When you test synthetic chemicals for their ability to cause cancer, you find that about half of them are carcinogenic. Until recently, nobody bothered to look at natural chemicals (such as organic pesticides), because it was assumed that they posed little risk. But when the studies were done, the results were somewhat shocking: you find that about half of the natural chemicals studied are carcinogenic as well.
This is a case where everyone (consumers, farmers, researchers) made the same, dangerous mistake. We assumed that "natural" chemicals were automatically better and safer than synthetic materials, and we were wrong. It's important that we be more prudent in our acceptance of "natural" as being innocuous and harmless.
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A recent study compared the effectiveness of a rotenone-pyrethrin mixture versus a synthetic pesticide, imidan. Rotenone and pyrethrin are two common organic pesticides; imidan is considered a "soft" synthetic pesticide (i.e., designed to have a brief lifetime after application, and other traits that minimize unwanted effects). It was found that up to 7 applications of the rotenone- pyrethrin mixture were required to obtain the level of protection provided by 2 applications of imidan.
It seems unlikely that 7 applications of rotenone and pyrethrin are really better for the environment than 2 applications of imidan, especially when rotenone is extremely toxic to fish and other aquatic life.
It should be noted, however, that we don't know for certain which system is more harmful. This is because we do not look at organic pesticides the same way that we look at conventional pesticides. We don't know how long these organic pesticides persist in the environment, or the full extent of their effects.
When you look at lists of pesticides allowed in organic agriculture, you find warnings such as, "Use with caution. The toxicological effects of [organic pesticide X] are largely unknown," or "Its persistence in the soil is unknown." Again, researchers haven't bothered to study the effects of organic pesticides because it is assumed that "natural" chemicals are automatically safe.