MEXICO-The monarch butterfly population dropped by more than a quarter in its Mexican wintering grounds this season due to cold weather caused by climate change, officials said Thursday.

The orange and black butterfly covered 2.91 hectares (7.2 acres) of pine and fir forest in the 2016-2017 season, compared to 4.01 hectares last year, a 27.4 percent fall, the government said. Last year, cold fronts and snow affected 100 hectares of woods in the central mountains where the butterfly spends the winter after traveling 4,000 kilometers (2,500 miles) from Canada. “The causes for this drop are mainly the extreme climate events,” Alejandro del Mazo, head of the national protected natural areas department, said at a news conference.  “There were deaths of monarch butterflies in the previous season and this, without a doubt, is one of the main causes for the reduction” this year, Del Mazo said.

The butterfly’s population had rebounded last year but seven percent were killed in a storm in March, around the time they make their journey back across the United States. The population in the states of Michoacan and Mexico is measured by the amount of forest it occupies. The monarch has been threatened by illegal logging in its Mexican habitat and the use of herbicides in fields of milkweed — the plant that it feeds on as a caterpillar — in the United States and Canada.

Mexico’s government has deployed a special police unit in the mountains to find illegal sawmills.