Organic Pesticides

(This is the next post in my “Organic” series. If you missed the first one, you can find it here.)

It may surprise you to learn that organic farmers use pesticides. The only difference is that, for the most part, their pesticides must derive from natural sources rather than a laboratory. Does this make them safer than synthetic ones?

Consider—copper sulfate, an organic pesticide allowed by the USDA, is more toxic than some synthetic pesticides; an overdose can cause anemia, liver disease, mutations and cancer. Arsenic is a natural substance, but is so toxic that it is banned by the USDA for use on organic crops. Nicotine-derived pesticides, another group of natural chemicals, are also considered too dangerous for use by organic farmers.


For our health and the health of the planet, there are more important issues than where a particular chemical comes from. We need to consider how specific the pesticide is, how much is needed to control a pest, and how long the pesticide persists in the environment.

Spinosad is made from chemical compounds found in Saccharopolyspora spinosa, a bacterium found in the soil. It’s bad news for bugs and slugs, but is considered relatively safe for humans. It also has a long half-life, continuing to protect stored grain for up to two years. Unfortunately, Spinosad isn’t selective when it comes to killing insects; it’s highly toxic to bees and other beneficials. Even dried residues can be lethal to honeybee colonies.

Spinosad isn’t the only organic-approved pesticides that kill bees. Copper sulfate (used as a fungicide), pyrethrins (insecticides derived from plants), horticultural oil (used on dormant trees to kill overwintering insects), and even diatomaceous earth (a white powder comprised of fossil diatom skeletons) are all bee-lethal. Clearly, extensive use of these approved chemicals is not good for Mother Nature.

On the other hand, a number of synthetic pesticides are very target-specific, killing the bad bugs without harming the good guys. So which is better—to use a natural, non-selective chemical, or a man-made, specific one?

(Happily, one pesticide used in organic farming is both very safe and very specific. Bacillus thuringiensis (Bt) is a bacteria that occurs naturally in the digestive system of caterpillars. It’s also found on leaves, in ponds, and pretty much anywhere you find bugs. When eaten by certain caterpillars, the bacteria produce toxins that kill the insects. These bacteria only infect caterpillars, and do not infect people, other animals, or even other kinds of insects. Different Bt strains affect other kinds of insects, including mosquito larva. However, Bt is the exception.)

Another issue is effectiveness. Many organic pesticides have a relatively low toxicity, meaning that the farmers have to use a lot of them, and reapply them often. It might be better to spray once with a synthetic chemical than to repeatedly apply a natural, but less effective, substance.

An article in Scientific American cited a study on organic vs. synthetic pesticide use:

Canadian scientists pitted ‘reduced-risk’ organic and synthetic pesticides against each other in controlling a problematic pest, the soybean aphid. They found that not only were the synthetic pesticides [a] more effective means of control, the organic pesticides were more ecologically damaging, including causing higher mortality in other, non-target species like the aphid’s predators.

Finally, contrary to a popular misconception, organic doesn’t mean pesticide-free. Not too surprisingly, the natural pesticides permitted on organic crops leave residues. “Studies have shown that copper sulfate, pyrethrins, and rotenone all can be detected on plants after harvest—for copper sulfate and rotenone, those levels exceeded safe limits” (Scientific American’s blog).

It seems that if you want food that has never been treated with pesticides, you should plan to grow it yourself.

Of course, there are the natural pesticides plants make for themselves. More on that next time.

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