While one study suggests populations have been falling steadily for 17 years, another says the species numbers are holding steady in the eastern United States, NewScientist.com reported Thursday.
Monarch butterflies migrate south to Mexico in the autumn, returning to the United States and Canada in the spring.
Earlier this year a study by Ernest Williams of Hamilton College in New York and others showed that winter populations in Mexico hit an all-time low in 2009-10.
But another study by Andy Davis of the University of Georgia, analyzing counts of migrating monarchs from two sites in New Jersey and Michigan for more than 15 years, found no evidence of a population decline at either site.
Many butterflies may die on the way to Mexico, which could explain the discrepancy, Davis said, with the monarchs rebounding from bad winters by breeding rapidly the following spring.
Williams disagrees and says the U.S. counts were conducted in the wrong places.
Most monarchs come from central states such as Nebraska, where their habitat and food are in decline, he says, while New Jersey and Michigan are on the edge of their range where their habitats are as yet undamaged.
A census from the core of the migration route would reveal a different picture, Williams says.