Monarch butterflies across North America are currently wending their way south for their Mexican holiday – but there may be fewer of them making the trip this year.
Each monarch butterfly, which weighs less than a paperclip, can power its little orange wings for hundreds of miles. The migration happens twice a year – heading south in the fall, north in the spring – but for each individual butterfly, it’s a one-way trip. The migrating females lay eggs for the next generation at the end of the journey.
Mark Garland, who tracks the butterflies in New Jersey with the Cape May Monarch Monitoring Project, worries that this year’s generation is smaller than usual.
“It's a little too early to tell, but there's a lot of concern that the monarch numbers are going to be very low this year,” Garland told the South Jersey TV news station NBC40.
It isn’t quite the worst season that the Cape May project has seen. This year, during the first week of monitoring, butterfly spotters observed just under seven monarchs an hour. That’s a big dip from the same week last year, when spotters saw almost 17 butterflies an hour; but 2004 was even worse, with 5.5 monarchs an hour. The worst first week in Cape May’s 21 years of census-taking occurred in 1998, when just the monarchs an hour were spotted.
However, “the peaking doesn't really start until later in the month, so we'll just have to wait and see,” Samm Wehman, also with the Monarch Monitoring Project, told NBC40.
Surveys of the Mexican overwintering populations of monarch butterflies show that numbers have been declining over the past decade. Iowa State University and University of Minnesota biologists John M. Pleasants and Karen Oberhauser investigated one possible cause: the destruction of milkweed – a plant that larvae feed upon – in the U.S. Midwest, where half of the Mexican overwintering population traditionally hails from.
In a paper published in the journal Insect Conservation and Diversity last March, Pleasants and Oberhauser suggested that a spike in herbicide use by farmers, thanks to the introduction of genetically altered “Roundup Ready” crops (which are tweaked to survive contact with the herbicide glyphosate) may be to blame. Based on data from Iowa, they estimated that milkweeds in the Midwest have declined by 58 percent from 1999 to 2010. During that same time period, there was an 81 percent decline in the monarch populations of the Midwest, correlating with a decline in the Mexican population.
“Taken together, these results strongly suggest that a loss of agricultural milkweeds is a major contributor to the decline in the monarch population,” Pleasants and Oberhauser wrote. “The smaller monarch population size that has become the norm will make the species more vulnerable to other conservation threats.”
What might explain trends in Iowa might not be the answer behind phenomena in New Jersey, however. Meanwhile, there are some signs of hope for this season at Cape May. Garland and his team have been seeing lots of caterpillars and eggs around, which could soon shore up the numbers.
“With female monarchs laying 200 or more eggs in their lives, the population can come back quite quickly,” Garland told NBC40. “And we've seen, after very low years within one or two years later, the population can come back up very well. The question is, is there a minimum number of monarchs that if the population drops that low, they won’t be able to rebound?”