MEXICO CITY — 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 directly affected 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 for weapons possession 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.

Ramon Paz, the spokesman for Mexico’s Association of Avocado Export Packers and Producers, said almost all avocados are grown on land that was not deforested to make way for plantations.

“We have been accused a lot of deforestation, but according to the information we have … between 85 and 90 percent of the area planted with avocado trees was previously occupied for other agricultural uses,” Paz said. “The vast majority of avocado orchards have been in agricultural use for many years” Paz said, adding they were usually corn fields.

Paz said his group, known as APEAM, was concerned about conserving forests, noting “we don’t want our product to be perceived as one that results from deforestation.”

Paz said the APEAM is working on getting its avocados certified as sustainable, and embarked four years ago on a project to plant about 250,000 pine seedlings annually.

Still, while prices have dropped from last year’s higher levels, avocados are much more lucrative than almost any other legal crop Mexican farmers can grow, and many landholders appear to be turning to avocados, legally or illegally.

However, the avocado trade also employs nearly 100,000 people in Michoacan, mostly produced by small farmers.

Paz said 80 percent of the state’s 23,000 avocado producers had plots of 12 acres or less.