The organization Journey North helps students and other citizens plot the paths of various migrating species.
It says this year's reports indicate that the decline in monarch numbers over the past 10 to 15 years appears to have been much steeper this summer.
Biologist Jeremy Kerr told the Ottawa Citizen that he thinks numbers are now down by as much as 90 percent.
Loss of habitat and pesticide use due to expanding agriculture is mainly to blame, according to experts.
Last summer’s extreme drought in the U.S. corn belt wiped out huge numbers of milkweeds, which the monarchs need to breed and feed.
Elizabeth Howard of Journey North says that was a fatal blow to many of the iconic fliers.
Monarchs typically live only four to five weeks, except for the generation that emerges in late summer. That’s the one that migrates the entire way southward to the species’ wintering grounds in the Mexican state of Michoacan.
The small number of southward migrants this autumn has caused even more concerns over the long-term future of the world’s longest-migrating butterflies.
Photo: Casey Overby