"Deforestation is the most likely culprit," Moeller said.
During breeding season, monarchs produce a new generation in about 30 days. The generation that will leave North America this fall for Mexico are the great-great-grandchildren of those who left there last spring.
Monarchs flitting across the state this summer are looking for swamp milkweed to use as worm nurseries. The butterfly will only deposit its eggs on milkweed leaves.
Many gardeners and farmers destroy the butterfly's host plant, which is considered a noxious weed in some states.
"The milkweed that used to be in the fields is gone," said Karen Oberhauser, an entomologist with the University of Minnesota Extension Service.
Some area butterfly lovers are attempting to bring the number of monarchs back up by providing a milkweed oasis in their backyards. Jo Amiot of Mankato, who carefully controls a little patch in her Mankato backyard, noted a small increase in butterfly numbers this summer.
"Last year, there were seven, and so far this year, 15," she said.
Author of "The Mystery of Monarch Metamorphosis," Amiot photographed the butterfly in all stages of its life cycle as her subject for the Minnesota Heritage Publishing book. In summers past, she's snapped pictures of up to 25 chrysalis from her backyard. In the summer of 2012, she found only two.
A productive breeding season this summer is essential for the population to recover. Monarchs breed across a large region, which could add to its comeback potential, according to Journey North News.
Moeller suggestions for helping monarchs include not using pesticides. Butterfly populations often take a direct hit from the poisons intended to kill destructive bugs.
"We need to provide monarchs with more habitat," Moeller said.
A little garden space with milkweed and nectar-producing flowers will help plant pollinators as well as butterflies. Monarch will find milkweed blossoms, even when they are growing in little gardens in the middle of big cities, he said.
"There's really good evidence that vegetables do better when they are planted near wildflowers," Moeller said. "So, help a monarch and you will help a tomato."