Agrichemical phaseout requires shift

10 March 2025

Opinion piece by AgResearch senior scientist Mark Hurst: The ongoing phaseout of agrichemicals traditionally used to control pasture pests in New Zealand presents a major challenge for our agricultural sector.

However, it also offers an opportunity for a shift in how we manage these costly pests, in a way that benefits farmers, our soils and the broader environment.

The Environmental Protection Authority (EPA) has proposed to ban chlorpyrifos, an insecticide from the organophosphate group that is used in New Zealand to control a broad range of insect pests on plants. The EPA considers that the risks of using chlorpyrifos now outweigh the benefits. Submissions on the proposed ban were called for and are now being evaluated.

Chlorpyrifos is considered the most effective chemical available to tackle the native grass grub, which is the most pervasive and costly pest for pastoral agriculture in New Zealand. It is also registered for use against another major pasture pest, the Manuka beetle.

The only other insecticide registered for use against grass grub in established pasture is diazinon and its agricultural use will be phased out in New Zealand by 2028.

A study by AgResearch a few years ago estimated that the impact of the grass grub, through reduced pasture production affecting revenue, was up to $380 million annually for dairy farms, and up to $205 million for sheep and beef farms, in “average years”.

Federated Farmers has raised concerns about the potential loss of chlorpyrifos from the farmers’ arsenal, saying it puts farmers in a precarious position without readily available alternatives.

While we must acknowledge that agrichemicals are going to be needed for the foreseeable future, increasingly we are seeing opportunities to research and develop biopesticides that harness naturally occurring organisms such as bacteria to target the pests.

This means opportunities to at least reduce chemical use, if not replace it, in more integrated approaches to pest management. The use of live organisms or derivatives, such as metabolites, are generally more specific to the target pest and have significantly less environmental residues and impacts.

AgResearch senior scientist Mark Hurst

One such example is AGR96X, a strain of the highly virulent Serratia proteamaculans bacterium, which is highly effective in killing the larvae of the New Zealand grass grub and the New Zealand Manuka beetle.

Through more than 20 field trials located throughout New Zealand and assessing different crop types, AGR96X has been shown to be as effective as synthetic products. The field trials have been enabled through Grasslanz Technology Ltd and Midlands Holdings Ltd, which both see the value of biologicals such as AGR96X for grass grub control.

Currently AGR96X (trade name yet to be confirmed) is being applied as an incorporated granule, which when ingested, the larvae die within 5-12 days. A comprehensive data package covering efficacy, production and usage is currently under review in the Agricultural Compounds and Veterinary Medicines process.

While biopesticides and newer, more selective, agrichemicals offer a lot of promise, they are not a silver bullet. Sometimes several products may be required to target multiple pests that are currently controlled by a single broad-spectrum agrichemical.

For effective grass grub control, growers should check their pasture for grass grub during late February to early March and decide if control measures are needed. The damage threshold in established pasture is 150-200 grass grub per square metre. Established pastures should be treated from March to April to minimise the risk of damage.

As key export markets demand the removal of broad-spectrum agrichemicals, such as chlorpyrifos, New Zealand’s pastoral farmers will need to adapt their pest management, following a similar path to the horticultural sector. This need for change was recognised by a further AgResearch study, which identified barriers that hinder practice change, including the need for new control options like biopesticides.

Pests like grass grub that are unique to New Zealand pose a challenge for product development because the market size is very small compared with cosmopolitan pests that damage similar crops all over the world.

Support is needed to achieve and maintain commercial production of new control options, as well as providing training and information to farmers so that they can make the best use of these new options.

If we can embrace the new opportunities offered by new selective biopesticides like AGR96X, then we all stand to benefit from reduced costs for our economy and less harm to our environment.

**The above article was first published by the Farmers Weekly**