkweed plants. This decline has had a greater negative impact on monarch butterfly populations than either deforestation or climate change. The authors concluded that reducing the use of intensive agricultural methods and GM crops should be the highest priority for monarch butterfly conservation.