The monarch butterfly population dropped by a quarter in its Mexican wintering grounds this season and US President Donald Trump's planned wall could affect its migration from Canada, Mexican authorities 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 the previous year, a 27.4 percent decline, the government said.

The monarch population is estimated by the area of forest it occupies in the states of Mexico and Michoacan, where it overwinters.

Last year, cold fronts and snow slammed 100 hectares of woods in the central mountains where the butterfly spends the winter months after traveling 4,000 kilometers (2,500 miles) across Canada and the United States.

"The causes for this drop are mainly the extreme climate events," Alejandro Del Mazo, Mexico's commissioner for protected areas, 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 some seven percent were killed in a storm in March, around the time they make their journey back across the United States.

Del Mazo warned that the butterfly's survival could be threatened by the massive wall that Trump says he plans to build across the 3,000-kilometer border.

The barrier could change the natural markers that guide the insect across the border, he said.

"Without a doubt, fragmenting the habitats and maybe causing changes to the tributaries and rivers that we share with the United States could have an impact," Del Mazo said.

"Along its route, this insect also needs to identify refuges, climate events and water that, if altered, could have some impact, but it's very early to say," he cautioned.

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 when in caterpillar form -- in the United States and Canada.

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