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Changing up chemical groups essential to preserve longevity of actives

Dr Garry McDonald, pictured, along with Dr Andrew Milgate and Professor Stephen Powles, say diversity in control tactics is the guiding principle of all resistance management strategies.
Photo: GRDC

Swapping MoA groups a key tactic to lower risks of resistance evolution in ag chemicals.

Diversity in control tactics is the guiding principle of all resistance management strategies and, for agricultural chemicals, this means rotating or alternating Mode of Action (MoA) groups.

The most effective way to delay or manage resistance is by integrating non-chemical control methods into pest, fungal pathogen or weed control programs.

But there are many situations where chemicals are still required and, for growers, rotating different MoA groups is essential to minimise the risk of resistance evolution.

CropLife Australias resistance management strategies provide detailed information about rotating relevant MoA.

These are designed to help growers maintain effective control of resistant pests in the field and also to minimise or delay the evolution of new resistances.

All agricultural chemicals have Mode of Action group labelling. PHOTO ChemCert.

All agricultural chemicals have Mode of Action group labelling. Photo: ChemCert.


The MoA group is shown on all chemical labels. This information is positioned immediately below the active constituent.

The group, shown in a black rectangle, includes one or more letters or numbers representing the MoA groups of each of the active constituents (see Table 1, below).

This is followed by the word FUNGICIDE, HERBICIDE or INSECTICIDE in capital letters.

The MoA group identifies the specific target site within an organism that the agricultural chemical affects, and is part an Australian labelling strategy to improve the overall management of resistance to agricultural chemicals. The groups are determined by three CropLife Australia Resistance Management Review Groups, in conjunction with international protocols, researchers, agronomists and growers. Current MoA tables for most products used in Australia are available on the CropLife Australia website.

Groups and resistance

When an organism becomes resistant to one agricultural chemical, the resistance can often extend to other chemically related agricultural chemicals with the same MoA. This is because chemicals within a common MoA target a common site within the pest organism.

Resistance often develops from a genetic modification affecting this target site. When this happens, the interaction of the agricultural chemical with its target site is impaired and the chemical loses its efficacy. Because all agricultural chemicals within a group share a common MoA, there is a high risk of cross-resistance to many chemicals in the same group.

Fungicide MoA groups are numbers, herbicides are alphabetic letters, insecticides use both. Numbers show the main MoA and letters show subgroups.

Within insecticides, the main group is a number, which is the principal indicator of MoA, but subgroups are also provided, shown as a letter (for example, imidacloprid 4A). Subgroups represent distinct agricultural chemical classes that are believed to have the same MoA. However, they are different enough in chemical structure or MoA with the target site that the chance of resistance selection is reduced compared to those of other subgroups.

The cross-resistance potential between subgroups is higher than that between different groups. It is advised growers avoid rotating only between subgroups/letters and look for options from other MoA groups/numbers.

GRDC Project Codes: UM00048, UWA00171, DAN00177

More information: Dr Garry McDonald, University of Melbourne, 0419521238,;APVMA Mode of action indicator; CropLife Resistance Management

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