The population of monarch butterflies has been steadily decreasing over the last 20 years, and it's just a matter of time before they completely disappear.

Although there is no definitive conclusion as to how and why these black and white insects are vanishing, previous research puts the blame on the flowering milkweed plant -- the only food eaten by monarch caterpillars.

Milkweed Availability

Professor Karen Oberhauser of the University of Minnesota, one of the authors of a study published in the journal Nature, says because farmers use more genetically-engineered crops and herbicides, the availability of milkweed has declined in the United States.

While climate change and extreme weather could be affecting the population of monarchs, Oberhauser says the simplest explanation is usually the best explanation. In science, this is called Occam's Razor.

Oberhauser established the argument: monarchs need milkweed, but the availability of the plant has declined. This decline correlates tightly with the size of the winter population. She says this is probably the most simple and right explanation for what has driven the dip in numbers.

However, a report by The National Academies of Sciences on genetically modified crops did not prove any link between disappearing milkweed and starving monarch caterpillars.

Additionally, a study by Hidetoshi Inamine of Cornell University suggests that the declining availability of milkweed may not even be the biggest threat to monarch butterflies.

Iname and his colleagues discovered that most of the decline in population is occurring when these insects turn around Canada to go south, when they are no longer dependent on milkweed. In fact, adult monarch butterflies feed on other plants and flowers laden with nectar.

He says that as some nectar-bearing plants are also waning in numbers, it could be a far greater threat to mature monarchs than the decreasing milkweed availability. The team's findings are issued in the journal Oikos.

Finding Solutions

Although there is a possibility that milkweed may not be the culprit for monarch decline, some scientists believe planting more milkweed could boost monarch populations as they travel across the Great Plains.

Bob Trout of the Loveland Initiative for Monarch Butterflies is leading a pack of conservationists that plant the edge of small ponds and riverbanks with milkweeds.

"We're trying to save the migration is what we're trying to do," says Trout.

Photo: John Flannery | Flickr