The effectiveness of plant protection products is determined by many factors, especially weather, as well as by the age and physiological state of the protected plants and agrophages.
Drought and low temperatures slow down the rate of life changes in both crop plants and pest organisms, which slows down and sometimes prevents the action of chemical as well as biological agents. The same effect can be observed in agrophages, at older stages after the intensive growth and development phase. The efficacy of plant protection products against older stages may be lower, as well as when using too low doses. Only the most susceptible individuals will die, which may result in population selection for resistance to the applied product.
Resistance is a naturally-occurring, unavoidable adaptation of the ability of individuals in a population to survive exposure to a plant protection product that would normally result in effective control of those organisms. The development of resistance is part of the evolutionary process. Any organism subjected to attempts to remove it from the environment by developing new adaptations will seek to adapt to the new conditions and survive. An important element in the development of resistance is selection pressure. In relation to plant protection, repeated use of products containing active substances with the same mechanism of action can lead to continuously increasing abundance, enhanced development and ultimately dominance of resistant agrophages, while eliminating susceptible individuals. There are many different mechanisms of resistance development. Some of these are specific to particular groups of agrophages. For example, so-called behavioural resistance applies to pests and involves the avoidance or escape of an insect from a site treated with an insecticide. Qualitative resistance and quantitative resistance are specific to fungal pathogens. The mechanism for the emergence of qualitative resistance is the inability of the active substance to act at its previous binding site, which is a consequence of a mutation in the gene controlling a specific metabolic pathway. The ability of the pathogen to survive despite contact with the toxicant, resulting from changes in multiple genes – this is quantitative resistance. Characteristic of this type of resistance is that it is not transmitted hereditarily, as a dominant trait, and after withdrawal of fungicides, the population will continue to develop in the primary direction. Other known mechanisms of resistance that can be attributed to different groups of agrophages are: site resistance (most often a point mutation in the gene sequence leads to such a change in the binding site of the active substance in the harmful organism that the substance cannot act), metabolic resistance (the ability to break down the active substance in the metabolic process into harmless metabolic products), penetration resistance (slower uptake of the active substance by the resistant agrophage or uptake of a smaller amount compared to a susceptible individual). Resistance of an agrophage to several active substances with a specific mechanism of action is referred to as cross-resistance. There is also the phenomenon of multiple resistance, where the pest becomes resistant to several substances with different mechanisms of action.
It is essential to distinguish between the concept of treatment ineffectiveness and agrophage resistance. In the event of treatment ineffectiveness, an advisor should be contacted to determine whether this is the result of resistance of the pest or another factor modifying the effect of the plant protection product. Use a rotation not only of active substances but also of chemical groups with different mechanisms of action. It would be optimal to use one active substance per chemical group, in the same crop, only once per season, but this is usually not feasible. A less effective pesticide should be used when the pest slightly exceeds the economic damage threshold, a more effective one when the threshold is exceeded significantly. When using a mixture of active substances, care should be taken that they belong to different chemical groups and that they are used in doses that ensure the effectiveness of the treatment, even if applied singly. Use plant protection products at the recommended doses indicated on the label. Doses that are too low quickly select populations of pests for resistance, doses that are too high are against the law, production economics, environmental protection and, in the long term, do not protect against the emergence of resistant individuals.