By analyzing 25 years of pesticide use data from the United States, researchers have found that the toxicity of pesticides to nontarget invertebrates, including pollinators, has increased markedly, even though the volume of pesticides used has gone down.
The study, published April 1 in Science, challenges the common assumption that the impacts of environmental pesticides have gone down over time.
"The use of pesticide as interpreted in media and by politicians and by scientists is that they very often talk about the amounts that are used, and that these amounts go down, implying that the environmental risk is also reduced," said first author Ralf Schulz, an environmental sciences professor at the University of Koblenz-Landau, in Germany.
"But on the other hand, we see that the toxicity of individual compounds increases," he continued. "And many of the compounds used these days are more toxic than the counterparts that have been used decades ago."
The idea that reducing the impacts of pesticides is only a matter of reducing the amount used is so pervasive that even some scientists outside the eco-toxicology field have fallen under the spell of that view, turning what was once an unproven assumption into what Schulz calls "a kind of paradigm."
The study claims that the driver of the increased toxicity of modern pesticides is the use of highly toxic pyrethroids and neonicotinoids, two classes of insecticide. Many of these compounds have been banned in other parts of the world, particularly Europe. A 2020 study in Environmental Health reported that more than one-quarter of pesticides used in the U.S. are forbidden in the European Union.
However, many of these pesticides are a relatively recent addition to agriculture. While pyrethroids have been around since the 1940s, becoming much more widely used in the 1970s and beyond, neonicotinoids weren't introduced until 1994. However, they now account for 30% of global insecticide sales.
The researchers sought to develop a means of measuring pesticide use in terms of total toxicity, not just the amount applied. To do so, they developed a mathematical model in which the mass of applied pesticide is multiplied by a toxicity factor for that pesticide, calculated using toxicity thresholds set by U.S. regulations.
The result gives a "total applied toxicity" for a pesticide against a given organism. The researchers calculated these values for 381 pesticides against eight groups of nontarget organisms between 1992 and 2016, looking for broad trends.
The results spell trouble for invertebrates.
While the researchers report that insecticide use went down 40% between 2005 and 2015, the total applied toxicity values against pollinators and aquatic invertebrates each increased by about 8% per year, achieving more than double their 2005 values in just 10 years.
While four types of pyrethroids drove the increased toxicity for aquatic invertebrates, the team found that neonicotinoids drove this increase in pollinators. This may be because bees pick up the pesticide orally from flowering plants, even though 80% of neonicotinoid treatments are done on seeds, not blooming plants.
One point of surprise for the researchers was the fact that even genetically modified organism crops that are designed to resist herbicides saw increases in total applied toxicity. This is because multiple herbicides are frequently applied to these crops to prevent weeds.
"If you look at glyphosate-resistant soybean, basically glyphosate can be used there, and other herbicides are not needed anymore," Schulz said. "But since there's so much resistance development against glyphosate, so many weeds have developed resistance. Agriculture basically ended up still using many other herbicides together with glyphosate, and that probably led to the situation where the herbicide toxicity was increasing."
The researchers also found that results for vertebrates and mammals were the opposite of those for invertebrates: Their total applied toxicity values decreased over time.
"I think [this] is a great success," Schulz said. "But, unfortunately, we have bought this success on the costs of the invertebrates and plants."
The results suggest the need to reevaluate pesticide usage on a global scale and give farmers and other agricultural workers incentives to reduce their use of highly toxic pesticides rather than just reducing the amount of pesticide used.
"I think what we probably should do is look at pesticide usage in agriculture more in terms of its toxicity," Schulz said. "And I know agriculture is under a lot of pressure. Very often, it's needed to use pesticides to fight against certain [pests], but I think we need incentives, and we need measures policy-wise that really allow the farmers to use reduced toxicity. That is key."
The study, "Applied pesticide toxicity shifts toward plants and invertebrates, even in GM crops," published April 1 in Science, was authored by Ralf Schulz, Sascha Bub, Lara L. Petschick, Sebastian Stehle and Jakob Wolfram, University of Koblenz-Landau.