To maintain the life of plant protection products and avoid resistance developing, pest and disease control products should be rotated. In fact the insecticide and fungicide action committees (IRAC and FRAC) have been promoting this practice already for years. Nevertheless it is well noted that plant protection products tend to get less effective over the years of use, even if they have been properly applied and rotated with other chemical groups.
Due to the increasing hurdles in the registration process, there are today less and less different active substances available with different modes of action. Just a limited number of products with different modes of action for the control of insects, mites, nematodes, diseases and bacteria have remained. Generally speaking, new active substances that are developed act on a single point of a metabolic process, which make them more selective and in many cases unique to one group of pest or disease to be controlled. This is good as they have probably less impact on non-target organisms, but it also means that the pest or disease can more easily develop resistance to the active. The reduction of available products and the potential for the development of resistance to new actives, seriously restricts the conventional chemical crop protection toolbox for the control of pests and diseases.
Biorational crop protection products are being promoted heavily now by the authorities and production chains as good potential alternatives. These products also require approval through an official registration process and more and more such products are being registered. Although this registration process is mostly a bit easier than that for chemical products, it is still quite a challenge to get a registration, but it does ensure that these biorational products have proven performance and are safe to use.
Certis leads the offer of biorational products for ornamentals, fruit and vegetables in Europe. These are defined as “registered plant protection products generally derived from the natural environment, offering improved benefits for plants, people and the planet, which are increasingly important factors for Integrated Crop Production to satisfy requirements of the value chain and consumers”.
In terms of resistance management biorationals offer major benefits. In general they contain an active component that acts on multiple sites, thus making it more difficult for the pest or disease to develop resistance. For example Cu2+ (from copper compounds) blocks multiple enzymes of fungi and bacteria that lead to a disorder of many metabolic processes in the interior of the cell. A biorational may also have a physical mode of action. For example maltodextrin prevents the insects from breathing by covering the spiracles they use to breathe; Bacillus amyloliquefaciens or Thrichoderma sp. protect the plant from disease pathogens by covering its surface. Biorationals can be natural living pathogens of the pests, to which no resistance has been found. For example, Beauveria bassiana is a natural fungus that penetrates insects killing many pests; Bacillus thuringiensis forms crystals which disrupt the intestine of caterpillars of lepidopteran species.
Biorationals also have the advantage that they do not carry the risk of administering a sub-lethal dose, which can increase decreased sensitivity of pests or diseases. Even where they have a similar mode of action, as is the case with natural pyrethrins extracted from chrysanthemum and synthetic pyrethoids. The natural product degrades easily, unlike synthetic more persistent pyrethroids. The time for a sub-lethal dose that might promote selection for resistance is so short that it is almost impossible for this to happen.
Of course the best way to avoid pest or disease populations running out of control due to decreased sensitivity, is to keep populations at low levels. This can be achieved by taking into account all known preventative control measures, like using non-sensitive varieties, crop rotation, avoiding optimal pest / disease conditions, using efficient monitoring and application systems, making use of natural enemies and so on.