Monarch butterfly populations in central-northern Florida have been declining since 1985 and have fallen by 80% since 2005.
He studies coauthor Jaret Daniels, associate professor in the University of FloridaThe Department of Entomology and Nematology, said that this decrease can also be seen in the declining number of monarchs in their wintering territories in Mexico.
"It's alarming in many different ways," said Professor Daniels. "This study shows the close connection between monarchs and milkweeds and highlights an abundance of dramatic losses in Florida that further confirm the decline of the monarch."
While the drivers of the decline are not clear, the researchers found that the shrinkage of algae populations and an increase in glyphosate consumption in the Midwest are contributing to the problem. Glyphosate is a herbicide often used to eliminate weeds which is lethal for euphorbia, the host plant of monarchs.
Less alfalfa means less habitat for monarchs, said study co-author Ernest Williams, professor emeritus of biology at Hamilton College in New York.
"A general pattern is that 95 percent of the corn and soybean products grown in the United States are Roundup Ready crops that resist glyphosate," said Professor Williams. "This has a national impact: what is really needed are patches of native vegetation and sources of nectar without pesticides.It is not just for monarchs but for all pollinators.
In the longest study on monarchs to date, a research team has examined plants of dairy plants for caterpillars and adult butterflies captured for 37 years – a period that included over 140 generations of monarchs. The study took place in a herbicide-free livestock grazing in Cross Creek, about 20 miles southeast of Gainesville.
Experts have discovered that the spring departure of monarchs from Mexico is planned to coincide with the optimal growth of asclepore in the southeastern United States.
If monarchs come too early to their breeding grounds, their host plants can be killed by frost. To maximize the chances of survival of their offspring, the butterflies must delay their arrival within a three-week window, Professor Daniels said. This is an incredible result for insects with a duration of between six and eight weeks.
The study is published in the Journal of Natural History.
Of Chrissy Sexton, Earth.com Staff writer