A Mexican environmentalist says studies show illegal loggers have clear-cut at least 24 acres (10 hectares) in the monarch butterflies' wintering grounds in central Mexico.

Writer and activist Homero Aridjis says the illegal logging went on unchecked between April and August, and occurred in one of the most important areas of the reserve.

Earlier, officials had said the reserve lost about 22 acres (9 hectares) due to illegal logging in one area this year, and said 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 Friday to stop all illegal logging in the butterfly reserve.