Organic pesticides may not always be the best choice

Organic pesticides may not always be the most environmentally-friendly choice of pest control, according to recent research.

Two new synthetic pesticides for controlling aphids were found to be less harmful to other species and more efficient than the two new organic pesticides tested in the study.

It is often thought that newer organic and natural pesticides are uniformly safer and therefore pose less risk to the environment than older, synthetic (manmade) chemicals used for pest control.

This study compared the potential risk of using six insecticides available in Canada to combat the soybean aphid, a severe pest that attacks commercially important soybean crops.

Two currently registered, synthetic insecticides (cyhalothrin-λ and dimethoate) were used as controls, and two, new synthetic insecticides (spirotetramat and flonicamid) and two, new organic insecticides (mineral oil and Beauveria bassiana (a fungus that kills the aphids)) were tested. The researchers assessed their environmental impact in terms of how harmful these pesticides are to non-target organisms, including insects that naturally control aphids.

Adults and larvae of the Harlequin ladybird Harmonia axyridis and adults of the flower bug Orius insidious, two natural enemies of soybean aphids, were exposed to the six insecticides in the laboratory. The two current synthetic insecticides were generally the most toxic to the ladybird and flower bugs in the laboratory. However, the two organic insecticides were more toxic than the two new synthetic insecticides.

In addition, experiments were carried out in four soybean fields. The overall environmental impact of each pesticide was estimated using the Environmental Impact Quotient (EIQ), which allows the toxicity of each pesticide to be ranked and considers the impact of the active ingredients in the pesticide on a number of organisms, including birds, fish, bees, in addition to other factors such as the toxicity to beneficial organisms, such as the ladybird and flower bug.

The conventional insecticide dimethoate and the two organic insecticides, mineral oil and B. bassiana had the highest environmental impact. The impact of the mineral oil insecticide, for example, was more than ten times greater than that of dimethoate because it has to be used in high doses.

The synthetic insecticides were the most selective - even the least selective synthetic insecticide, dimethoate, was still more selective than the organic insecticides.

The researchers suggest that certain organic management practices are not necessarily more environmentally sustainable than conventional systems. An integrated pest management approach might be more suitable, as such a system is flexible enough to include whichever practices have the smallest environmental impact.