Researchers at Cornell University who are studying the decline of Monarch butterflies have determined that it's not only a lack of milkweed, herbicides, and genetically modified crops that are contributing to the drop in Monarch populations. The scientists report that Monarchs also suffer from sparse sources of nectar in autumn, poor weather, and a decline in habitat.
Anurag Agrawal, professor of ecology and evolutionary biology, says the butterflies are having problems during the time they are moving from the United States and southern Canada to their overwintering grounds in Mexico. According to Agrawal's study, milkweed is an important food source for the Monarchs in summer, but not as they leave for their annual migration south in the fall. Agrawal said his research finds that a "lack of milkweed, the 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."
Although there's been a large decline in the Monarch population, there are six times as many Monarchs at their sites in Mexico as there were two years ago. Agrawal says the increase in population is due to improved weather conditions and an end to severe drought in Texas. He also says that a constant decline caused by a lack of nectar sources, use of insecticides, or habitat loss can conspire with large annual fluctuations in the Monarch population --mostly due to weather conditions -- and push the butterflies to dangerously low numbers. To save the Monarchs, says Agrawal, "we have to get the science right."
Additional information is at https://cornell.box.com/v/Monarch