Lack of autumnal nectar sources, habitat loss, weather, and insecticide, may be causing a decline of monarch butterfly‘s population, according to researchers from Cornell University. Previous theories have suggested that butterflies were being affected by a lack of milkweed, but the study could not find evidence to support it.

A team conducted Hidetoshi Inamine, a graduate student of evolutionary ecology, have analyzed “years of data” registered by the World Wildlife Fund and citizen-scientists in North America. They studied the monarch life cycle, to better understand what is leading them to a population decrease.

A monarch touches down in the ecosystem of this purple thistle flower. Source: Discovery News/ISTOCKPHOTO/THINKSTOCK

Scientists did not find evidence to support the latest theories suggesting that a limitation of milkweed, may be affecting the monarch, during its summer breeding season in the midwestern and northeastern United States. Results were published April 4 in the journal Oikos.

By contrast, the team encountered complications in the butterflies’ transition from the U.S. and southern Canada, to overwintering grounds in Mexico, according to a press release issued by Cornell University. They explained that milkweed is only a food source for caterpillars, during summer.

However, the favorite plant of monarchs is not a food source when they carry out a massive southern migration in autumn. According to Anurag Agrawal, Cornell professor of evolutionary ecology and senior author of the paper, the key to understanding the population’s decline, may be laying on fall flights.

“Lack of milkweed, the only host plant for monarch butterfly caterpillars, is unlikely to be driving the monarch’s population decline, as the problem appears to occur after they take flight in the fall,” said Agrawal in a statement.

Even when monarchs are facing threats in Mexico, “population is six times what it was two years ago”

Within an average year, four generations of the most popular American butterfly flight across North America, on a 2,000-mile journey that starts in early spring, at the Mexican wintering grounds.

The first generation flows through Texas and Oklahoma while the next generations move into the Midwest and Northeast until early fall, when a fourth generation comes back to the “mountainous, high-altitude Oyamel fir forests”, located in Mexico, according to researchers.

Researchers have suggested that monarch populations have been declining during the last years. According to Professor Agrawal, a “consistent decline” at overwintering locations in Mexico is causing concern. However, the population is currently six times bigger than two years ago.

A theory suggests that “improved weather and release from severe drought” in Texas, may be contributing to a population regrowth. At the same time, other elements such as lack of nectar sources, habitat loss or insecticide may be conspiring against monarchs.

“Given the intense interest in monarch conservation, the blame being put on herbicide use and the national dialogue about potentially listing monarchs under the endangered species act, we have to get the science right,” said Agrawal.

Butterflies play a relevant role in the pollination of flowering plants since they move pollen from plant to plant. Moreover, they are an important element of the food chain, since they are the prey of birds and bats in some regions, according to the Butterfly Conservation Organization.

Source: Cornell University