MEXICO CITY (Reuters) - Hundreds of acres of forest that are a home to the imperiled Monarch butterfly have gone up in flames in the last four days, Mexico's environment ministry said on Wednesday.

Conservationists say the destruction of the winter habitat by the blaze -- which is still burning -- could threaten hundreds of millions of Monarchs with extinction.

The bright orange and black butterflies have captured the imagination of environmentalists for their migration from Mexico to Canada and back and draw tourists to their Mexican sanctuary from November to April when they festoon the trees.

Eight hundred and fifteen acres have been burned in a 2,100 square-mile nature reserve in mountainous central Mexico, the ministry said in a statement. The fire began Sunday afternoon and was 75 percent under control on Wednesday, it added.

Scientists say the blaze will dry out the forest and thin the foliage canopy.

``The butterflies use the forest as an umbrella and a blanket,'' said Lincoln Brower, a biology professor at Sweet Briar College in the U.S. State of Virginia. ``If they get wet, they lose their tolerance to freezing,'' he told Reuters by telephone.

The area burned in the last four days is part of a nature reserve set aside in 1986 by Mexico as a home for butterflies fleeing harsh northern winters.

The environment ministry said the fire is outside the area designated a ``core zone'' for the butterflies.

But Brower said research shows colonies of Monarchs also reside in the outer ``buffer'' zone, currently in flames.

``Wonder Of Biology''

The director of the local forestry commission, Rene Orozco, told Reforma daily that Tuesday's fire damage had not endangered the equilibrium of the nature reserve.

But scientists and environmentalists were not so certain.

``The migration and overwintering biology could collapse completely in the eastern United States. As far as we know they don't know where else to go,'' said Brower. He has studied the Monarchs for 45 years and calls their overwintering ``one of the world's wonders of biology.''

Juan Carlos Cantu, biodiversity director at Greenpeace Mexico, said fires ruin forever the Monarchs' Mexican home.

``If we destroy their home here, it wouldn't be worth protecting them outside Mexico,'' he said.

Monarchs breed in the spring and summer on the gulf coast of the United States and in the Great Lakes region.

All Monarchs from east of the Rocky Mountains winter in Mexico in an area one millionth the size of their northern breeding grounds, Brower said.

Cantu and Brower both said Mexico lacked the resources to protect nature and failed to enforce forestry regulations.

``Mexico is not prepared to protect its reserve areas from fire,'' he told Reuters. ``It doesn't have the infrastructure, it doesn't have the trained personnel, and it doesn't have the culture of educating communities on how to prevent fire.''