A study that appears in the current edition of PLoS One suggests that organic pesticides are not necessarily synonymous with "green" pesticides. Environmental science researchers from the University of Guelph examined the effectiveness and environmental impact of two conventional pesticides, two "risk-reduced" synthetic pesticides, and two organic pesticides on soybean crops.

To quantify the consequences of using each pesticide, the researchers relied on a database of environmental impact quotients that rank the active ingredients based on such factors as leaching rate into soil, runoff, toxicity from skin exposure, consumer risk, toxicity to birds and fish, and duration of the chemical in the soil and on plants.

In addition, they carried out two years of field tests to determine how well each pesticide worked at killing aphids, the intended target, while leaving the aphid's natural predators, such as ladybugs, unharmed.

The conventional synthetic pesticides were Matador 120E® and Lagon 480®, both neurotoxins; the "risk-reduced" synthetic pesticides were Movento® and Beleaf®, a fatty acid biosynthesis inhibitor and a neurotoxin, respectively. The organic pesticides examined in the study were Superior 70 oil®, a mineral oil that is meant to smother the aphids, and Botanigard®, a fungus that is meant to infect and kill insects.

On a per-weight basis, all six of the pesticides had similar environmental impact factors (ranging from 8.7 to 47.2) with the organic variants being in the bottom half of this ranking.

When the data was converted into the environmental impact on a field use basis (pounds needed per acre), the organic pesticides did not fare so well. The mineral oil had by far the largest environmental impact factor, a whopping 280.2 rating; the next most damaging pesticide only garnered a 12.5 environmental impact factor. The fungus did better, but still ranked fourth out of six in terms of least environmental impact.

The authors conclude that, when compared to synthetic pesticides, the organic variants were not as effective because they also killed off a large number of the aphid's natural predators.

The paper cautions that those seeking to be kind to the environment should not fall for the simplistic belief that organic is environmentally friendly—a comprehensive review of pesticides is needed to determine which is truly best for a given use.

"The consumer demand for organic products is increasing partly because of a concern for the environment," said one author, Rebecca Hallett. "But it's too simplistic to say that because it's organic it's better for the environment. [...] It's a simplification that just doesn't work when it comes to minimizing environmental impact."

PLoS One, 2010. DOI: 10.1371/journal.pone.0011250