Studies found that illegal loggers clear-cut at least 24 acres in the monarch butterflies' wintering grounds in central Mexico this year, a Mexican environmentalist said.
Writer and activist Homero Aridjis said the illegal logging went on unchecked between April and August and occurred in one of the most important areas of the reserve.
Earlier, Mexican officials had said that the reserve lost about 22 acres due to illegal logging in one area this year and that a number of arrests were made.
Illegal logging had fallen to almost zero in 2012. The butterflies depend on the pine and fir forests west of Mexico City to shelter them against cold and rain.
Aridjis called on authorities to stop all illegal logging in the butterfly reserve.