Daily temperatures are cooling, leaves are changing and one more unavoidable tradition occurs: Monarch butterflies return to Mexico.
But this year, fewer of them will be making their way south, according to findings by the University’s Monarch Larva Monitoring Project.
The project sends volunteers in search of milkweed plants, where monarch butterflies place their eggs. Since studies began in 1997, the average amount of milkweed plants holding monarch eggs is 17 percent. In the 2002 results, only 12 percent of the plants are home to the young insects.
The report comprises findings of 264 volunteers who gathered information from stations across the United States and Canada. The project originated with meetings of 17 volunteers who are now spread among Midwestern and Northeastern states.
Officials theorize a number of explanations for the colorful insects’ sharp decline this year.
Perhaps the most damaging events, a drenching January storm and irregular late-spring freezes, have killed millions of the monarchs in their usually comfortable winter environment.
While many insects can survive sub-freezing weather, University ecology professor Karen Oberhauser said monarchs are especially susceptible to harm in freezing temperatures for more than a few hours.
A monarch researcher since 1984, Oberhauser said she has seen results much lower than this year’s.
After 1998, when a record-low 3.6 percent of milkweed plants were found with monarch eggs, the butterflies bounced back to produce healthy numbers the following year.
Oberhauser said with any luck, next year will reflect the same pattern.
“It’s really difficult to predict how this year will affect next year. There’s so many factors involved,” Oberhauser said.
Monarch butterflies are believed to begin the journey north from Mexico in late March. Reports suggest the insects make the return journey in November.
Oberhauser has directed the Monarch Larva Monitoring Project since 1997. The project received a welcomed boost with a National Science Foundation grant, part of which helped recruit new volunteers for the project.
Spending two to three hours weekly collecting data, volunteers post their findings on the Internet.
To present findings and recruit future data gatherers for the project, Oberhauser and her team traveled to North Carolina earlier this year.
“We’d like to have a good representation in how our map is spread. It’s really important that we pick up some people in the South. There are big holes in the South and it’s really important in understanding the second generation of monarch,” she said.
Besides counting, future investigations will examine the kinds of plants monarchs prefer to lay eggs on and how weather affects the yearly number of monarchs.