The police in the Michaocan towns of Angangueo and Ocampo were called in late Monday after the head of the management council of the El Rosario butterfly reserve vanished.
Prosecutors did not specify what made them suspect the police might have information on the whereabouts of Homero Gómez. Activists worry that Gómez' disappearance could be related to retaliation by illegal loggers.
Michoacan state police took over protecting the two towns while the local police were away.
Gómez is a former communal land officer who has led efforts to preserve the pine and fir forests where the butterflies spend the winter.
He posted his last video of himself in the El Rosario reserve Jan. 13, encouraging tourists to visit the reserve.
Monarchs need healthy tree cover to protect them rain or cold weather in the pine and fir mountaintop forests in Mexico where they spend the winter.
Millions of monarchs make the 3,400-mile (5,500-kilometer) migration from the United States and Canada, and then return, each year, though no single butterfly lives to make the complete trip.
Mexico has clamped down on illegal logging, which was once a major threat to the reserves but which has fallen to about one-third last year's level.