Monarchs may be facing more threats than researchers once thought. Not only are monarch butterflies suffering from a lack of milkweed and the threat of herbicides and genetically modified crops, but they are also suffering from sparse autumnal nectar sources as well as weather and habitat fragmentation.
In any given year, a total of four generations of monarch butterflies traverse much of North America over a 2,000-mile trek beginning in early spring when they leave their Mexican wintering grounds. In the first generation, millions of monarchs fly through Texas and Oklahoma. The following generations move into the Midwest and Northeast. When autumn hits, though, the fourth generation returns to Mexico for the winter.
The monarch butterfly population reached an all-time low about two years ago. However, the population today is about six times what it was then. This can be largely attributed to improved weather and release from the severe drought in Texas.
With that said, there are still threats that face the butterfly population, and that's why the researchers in this latest study decided to investigate the issue a bit more closely.
"Thanks to years of data collected by the World Wildlife Fund and citizen-scientists across north America, we have pieced together the monarch life cycle to make inferences about what is impacting the butterflies," said Anurag Agrawal, professor of ecology and evolutionary biology at Cornell University and senior author of the study.
The researchers actually didn't find evidence to support the "milkweed-limitation hypothesis" during the monarch's breeding season during the summer in the Midwestern and northeastern U.S. Instead, they found problems in the transition from the U.S. and southern Canada to the overwintering grounds in Mexico.
So, what's happening? It's likely that the monarchs aren't finding enough nectar to feed on as they travel long distances. In addition, threats such as habitat loss and insecticide can also pose problems to the traveling monarchs, especially as populations of these butterflies continue to decline.
"Given the intense interest in monarch conservation, the blame being put on herbicide use and the national dialog about potentially listing monarchs under the endangered species act, we have to get the science right," Agrawal said.
The findings were recently published in the journal Oikos.