A new study apparently casts doubt on the supposed unambiguous relationship between the adoption of genetically modified (GM) crops and the decline in monarch butterflies.
A study just published in the peer-reviewed journal PNAS suggests that recent declines in monarch populations are part of a much longer trend, and that herbicide-tolerant GM crops are unlikely to be the prime culprit. But it stops short of absolving agrochemical use, or providing a definitive explanation for the dwindling numbers.
The claim that GM crops might be responsible for the decline of monarchs was one of the few plausible reasons for opposing their cultivation from an environmental perspective.
The dramatic decline in monarchs in recent years has left ecologists increasingly desperate, and some campaigners have demanded that the butterfly be added to the Endangered Species List.
Monarchs are charismatic and beautiful insects and their long journey from feeding grounds in the United States and Canada to their overwintering locations in the forested mountains of Mexico is one of the world’s great animal migrations.
They have also been adopted as a cause celebre by anti-GMO activists because of the compelling argument that the use of herbicide-tolerant GM crops must be driving the decline of the milkweed plant — the main food source for monarch larvae.
“In addition to threats from more frequent and harsher weather events, monarchs are still severely jeopardized by the ever-increasing pesticides used with genetically-engineered crops, destroying their habitat,” George Kimbrell, senior attorney at the anti-GMO group the Center for Food Safety, claimed in a 2017 press release.
The unmistakeable profile of the monarch also has been adopted as the logo of the non-GMO project, which certifies large numbers of grocery products (including, infamously, some of non-biological origin like salt) as not containing genetically modified material.
A 2013 news story in the Guardian stated: “The use of herbicides destroying milkweed is directly linked to the mass cultivation in the great plain states of the US of genetically modified soybean and corn crops with inbuilt resistance to chemicals that the rest of the plants in the areas sprayed do not have.”
But the PNAS study throws that contention into doubt after looking at longer-term historical data of monarch populations and milkweeds.
Previous data about monarchs goes back only to 1993, whereas the scientists — led by Jack Boyle, a biologist at the College of William and Mary in Williamsburg, VA — used digitized records from museums and herbaria to collect information for 1,191 specimens of monarchs and 39,510 specimens of milkweed over a 116-year period dating back to 1901.
Boyle and colleagues found that both monarchs and milkweeds increased during the early 20th century and recent declines are actually part of a much longer trend beginning around 1950.
“Herbicide-resistant crops are clearly not the only culprit, and likely not even the primary culprit,” the paper states. “Not only did monarch and milkweed declines begin decades before GM crops were introduced, but other variables, particularly a decline in the number of farms, predict common milkweed trends more strongly over the period studied.”
The authors conclude by stating: “By no means do our results absolve from blame agrochemical use in general, or herbicide use in particular; nevertheless, it is clear that focusing on GM crops in particular at the expense of other potential drivers will hinder our ability to address and reverse the worrying declines in these species.”
So what was driving the post-1950s decline in both milkweeds and monarchs? Boyle and his team suggest that the overall trend away from family farms and towards industrial agriculture has likely had a long-term impact on the habitat for milkweed.
The authors state: “Indeed, the timing of monarch and milkweed declines roughly corresponds to the mid-century agricultural revolution in the United States which saw greatly increased mechanization and chemical inputs to farmland.”
However the paper — as does other work we reported recently — leaves a lot of unknowns. “We tested the role of farm size, herbicide use and fertilizer use,” said co-author Josh Puzey. “Collectively, the model explained less than 20 percent of the variation in milkweed abundance. In other words, there’s a lot of variation in milkweed abundance that’s not explained by farm size, herbicide use or fertilizer use.”
He added: “I think the main takeaway from this paper is if we really want to understand why milkweed is declining and why the monarch is declining, we need to figure out what is going on with that other 80 percent not explained by our model. What is causing this massive variation is simply not explained by these agricultural parameters.”
Meanwhile, there’s been a surprising and very welcome rebound in monarch populations monitored in their overwintering grounds in Mexico. The area occupied by monarchs totalled 6.05 hectares in 2018-19, the largest since 2006-7 and a 144 percent increase over last year. The lowest-ever monarch wintering population was measured in 2013-14 when a mere 0.68 hectares was occupied.
Despite that heartening report, no one thinks that the threat to monarchs is over. Numerous ecologists have warned that the rebound in monarch populations is unlikely to be replicated next year. “Not even close,” the director of Monarch Watch told the Guardian.