Where Have All The Monarchs Gone?

Sep 20, 2013

A monarch on its favorite habitat, the milkweed.
Credit Creative Commons/Rachel Ford James

A lot of people are asking this question, concerned at the diminished numbers of this most charismatic butterfly.  Not many schoolchildren this fall will be able to watch caterpillar transform into chrysalis and then glorious adult—metamorphosis in action.

Monarchs are celebrated for their fall migration to Mexico, but the population that spends the wintering there is experiencing a decline. In fact, this past winter it was the lowest on record.

The main cause of this decline in the U.S. is the loss of milkweed habitat, which in turn is a result of increasing use of herbicides. As every gardener knows, milkweed sprouts in garden soils.

Back when prairie grasslands were converted to corn and soybean cropland, milkweed flourished in and among the crops as well as at field edge. Since milk weed is the monarch's one and only food source, Monarch caterpillars thrived.

In the mid 1990s herbicide use increased, including the introduction of crops genetically modified to tolerate glyphosate, an herbicide that kills all non-crop plants including milkweed. Today 90% of corn and soybean crops are genetically modified to survive this spraying.  

Additionally, last summer's excessive heat and drought in the central and Great Lakes states—where most monarchs breed—added further challenges for the butterfly. Their eggs don't survive temperatures in the high 90s, and drought withers milkweed along with other nectar plants.

Increasing weather extremes present survival challenges beyond herbicides for a species that's known and celebrated nationwide.

It's also a species widely studied in a search for answers relevant to all pollinating insects, not just the iconic monarch.