It's "common knowledge" that organic (naturally-derived) pesticides are safer for the environment than synthetic pesticides. It's careless to make such a broad generalization; Yuan-Ping Pang and coworkers have worked towards developing synthetic pesticides that kill soybean aphids, but do not harm people.
Your average crotchety chemist scoffs at the organic/synthetic debate, pointing out (correctly) that both kinds of pesticides are based on carbon atoms, and throws out the "there's no such thing as an inorganic tomato" zinger. On the other hand, much research does in fact demonstrate the environmental advantages of organic pesticides, and even the benefits of reducing pesticide use altogether.
However, such evaluations should be applied to the specific study under investigation, and should not be over-generalized. Results can vary with crop and location; for example, experiments with apples were different in the United States relative to New Zealand.
Governmental policies that encourage the use of organic pesticides, even when their efficacy hasn't been rigorously proven, are based on questionable science that may at times defeat the intent (a more sustainable future). Rebecca Hallett (University of Guelph, Canada) and coworkers have reported an example of soybean aphid management wherein organic pesticides were more harmful than synthetic pesticides.
The pesticides.
The scientists studied six pesticides. Two of them (cyhalothrin-lambda and dimethoate) are approved synthetic pesticides.
Two of them (spirotetramat and flonicamid) are synthetic pesticides not yet approved in Canada. The other two (mineral oil and Beauveria bassiana) are approved organic pesticides.
All six have unique modes of action (they are toxic to soybean aphids, an insect pest, by different mechanisms). For example, cyhalothrin-lambda is a neurotoxin that targets sodium channels, and mineral oil disrupts oxygen exchange (breathing).
Laboratory toxicity tests.
In a laboratory, they studied the contact toxicity of these pesticides (approximate field exposure levels) against two enemies of soybean aphids, Harmonia axyridis (a ladybug beetle) and Orius insidiosus (a minute pirate bug). Both of these critters are commonly used to control agricultural insect pests.
Farmers don't want these insects to die, because they feed on insect pests. If any of the six pesticides harm these critters, that reduces the utility of the insecticide.
The scientists found that both of the currently registered synthetic pesticides were toxic to the beneficial insects. Seventy-two percent of the insects were killed.
The remaining four pesticides were less toxic. Of these four, the two organic pesticides (24% mortality) were more toxic than the other two synthetic pesticides (13% mortality).
Field toxicity tests.
The scientists then extended their toxicity study to a two-year study in the field at five agricultural sites in southwest Ontario. All four of the synthetic pesticides were similarly effective against soybean aphids after one and two weeks (roughly 60% to 90% mortality, respectively), with the exception of dimethoate, which was less effective after two weeks (roughly 50% mortality).
Neither of the organic pesticides were as effective against soybean aphids (roughly 10-30% efficacy). Furthermore, the organic pesticides were the least selective against soybean aphids; they killed more helpful insects than two of the four synthetic pesticides.
Cyhalothrin-lambda (approved synthetic pesticide) and spirotetramat (unapproved synthetic pesticide) were more selective for soybean aphids than the other two synthetic insecticides and the two organic pesticides. It's clear that in terms of pest control, in these experiments, the two organic pesticides aren't very effective.
Environmental impacts.
What are the environmental impacts of these six pesticides? The scientists considered toxicity to birds and bees, degradation rate in the soil, and other relevant factors.
The impacts of both mineral oil (organic) and dimethoate (unapproved synthetic) were high. Both had a greater impact than any of the other four pesticides.
Implications.
In terms of lethality against soybean aphids, selectivity against soybean aphids, and environmental impact, cyhalothrin-lambda (approved synthetic pesticide) and spirotetramat (unapproved synthetic pesticide) are superior to two organic pesticides. It's clear that organic pesticides are not always more beneficial than synthetic pesticides.
This doesn't mean that the aforementioned crotchety chemist was right, that organic pesticides are just a "fad." These results instead suggest that integrated pest management strategies, which use pest control of any origin and design as long as it's effective and sustainable, is worth considering rather than a blanket dismissal of either synthetic or organic pesticides.
for more information:
Bahlai, C. A., Xue, Y., McCreary, C. M., Schaafsma, A. W., & Hallett, R. H. (2010). Choosing Organic Pesticides over Synthetic Pesticides May Not Effectively Mitigate Environmental Risk in Soybeans PLoS ONE, 5 (6) DOI: 10.1371/journal.pone.0011250