Saturday, 26 June 2010

Environmentally Unfriendly Organics



The pedantic hemisphere of my brain gets irritated by people saying they ‘don’t use any chemicals’ on their garden. What they mean of course is that they don’t use chemicals produced by human chemists.

Most people who ‘don’t use any chemical’s are happy to slap on a range of substances called ‘organic’ or ‘natural’. Don’t get me wrong. I’m all for using chemicals that are not exceedingly toxic when applied and are more likely to break down quickly and not leave toxic residues.

In general, this includes most things we consider organic. So that’s good. But do take care. The best thing is to put up with a few bugs nibbling your leaves but in some cases this will mean no food for you or a particularly unattractive garden. This is when many people resort of potions and lotions.

A study by Professor Rebecca Hallett and PhD student Christine Bahlai from the University of Guelph, in Canada, found that sometimes a quick, sharp attack on a nasty bug with a human-produced chemical can be better than lathering a plant in apparently harmless ‘natural chemicals’.

As their media release puts it “Consumers shouldn't assume that, because a product is organic, it's also environmentally friendly”.

Because we often need to use higher doses of organic products to kill anything, say Hallett and Bahlai, they can have a higher environmental impact than conventional pesticides.

Six different pesticides were tested on soybean aphids, a serious pest of soybean crops in North America. Four of them were ‘synthetic’ – that is, we made them. Two of these were described as conventional, of the kind commonly used by farmers. The other two were new products, designed to reduce environmental impact.

The two organics were a mineral oil-based pesticide and a fungus that infects and kills the aphid.

To assess the impact of each pesticide (and you’ll note that the plant oil and fungus spread are still technically pesticides because they kill pests!) the Guelph researchers measured runoff and leaching rates, toxicity to human skin, toxicity to birds and fish, and so on. They also tested whether the pesticides killed predators of the aphids, like ladybirds, the ‘good bugs’.

Taking all this into account the mineral oil organic pesticide had the biggest net impact on the environment. The oil works by smothering the aphids and this requires a very heavy application.

In addition, the mineral oil and the fungus solution were less effective because they also killed the good bugs, reducing this important regulator of future aphid growth.

The researchers conclude that we should make our selection based on the total environmental impact ‘rather than if it’s simply natural or synthetic’.
Less elegantly but quite correctly they stress that ‘policy decisions must be based on empirical data and objective risk-benefit analysis, not arbitrary classifications.

If you want to read more about the study, it’s published in the open-access journal PloS One.

Image (by Jaime Plaza): The Palace Rose Garden in Sydney’s Royal Botanic Gardens where we no longer use ‘nasty chemicals’. Although we are using predatory wasps to control aphids, we do apply a ‘low toxicity’ mineral oil to control Black Spot. It’s a complex business!

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