Plants and invertebrates face increasing applied pesticide toxicity

Increased efficacy of pesticides comes along with decreased applied amounts in agriculture – but does this translate to lower risks to non-target species? The answer is NO, if you ask scientists at the University of Koblenz-Landau who recently published a study in Science assessing changes in the use of 381 pesticides and toxicity to eight non-target species groups over the course of 25 years. In our blog, the authors explain the shifts in applied pesticide toxicity they found, and which species are increasingly at risk.

Global pesticide sales keep increasing to more than 240 billion US$ annually, while reports of environmental and human health issues continue to accumulate. The recent neonicotinoid bans in Europe and glyphosate lawsuits in the US illustrate that pesticides – although highly effective for their purpose – can come along with underestimated risks.

Why pesticides have become more effective

Did you know that  it takes more than 10 years and costs more than 250 million US$ to develop, register, and market a new pesticide? This is a huge investment made by industry prior to generating any sales. What are the industry’s strategies to make this process pay off? Today industry praises the development of highly efficient pesticides. Most novel pesticides are so effective that only small amounts are needed to control pests – as small as few grams per hectare, as compared to a few kilograms for pesticides developed in the 1970s. Over the past decades, the amount of pesticide needed to get the job of weed or insect control done has decreased by several orders of magnitude. This is what industry or agricultural scientists would call the efficacy of a pesticide: the amount needed to control a given pest. Higher efficacies, or smaller amounts needed, imply higher precision and thus a higher-quality, technologically improved product, which likely can be sold for more money per unit of mass.

Photo showing a field with a tractor applying pesticides by spraying it onto the field surface. Next to the field is a small vegetation buffer strip which leads into a creek with water, and in the background are trees.
Using less pesticide mass, but applying more toxicity: How does that work? (photo by R. Bereswill)

A story of linking pesticide efficacy with environmental impact

Increased pesticide efficacies and lower amounts of pesticide applied sound good at first glance, right? It is proof for technological improvements over the past decades, many scientists, media outlets, NGOs, and politicians have agreed. Narratives have been created that link these reduced amounts of pesticides needed with reduced impacts on vertebrates (such as birds or fish) and mammals, aiming to illustrate an apparently environmentally benign character of pesticides. One of the more recent chapters of this narrative, and one that shows still a fast growth globally, is a story about genetically modified (GM) crops. GM crops have been advocated to come along with a reduced dependency of farmers on classical chemical pesticides and thus reduced environmental impacts. 

BUT: This story about efficacy misses one crucial piece: If a pesticide is more effective in killing a target pest such as insects in an agricultural field, it will usually also show higher toxicity to non-target insects living in field margins or other non-target ecosystems such as adjacent surface waters. Higher toxicity of pesticides likely translates to more unwanted side effects on the thousands of species living in nearby terrestrial and aquatic environments. The biodiversity in agricultural ecosystems – the largest terrestrial biome on earth – is at stake. Ideally, these non-target species should not be affected negatively by pesticides. This is why elaborated environmental risk assessments are conducted for pesticides. But do these assessments sufficiently protect biodiversity?

New study disentangles the relationship between pesticide efficacy and environmental impact

A new study by environmental scientists from the University in Landau published in Science challenges the claim of pesticides improving their environmental profile over time. The study shows that for plants and insects, the applied pesticide toxicity in agriculture, i.e. the total amount of pesticides applied expressed as toxicity, has substantially increased between 2004 and 2016, and that this pattern is even relevant in GM crops that were originally designed to reduce pesticide impacts on the environment.

“We have taken a large body of pesticide use data from the US and have expressed changes of amounts applied in agriculture over time as changes in total applied pesticide toxicity. This provides a new view on the potential consequences that pesticide use in agriculture has on biodiversity and ecosystems.” 

Ralf Schulz, environmental scientist in Landau and lead author of the study

The amount of insecticides used in US agriculture has decreased substantially by more than 40% between 1992 and 2016. Fish, mammals, and birds face lower applied toxicities than in the 1990s, because insecticide classes such as organophosphates, which show high vertebrate toxicity, are used in lower quantities today. Aquatic invertebrates and pollinators, such as honeybees, yet experience the opposite: Despite reduced applied amounts, applied toxicity for these species groups has more than doubled between 2005 and 2015. A shift in the classes of insecticides used toward increased usage of pyrethroid and neonicotinoid insecticides is responsible for this trend.

The applied toxicity increases for herbicides as well, alongside the applied amount. In this case, terrestrial plants are facing the highest increase in applied toxicity. Since plants and pollinators are ecologically strongly connected, simultaneously increasing applied toxicities in both groups alert to potential strong overall negative effects on both plant and insect biodiversity.

GM crops have originally been developed to reduce the dependency of agriculture on chemical pesticide use. The results of the new study, however, clearly reveal that even in the two most important GM crops in the US, corn and soybean, the applied toxicity increases, along with increasing GM adoption, at the same rates as for conventional crops. 

The results of the study likely apply to many other regions dominated by modern agriculture, but in many jurisdictions the data for a thorough evaluation of trends in applied toxicity are not publicly available.

“These results challenge the claims of decreasing environmental impact of chemical pesticides in both conventional and GM crops, and call for collective action to reduce the pesticide toxicity applied in agriculture worldwide.”

Ralf Schulz, environmental scientist in Landau and lead author of the study

The paper “Applied pesticide toxicity shifts toward plants and invertebrates, even in GM crops” is authored by Ralf Schulz, Sascha Bub, Lara L. Petschick, Sebastian Stehle, and Jakob Wolfram, and published in Science. DOI: 10.1126/science.abe1148