Texas A&M researcher Craig Wilson said reports by the World Wildlife Fund and Mexico's Michoacan state show Monarch numbers will be down almost 30 percent in 2012 as they make their annual trek north into the United States.
The figures show an alarming decades-long decline in their numbers, Wilson says.
"Last year's severe drought and fires in the region no doubt played a part, resulting in less nectar for the Monarchs as they migrated south," he said in a university release.
"But estimates show that each year, millions of acres of land are being lost that would support Monarchs, either by farmers converting dormant land for crop use -- mainly to herbicide tolerant corn and soybeans -- or the overuse of herbicides and mowing.
"Milkweed is the key plant because it's the only plant where the female will lay her eggs," Wilson said.
The loss of such lands is a critical issue for the Monarchs' survival, he said.
"We need a national priority of planting milkweed to assure there will be Monarchs in the future," Wilson said. "If we could get several states to collaborate, we might be able to promote a program where the north-south interstates were planted with milkweed, such as Lady Bird Johnson's program to plant native seeds along Texas highways 35-40 years ago.
"This would provide a 'feeding' corridor right up to Canada for the Monarchs."