Organic pesticides carry other environmental risks

A University of Guelph study in soybean fields suggests organic pesticides may not always be as easy on the environment as synthetic bug killers would be.

Its "too simplistic" to say an organic product is by its nature better for the environment, Hallett, a professor in Guelphs school of environmental sciences, said in a university release Wednesday.

"Organic growers are permitted to use pesticides that are of natural origin, and in some cases, these organic pesticides can have higher environmental impacts than synthetic pesticides, often because they have to be used in large doses."

The study, which Hallett and PhD candidate Christine Bahlai published last week in the peer-reviewed online journal PLoS One, examined two conventional synthetic products commonly used by soybean farmers; two new "reduced-risk" synthetic pesticides; a mineral oil-based organic pesticide that smothers aphids; and an organic fungal product that infects and kills insects.

Hallett and Bahlai gauged the products effectiveness on soybean aphids against the environmental impact quotient, a database indicating impact based on factors such 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 the plant.

But the Guelph study also looked at how well each pesticide targeted aphids while leaving aphids predators, ladybugs and flower bugs, unharmed.
Ladybugs and flower bugs are considered to be reducing environmental impact, as they naturally protect soybean crops by regulating aphid populations and growth and thus reduce the amount of pesticides needed, Hallett said.

"We found the mineral oil organic pesticide had the most impact on the environment because it works by smothering the aphids and therefore requires large amounts to be applied to the plants," she said.

The mineral oil-based and fungal products were less effective than the synthetics because they killed ladybugs and flower bugs as well, the study said.

"Ultimately, the organic products were much less effective than the novel and conventional pesticides at killing the aphids, and they have a potentially higher environmental impact," Hallett said in the universitys release.

"In terms of making pest-management decisions and trying to do what is best for the environment, its important to look at every compound and make a selection based on the environmental impact quotient rather than if its simply natural or synthetic."