MEXICO CITY (AP) — Mexican environmental inspectors said Wednesday that they found 7.4 acres of illegal avocado plantations in the Monarch butterfly wintering grounds west of Mexico City.

It's apparently the first time that a wave of avocado planting has had a significant, direct effect on the heart of the Monarch area, a protected nature reserve.

Monarch butterflies migrate from the U.S. and Canada to pine and fir forests that thrive at about the same altitude as prime avocado-growing land.

Previously, deforestation linked to lucrative avocado planting had been seen in areas to the west and south of the reserve.

But on Wednesday, the environmental protection office said one man had been arrested at the site.

In April, police found that a 91-acre swath of pine trees had been cut down in the nature reserve of Valle de Bravo, a bit east of the butterfly reserve, to plant avocado trees.

Without pine trees to provide thermal cover and roosting sites, the butterflies can freeze to death. While the Monarch is not in danger of extinction, its amazing 3,400-mile migration is endangered. No butterfly lives to make the round trip, and experts are still studying how they "remember" the route.

Previously, experts had estimated that Michoacan — the state where part of the reserve is located, and the biggest avocado-producing state in Mexico — loses about 15,000 to 20,000 acres of forest land annually to avocado plantations.