Despite reaching dangerously low numbers in the past, monarch butterfly populations are on their way to recovery as more and more of the orange and black insects have been spotted in recent months.

The Natural Resources Defense Council (NRDC) released its latest study on Friday that highlights the progress monarch butterflies have made in terms of their recovery. According to the group's estimates, the total number of insects that joined the annual journey south this year could reach as many as 150 million.

Researchers count the number of migrating monarch butterflies by measuring how much of the forest they cover after they arrive in Mexico from areas in Canada and the United States. In December, the insects were able to cover four hectares (9.9 acres) of the forest, which is larger compared to the 1.13 hectares (2.8 acres) recorded the previous year.

While the monarch butterfly numbers are up this year as estimated by experts last year, they are still considered to be well below the high of 1 billion insects set 20 years ago.

The NRDC said the comeback of the monarch butterflies could likely be caused as a result of favorable weather in recent months. Efforts to plant more milkweed plants have also allowed the insects to have enough food during their yearly migration.

"Today's news provides a hopeful indication that we are helping them head in the right direction and curbing the loss of this magnificent butterfly," Sylvia Fallon, a researcher from the NRDC, said. "But we must be careful not to declare victory too soon."

The findings of the NRDC report were largely based on data that shows how much of the Oyamel forest in Mexico is covered by the monarch butterflies during winter.

Researchers said populations of the insects were placed in danger over the past few years because of habitat loss, which is caused in large part by the growth of housing development projects, farmland expansion and the destruction of natural landscapes along the monarchs' typical migration path.

In the United States, for example, farmers and ranchers tend to destroy milkweed plants because they are particularly poisonous to cattle. However, these plants are important to the growth of monarch butterfly populations because they serve as a food source for the insects' larvae.

The U.S. government has initiated efforts to reintroduce milkweed plants on an area of about 1,160 square miles within five years to help monarch butterflies recover their numbers. Aside from planting more milkweed plants along the insects' migration route, the government has also established pesticide-free areas.

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