Comment: Soil scientist Dr Jacqueline Rowarth takes a look at the concerns around the use of chemicals in food production.

Chemical-free and spray-free are common descriptors for food produced under organic principles. The 'free' refers to absence of synthetic chemicals. What are considered to be natural pesticides are allowed. This article considers some of the concerns around the use of chemicals in food production.

What you need to know about glyphosate
Dr Jacqueline Rowarth: A matter of perspective
Dr Jacqueline Rowarth: Regenerative agriculture - context is everything
Dr Jacqueline Rowarth: Economists needed in freshwater plans

The Facts

• All chemicals sold in New Zealand, whether synthetic or natural, must be approved by the Environmental Protection Authority and Ministry for Primary Industries (MPI).


• Almost 100 per cent of the pesticides found in the American diet are chemicals that plants produce to defend themselves.

• Some of the most toxic chemicals in the world are natural.

• MPI analyses foods regularly and has found very few instances of synthetic chemicals exceeding threshold concentrations and none posing health concern.

• Food production in New Zealand is regulated and results in food that is safe to eat.

The Concern

Chemical-free is used to indicate 'natural' rather than synthetic. It is a common misunderstanding that chemical-free means that no pesticides have been used.

MPI has Technical Rules for Organic Production on its web site, showing all the chemicals and forms of chemical permitted, whether as animal feed additives, soil amendments, or pesticides. Some of the pesticides are included at the end of this article.*

Many of the approved pesticides are processed in order to quantify dose rates of active ingredients. The goal with all approvals is to ensure that users and end products are safe.


However, whether the production system is conventional or organic, most of the pesticides eaten come from the plant's own defence systems.

This was the conclusion in research published in 1990: 99.99 per cent by weight of the pesticides in the American diet are chemicals that plants produce to defend themselves.

Many of the newer plant varieties in production systems have been selected for low ability to create toxins, which influence flavour.

In 2002 in New Zealand, zucchini from an older plant variety reacted to an aphid attack by creating cucurbitacins. In a conventional production system the aphids would have been treated with a targeted insecticide.

Natural insecticides tend to be broad spectrum, and therefore can affect beneficial insects such as the ladybirds that feed on aphids.

The result of the cucurbitacins which the zucchini produced as defence against the aphids was sickness and hospitalisation for those who ate the zucchini (the details are on the website, authored by Professor Thomas DeGregori).

Professor DeGregori commented that "we often worry about toxicity resulting from spraying crops but rarely are we as concerned about those from not spraying them".

Dr Jacqueline Rowarth. Photo / Supplied
Dr Jacqueline Rowarth. Photo / Supplied


Residues of applied chemicals are found in most food, whatever the production system, but at very low concentrations.

Increasing sensitivity in analytical detection means we know more about what we are eating. MPI's Total Diet Survey 2016 (released in May 2018), helps with understanding the safety of our food.

The selected food types (over 130 of them) represented the most commonly consumed foods for the majority of New Zealanders and were analysed for 301 agricultural chemicals.

None of the estimated dietary exposures to agricultural chemicals represented a risk to health, with the highest dietary exposure reflecting less than 3 per cent of the respective health-based guidance value.

The majority of the agricultural chemicals analysed were not detected and therefore calculated to have a zero exposure.

In the MPI results, production system was not identified. No foods were found to be contaminated at a level to cause health concerns.

USDA data allows deeper analysis. It, too, shows that food supply is safe when it comes to the issue of pesticide residues, whether produced organically or conventionally.

The USDA Organic Program rules state that if a synthetic pesticide residue is detected on an organic product at 5 per cent or less of the EPA tolerance, it is considered "unintentional" and thus not a violation of the organic certification standard.

Of the organic samples from the US, 99.2 per cent would have met the 5 per cent or less.

Of note is that 96.3 per cent of the "conventional" samples from the US would also qualify if they had been organic samples being tested for compliance.

The take home message is that New Zealanders have choice in the type of food they buy and can rest assured that food is safe to eat, whatever production system has been used.

- Dr Jacqueline Rowarth CNZM CRSNZ HFNZIAHS has a PhD in Soil Science and has held various academic and government positions across the environment, agriculture and business spectrum. She is currently a farmer-elected director on the boards of DairyNZ and Ravensdown.

*Some of the MPI-approved chemicals

Pyrethrins (which come from the seed cases of the pyrethrum plant Chrysanthemum cinerariifolium), rotenone (found in roots of some legumes and the main active ingredient in derris dust), quassia (from the Quassia tree) and azadirachtin (form the Neem tree) as insecticides. Ferric phosphate is an approved molluscicide. Calcium polysulphide (lime sulphur) is approved as a fungicide, insecticide and acaricide. Calcium hydroxide and copper in various forms are approved fungicides.

Bacillus thuringensis is approved as a biological pest control. (The Bt gene inserted into some crops through genetic modification (GM) processes allows them to be produced without insecticides, but GM is not allowed in organic production systems or in NZ production systems at all.) Diammonium phosphate is approved as an insect attractant in traps, but not as a soil amendment (fertiliser).