In recent years, news releases from Mexico based on aerial imagery have claimed that illegal logging inside the reserve has been reduced to virtually zero. But these reports are incomplete and misleading, as they measure only visible deforestation in the core zone of the reserve, about a fourth of the protected 217-square-mile area. Forest loss in the surrounding areas is not reported, nor is the magnitude of local logging, in which individual trees are removed from the core area. This “selective cutting” is not visible in the aerial imagery, and yet it alters the forest microclimate and increases monarch butterflies’ risks of starving and freezing.

Ecotourism is presenting an additional threat to the butterflies as its popularity increases. As regular visitors to the monarch colonies, we have seen the conditions in these areas deteriorate, for butterflies and tourists. Along the heavily used trails that lead to the colonies in Piedra Herrada and Sierra Chincua, extensive areas of vegetation have been killed. Excessive dust, which now rises into the air with each step, is hazardous to the butterflies because it clogs their respiratory orifices. When we visited the Piedra Herrada site this February, along with former President Jimmy Carter, a welcome sign on the trail leading to the butterflies read, “No more than 20 people in the Sanctuary.” And yet we counted 24 tourist buses in the parking lot.

So what can be done? Ecotourism is an important part of the local economy, but we must make sure that its costs in habitat degradation and increased butterfly mortality don’t kill the goose that lays the golden egg. The Mexican government has made strides in reducing much — but not all — illegal logging, and needs to do more. The United States, for its part, should re-examine the extent to which industrialized herbicide-based agriculture is destroying the flora in the Midwestern United States that monarchs depend on in the spring and summer. In addition to all of this, we simply need better data on the butterflies.

Measuring the size, location and survival rate of each colony that winters in Mexico is the responsibility of World Wildlife Fund Mexico and the staff of the Monarch Butterfly Biosphere Reserve. Sites are measured in December, and the total areas in each of the major locations are reported, usually in February or March. But, although W.W.F. Mexico has more specific data, the published reports are not detailed enough to allow scientists to review the status of individual colonies or to evaluate the butterflies’ survival in relation to forest condition. W.W.F. Mexico must release data on where the butterflies roost each year, whether each colony is increasing or decreasing in size, and what the mortality rates are at each site.