But across the midwestern US, this familiar seasonal arrival has become more of a rarity of late. Now research shows that restoring the milkweed plant – essential food for growing caterpillars – to marginal agricultural land will be key if we want to see the monarch return to this region in significant numbers.

Over the last two decades the eastern migratory population of monarch butterflies, which breeds in the US and Canada but migrates to Mexico for the winter, has decreased by 84%. This dramatic decline is almost certainly due to loss of milkweed. Previously this plant grew vigorously amongst corn and soy crops, which cover 42% of the landscape in the Midwest. But in recent years corn and soy have been modified to withstand the application of the herbicide glyphosate. As a result, milkweed is now scarce in the corn and soy fields, and vast tracts of monarch habitat have been lost.

The United States Fish and Wildlife Service is now considering whether the monarch should be listed as a threatened species under the Endangered Species Act, and across the country conservationists are looking at ways of restoring monarch populations.

Wayne Thogmartin from the United States Geological Survey and his colleagues developed scenarios for re-introducing milkweed to the Midwestern US landscape. They used a model to assess how effective each of those scenarios is likely to be in terms of increasing monarch numbers.

Of the 98 million hectares of land they considered, 46% was devoted to agriculture, not including pasture or hay-land; the majority was corn and soy fields. Approximately one quarter of the landscape was forested (providing some milkweed, but not a principal monarch habitat), whilst urban environments made up a little over 5% of the region. Rights of way – roads, power line corridors, rail lines and the like – covered about 6% and grasslands formed around 14% of the landscape. The remaining 6% was water.

The scientists considered 218 milkweed regeneration scenarios. Of those 218, only 16 were capable of restoring the 1.3 billion stems of milkwood considered necessary to abate the risk of extinction faced by the monarch population. One of these scenarios converted all marginal agriculture to conserved status, and the other 15 converted half of marginal agriculture to conserved status, with the remaining milkweed boost being provided by other land types.

Scenarios without significant agricultural participation were all found to be insufficient for attaining the population goal. "Agricultural lands are essential to reaching restoration targets because they occupy 77% of all potential monarch habitat," Thogmartin and his colleagues write in their paper in Environmental Research Letters (ERL) .

However, this doesn't necessarily mean that soy and corn farmers have to allow milkweed back into their fields. "There are marginal lands that farmers are currently incentivised to plant," said Thogmartin. "Removing the incentives to grow crops on these poorly producing lands could add more grass to the landscape, leading to more habitat for monarch butterflies and other species too." Meanwhile, discouraging farmers from spraying field borders could also help milkweed to creep back in.

Saving monarch butterflies could have the added benefit of saving many other species. "It’s likely that the decline in monarch butterflies is symptomatic of declines seen in other insects, including species that we rely upon to provide pollination for our food, and sustenance for the wildlife we care about," explained Thogmartin. As American conservationist and philosopher Aldo Leopold once wrote, "To keep every cog and wheel is the first precaution of intelligent tinkering".

Related links

Related stories