4.5 Mode of Action (MoA) groups

Historically, pesticides have often been classified according to their chemical groups and this is useful for understanding the properties of a given compound (as above). 

However, the first entry given for most compounds in the Pesticide Manual3 is the mode of action (MoA) group, which possibly represents the most useful pesticide classification for biologists.

MoA entries may be something like: ‘FRAC G1’, ‘IRAC 2A’ or ‘HRAC G’. From a pesticide industry point of view, one of the most important threats to product sustainability and innovation is the onset of resistance (see section 4.6). 

Research-based companies collaborate (under the auspices of CropLife International) in order to develop better understanding of MoA mechanisms, and thus create a “common good” by mitigating the onset of resistance. 

Currently, there are four specialist committees:

• Fungicide Resistance Action Committee (FRAC)
• Insecticide Resistance Action Committee (IRAC)
• Herbicide Resistance Action Committee (HRAC)
• Rodenticide Resistance Action Committee (RRAC)

MoA describes the way a pesticide attacks some biological process (often a certain biochemical pathway in a particular kind of living cells) within the pest. For example:

• Selective herbicides might attack specific photosynthetic process in the chloroplasts of susceptible plant cells (i.e. weeds not crops).
• Pyrethroid and neonicotinoid insecticides (NNI) attack nerve cells (and have a fairly broad spectrum).
• Phenylamides that attack specific nucleic acid synthesis pathways in Oomycetes such as Phytophthora.

Classification of pesticides by using MoA is important for:

• Resistance management (often effective by rotating 3 or more MoA on a seasonal basis);
• Understanding the biochemical pathways by which a substance is effective, thus:
  • Determining its likely effects (and often speed of action) on the target pest;
  • Providing a convenient classification of pesticides for biologists.

Having entered an organism, pesticides are often metabolised – or changed - into one or more different chemicals. The metabolites (changed products) may be either more toxic or less toxic than the original pesticide ingredient. 

Given enough time, an organism may be able to metabolise certain pesticides to non-toxic metabolites, and survival or death may depend on the rate of metabolism before the toxic activity is complete or irreversible. 

On the other hand, some pesticides are effective only after they have been metabolized into a lethal compound in the organism.

The MoA will often determine the Spectrum of action: the degree to which a pesticide discriminates between target and non-target organisms. A selective pesticide affects a very narrow range of species other than the target pest. 

The chemical itself may be selective in that it does not affect non-target species, or it may be used selectively in such a way that non-target species do not come into contact with it. 

Non-selective pesticides kill a very wide range of weeds, insects, plant disease organisms, etc.