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This week's Book of the Week feature is The Myths of Safe Pesticides by André Leu.

Myth #3: Breakdown

“Modern pesticides rapidly biodegrade.”

One of the major pesticide mythologies is the belief that most modern agricultural chemicals rapidly biodegrade and leave few if any residues. We are misled into believing that they break down and do not persist in our food like older chemicals such as DDT.

The following is a claim by the main food regulator in Australia and New Zealand, FSANZ, and is typical of the claims by many nations’ regulators. They state, “Organophosphorous pesticides, carbamate pesticides are mostly biodegradable, and therefore do not concentrate in the food chain. Synthetic pyrethroids . . . are generally biodegradable and therefore tend not to persist in the environment.” These types of statements give the false impression that few agricultural pesticides persist in our food and environment.

In fact, most agricultural and veterinary chemicals leave residues in food. That is the reason why tolerances for maximum residue limits (MRLs) and the acceptable daily intake (ADI) are set for these poisons.

The data presented in the United States President’s Cancer Panel 2010 report indicating that only 23.1 percent of food samples had zero pesticide residues is reasonably consistent with the data from testing in most countries. This means that the overwhelming majority of foods contain pesticide residues.

Many of the current chemicals, including some of the synthetic pyrethroids, organophosphates, carbamates, and herbicides such as atrazine are as residual as the mostly banned older chemicals such as the organochlorine group that includes dieldrin, DDT, chlordane, heptachlor, lindane, and aldrin.

Metabolites of Pesticides

One of the biggest myths is the assumption that once a chemical degrades it disappears and is harmless. Most agricultural poisons leave residues of breakdown products or daughter chemicals when they degrade. These breakdown products of chemicals are also called metabolites. Where there is research, it shows that many of the metabolites from agricultural poisons cause health and reproductive problems.

A substantial number of agricultural pesticides—such as organophosphates like diazinon, malathion, chlorpyrifos, and dimethoate—become even more toxic when they break down. These metabolites are known as oxons. Scientists at the Cooperative Wildlife Research Laboratory at Southern Illinois University and the Western Ecology Research Center of the U.S. Geological Survey in Point Reyes, California, found that the oxons can be up to one hundred times more toxic than the original pesticide.

"In this study the oxon derivatives of chlorpyrifos, malathion, and diazinon were significantly more toxic than their respective parental forms. Chloroxon killed all of R. boylii tadpoles and was at least 100 times more [toxic] than the lowest concentration of chlorpyrifos which resulted in no mortality. Maloxon was nearly 100 times more toxic than malathion and diazoxon was approximately 10 times more toxic than its parent. This is consistent with other studies that have compared parent and oxon forms."

Dimethoate is a good example. Dimethoate is a systemic pesticide because it is absorbed into all the tissues of the plant, including the edible portions such as all the flesh of fruits, stems, tubers, and leaves.

Contrary to popular belief, because systemic poisons are absorbed into the flesh, and consequently every part of the plant is toxic, washing or peeling the surface of the food only removes a small percentage of the poisons on the surface. It will not remove the bulk of poison, which is inside the food.

Dimethoate is widely used as a fruit fly treatment because it is so residual that even after two weeks any maggots that hatch from eggs inside the fruit will be killed by the poison residues in the edible portion of the flesh. Dimethoate breaks down to an even more toxic metabolite called omethoate. Omethoate is also used as a pesticide and consequently, unlike the vast majority of metabolites, it has been researched and has an LD. According to the WHO, omethoate has an LD of 50 mg/kg, whereas dimethoate has an LD50 of 150 mg/kg. This means that as the dimethoate decays within the treated food, it becomes 300 percent more toxic as omethoate.

Under the WHO classification of hazards it goes from being a moderately hazardous to a highly hazardous pesticide. Several countries have withdrawn or are in the process of withdrawing omethoate from use as a pesticide due to its high toxicity and its persistence. Other countries are still debating whether to ban dimethoate. All food that is treated with dimethoate will end up with residues of the more toxic and persistent omethoate as well as a number of other toxic metabolites that are generated as the dimethoate breaks down.

Learn more about The Myths of Safe Pesticides here

About the Author

André Leu previously served as president of IFOAM — Organics International and is currently the international director of Regeneration International. Leu has over 40 years of experience in all areas of organic agriculture, from growing, pest-control, weed management, marketing and post-harvest transport to grower organizations, developing new crops and education – not only in his home country Australia, but across Asia, Europe, the Americas and Africa.

He has written and published extensively in magazines, newspapers, journals, conference proceedings and newsletters in print and online on many areas of organic agriculture including climate change, the environment and the health benefits of organic agronomy.

Leu and his wife, Julia, run an organic tropical fruit orchard in Daintree, Queensland, Australia.

More By André Leu

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