Resistance is all about survival. Weeds, insects and fungal pathogens will employ every move in the book to prevent agricultural chemicals from beating them.
Biological cells use enzymes to manufacture essential products, such as proteins, fats, carbohydrates and nucleic acids. Enzymes function using a lock and key mechanism whereby an input substrate key fits into the enzyme lock (as shown in Figure 1A).
Agricultural chemicals target these essential enzyme pathways in the cells of organisms by mimicking the substrate key (as shown in Figure 1B). In this way, they block the target binding site and prevent the normal enzyme function that is essential for life.
Manipulating rates is not the solution to managing resistance. Instead, optimise application and rotate chemical Modes of Action (MoA) in combination with non-chemical integrated pest management options.
Chemical resistance can evolve in many different ways whenever a population of organisms is repeatedly exposed to the chemical. The major mechanisms of resistance are: a) target-site resistance; b) target-site over-production; c) metabolic resistance; and d) chemical isolation. Resistance may also result from behavioural responses or physical barriers to prevent a chemical moving into the organism.
The type of resistance and the speed of resistance development will be affected by population size, the method of reproduction, genetics, generation time of the pest and the level of exposure (rate, number of applications and length of residual activity).
Target-site resistance is when the organism changes the shape of the lock that enables normal cell function so that the organism can continue to grow (as shown in Figure 2). As the lock has changed shape the chemical can no longer bind and hence there is resistance. The level of resistance will depend on how much the lock has changed shape. There may only be partial resistance, particularly in fungal pathogens. If the change in shape negatively impacts the enzymes function, there is a fitness penalty.
The change in shape affects all agricultural chemicals in the mode of action (MOA) group to a greater or lesser extent this is called cross-resistance.
To alter the lock and still produce the essential product is a big change for an organism. It requires a high selection pressure involving repeated exposure to high levels of the chemical. If the lock is changed too much, the substrate (key) will also not fit the lock causing a fitness cost to the organism.
Target site over-production
Another common way resistance evolves is when more of the target site enzyme is produced by the cell. This is an inherited change caused by a variation in the control circuit that produces the enzyme.
As there are more locks for the keys to bind to, more chemical is needed to achieve the same level of control. As for target site resistance, all other chemicals in the same MOA group are affected. As over-production is costly to the cell, there is a fitness penalty of some degree.