Monarch butterflies need milkweed to survive. Declining populations of this iconic butterfly have been driven by many factors, but until recently, one undisputed factor has received the most attention — the loss of milkweed in the wake of the near-universal adoption of Roundup Ready corn, soybean, cotton canola, alfalfa, and sugarbeets. And these are serious losses, as mentioned in our reporting about research from 2018 that found monarch populations in parts of Florida started declining as early as 1985, and are now down 80 percent.
But now a new possible explanation has surfaced in an open-access paper published in Frontiers in Ecology and Evolution (Halsch et al, 2020). In a survey of 227 samples of milkweed collected across 19 sites in California’s Central Valley, there were an average of 9 pesticide compounds on each milkweed plant. A total of 64 pesticides were found, including 25 insecticides, many known to be toxic to butterfly larvae and/or adults.
One insecticide (chlorantraniliprole) was found on 91% of the samples! In fact, 58 samples of milkweed contained the insecticide chlorantraniliprole (rynaxpyr) at levels that could deliver to a visiting monarch a dose exceeding the LD-50 for this pesticide (dose that would kill one-half of an exposed population). In a EurekAlert! post on the new study, on of the study co-authors acknowledges that “the ubiquity and diversity of pesticides we found in these milkweeds was a surprise” (University of Nevada, Reno; 2020).
Back in the Frontiers in Ecology and Evolution paper, the authors conclude that:
“Our primary finding is the ubiquity of pesticides in milkweeds in an early summer window of time that monarch larvae are likely to be present in the area.”
Then, they cautiously state:
“…these results are consistent with the hypothesis that pesticide exposure could be a contributing factor to monarch declines in the western United States” (Halsch et al., 2020).
So, based on this research, the average monarch butterfly traversing California’s Central Valley on a warm spring or early summer day is exposed through its diet to roughly the same number of pesticides that the average California resident is from their daily diet, with two exceptions.
People following the government’s advice to consume well over half-dozen servings of fruits and vegetables daily will be exposed to far more than 9 pesticides daily (20 or more on average), if they choose mostly conventionally grown fresh fruits and vegetables.
If they choose mostly organic produce, they will likely consume fewer than 9 pesticide residues in their daily diet, and at far lower doses.
These sobering facts lead Hygeia Analytics to offer this hypothesis:
The adverse human health impacts of pesticide use in California will not be significantly reduced until the impacts of pesticides on monarch butterflies are markedly reduced, and vice versa.
Halsch CA, Code A, Hoyle SM et al. “Pesticide Contamination of Milkweeds Across the Agricultural,Urban, and Open Spaces of Low-Elevation Northern California,” Frontiers in Ecology and Evolution, Vol 8, Article 162. June 2020.
University of Nevada, Reno; “Milkweed, only food source for monarch caterpillars, ubiquitously contaminated,” Eureka Alert, June 8, 2020