What is Pesticide Resistance?January 24, 2021
Living organisms by definition are not homogenous. Species constantly evolve, including your vegetable and fruit plants. Therefore, it is crucial to understand what your plants are and aren’t resistant to.
Pesticide resistance is the natural and transmissible capacity of certain plants to survive exposure to this pesticide product. How can it be explained that entire populations of pathogenic microorganisms, weeds, or pests are resistant to a particular active substance? This result can be explained by the selection pressure exerted by repeated applications of:
- The same active ingredient
- The same fungicidal mode of action
- The same product
This mechanism progressively favors individuals with little or no sensitivity to these products to the detriment of others. Products such as the SNS 209 Organic Systemic Pesticide take advantage of this mechanism to control pets.
The resistance of a pest to certain phytosanitary products is a major concern for all agricultural professionals. To prevent and manage the development of resistance, it is essential to understand its mechanisms.
What is resistance? How do you control it?
We can define pesticide resistance in two different ways.
It is a selected breed of insects or other organisms whose organisms can tolerate the doses of deadly poison. This is true for the vast majority of individuals in the natural population.
Resistance may also be permanent. It’s the case with a hereditary adaptation of a pest to a limiting agent used. This is resulting in lower mortality or sensitivity to that agent than the original one.
Both definitions state that pests respond to treatments aimed at reducing their numbers by “producing” resistant breeds. These are no longer effective against the preparations used so far.
The importance of the problem of selection of resistant pest breeds is reflected in the creation of special working groups called “Insecticide Resistance Action Committee” in the mid-1980s. (IRAC) and the Fungicide Resistance Action Committee (FRAC). The latter brings together representatives of phytopharmaceutical companies and scientific institutions. All are dealing with resistance problems, especially with how to prevent this process.
The phenomenon of selection of resistant pest breeds is a problem of field, greenhouse, and orchard cultivation. These last two environments are particularly vulnerable to this phenomenon. That is mainly because of the intensification of production.
Consequences for production
The selection of resistant breeds is a serious problem for several reasons.
Among the most measurable is the economic aspect. The occurrence of the phenomenon of resistance makes it difficult to fight many pests. It creates a necessity for:
- Repeated treatments
- Increase doses
- Costly synthesis of additional measures with a different mechanism of operation
An important problem posed by the selection of resistant pest breeds is also the ecological aspect.
- Repeated treatments
- Increasing doses
- Introduction of new substances
- Increasing environmental risk
The selection of resistant races of different organisms may also pose a threat to human health, both directly and indirectly. This might be the case of combating certain arthropod species. Especially those that threaten human health by transmitting pathogenic microorganisms.
The selection of plant resistant breeds
The factors determining the selection of resistant breeds are:
The genetic diversity of the population depends on the allele systems of the immunity-related genes. It is a very complex mechanism and can run naturally. But it can also be controlled by humans with modern knowledge and technology, although it is difficult. This is called genetic engineering.
The application factors are:
- The size of the population subject to selection
- Method of application
- Dose of the pesticide
- Scope of application of the pesticide
- Type of product used
- Type of measures applied in the past
Biological and ecological factors are determined by:
- The number of generations per season
- Pest population density
- Spatial isolation (e.g. areas surrounded by natural barriers)
- The variability of ecological conditions in time and space
The rate of selection of resistant breeds is very different and depends on:
- The number of generations during the growing season
- Frequencies of the selection factor
- Homogeneity and strength of the preparation
- Biological properties of the selection factor
The mechanisms of resistance
Several types of resistance can be defined and differentiated:
- Simple resistance – only to one pesticide; occurs very rarely.
- Cross-resistance – when the use of a pesticide makes insects and mites resistant to other insecticides and acaricides from the same chemical group or even another (e.g. DDT, pyrethroids, organophosphorus preparations), this also applies to many other target organisms. In this case, you may need a spider mite pesticide and therefore be in search of the best pesticide for spider mites. Look for a pesticide spray or pesticide concentrate that is safe and effective.
- Multidirectional resistance – occurs when a population is resistant to many pesticides or even to many groups of pesticides because of the selection of many resistance mechanisms.
Resistance to phytosanitary active ingredients involves mechanisms that can be very complex. Those include:
- Biochemical mechanisms
- Physiological mechanisms
- Behavioral mechanisms
Beware: any failure in the field is not synonymous with resistance.
The demonstration of a loss of sensitivity of a population to pesticides is carried out in the laboratory under conditions very different from those encountered in reality.
It doesn’t therefore necessarily translate into practical resistance. This is mainly because the active ingredients are :
- Formulated within a product and sometimes associated with each other
- Used within the framework of reasoned programs
- Applied according to the parasite pressure or the weather
Pest resistance by modification of the target
The target of a phytosanitary product is generally a protein called the target protein. It’s necessary for the development of the pest. This is the case, for example, with fungicides of the IDM family (demethylation inhibitors). It acts by inhibiting one of the enzymes involved in the synthesis of ergosterol. This compound is specific to the cell membrane of fungi and is essential to its integrity. The best organic pesticides work in a similar way.
A mutation can sometimes change the structure of this target protein. It’s preventing the active ingredient from binding normally and playing its role as an inhibitor. Individuals in a population with this trait are then resistant to the active ingredient. In plant pathogens, the evolution of fungicide susceptibility appears to be mainly based on this mechanism when decreases in efficacy in practice are observed.
Resistance by overexpression of the target
Alternatively, the enzyme targeted by the active ingredient may not be modified, but multiplied in the cells. This is known as target overexpression.
Faced with such a phenomenon, the normal quantity of active ingredients is not enough to inhibit the activity of the enzyme and cause the death of the pest.
Resistance by overexpression of the target has been demonstrated in particular in the United States in glyphosate-resistant amaranth and ryegrass for the weedy part, and in Zymoseptoria tritici and IDMs for the pathogenic microorganism part.
Resistance by detoxification
Like all living organisms, bio aggressors have purifying functions:
These allow them to get rid of their own metabolites and also foreign molecules, such as those used in plant protection products. The implementation of these functions can prevent part of the active ingredient from reaching its target. This is called resistance by detoxification.
This phenomenon is frequent in insects and weeds. In pathogenic fungi, this mechanism is identified in Botrytis cinerea with regard to hydroxyanilides. It’s suspected to be at the origin of the few rare cases of resistance to multisite fungicidal active ingredients. In this case, you may need the best pesticide for mites or the best pesticide for spiders. Choose the right mite pesticide to ensure it works and is safe for your plants.
Resistance by excretion
This resistance mechanism involves an increased efflux of active ingredients via overexpression of transport proteins. This mechanism is non-specific to a chemical family and is encountered in strains of pathogenic microorganisms of the MultiDrug-Resistant type (MDR).
Although identified frequently in the laboratory, the excretion of active ingredients does not seem to be related to a decrease in product efficacy in practice, at least in plant pathogenic microorganisms.
It is possible for several resistance mechanisms to coexist in the same individual. Here, it is referred to as multiple resistance. For example, this is observed in populations of vulpine animals resistant to different ACCase inhibitors through target modification and detoxification. The accumulation of several resistance mechanisms can also be observed in pathogenic microorganisms in relation to:
- The same mode of action. For example, several mechanisms of resistance to IDM are observed in Zymoseptoria tritici. Various combinations of many target mutations, overexpression of target number and overexpression of membrane transporters.
- Different modes of action. For example, Zymospetoria tritici populations may be resistant to the QoI/strobilurin mode of action (G143A mutation at the QoI/strobilurin target) and the IDM mode of action.
Examples of pesticide resistance
The most obvious examples of the organic pesticide resistance problem are the spotted pear honey and scab of apple trees.
The honeysuckle has become immune to two important groups of the synthetic pyrethroids and organophosphorus pesticides. There were situations in which further use of these products caused an increase in the number of people immune to the problem.
This situation happened for two reasons: the pesticides used were ineffective in combating resistant pest breeds and at the same time destroyed the enemies of natural honeysuckle, which play an extremely important role in reducing the number of this species. The effectiveness of the pesticides’ action is particularly strong in the second half of the growing season.
A similar phenomenon occurred after a period of uncritical and widespread use of benzimidazole preparations in the fight against scab. Preparations that seemed to solve the problem of fighting this disease forever proved to be ineffective already after four years. In some orchards, their use even increased the problem. It happened because the perpetrator of the disease, the fungus Venturia inequalis, became immune to this group of preparations.
Sometimes it gives the effect of several or even several dozen years. Such an action is also in the interest of practitioners. Many of the initially good pesticides were lost by irrational use, not paying attention to the recommendations and warnings given. It’s therefore crucial to understand how these work as well as when to apply an organic weed killer.
Fruit and vegetables that show pesticide resistance
The World Health Organization (WHO) recommends eating seven servings of fruit and vegetables a day. In the latest pyramid of nutrition, they’re the foundation of our diet. No wonder, these are some of the healthiest products available. At least in theory. In practice, most of them contain toxins from environmental pollution and pesticides.
Is there a way to protect ourselves from harmful substances in agricultural crops? To some extent, yes. First, you can buy food from a trusted farmer who does not spray. However, this solution is logistically tricky. Another option is ecological shopping, but you have to pay more for it. The third way is to choose products according to their resistance to pesticide absorption. You can also look for farmers who use vegetable safe pesticide or natural pesticide for vegetables. Those that use chemical free pesticide products would be another option.
The list of these fruits and vegetables is published every year by the American Environmental Working Group (EEC). The EEC rankings contain 48 items sorted by pesticide content. Among them, experts select the so-called pure fifteen and the dirty twelve – i.e. products least and most contaminated by spraying.
Ways to delay pesticide resistance
Nature responds to our actions by counteracting them. The process of selection of resistant pest breeds is unavoidable, but it can be significantly slowed down or delayed by the following rules:
- Application of measures only when absolutely necessary (in the case of pests, when the danger thresholds are exceeded)
- Use of pesticides according to the doses established during the tests
- Observation of the rotation of preparations – i.e. alternating between those with different mechanisms of action
- Discontinue treatment with agents with a similar mechanism of action if the resistance of the local population of pests to compounds from a given chemical group is found
- Protection of beneficial fauna, especially predators and parasitoids, because they play a very important role in reducing the number of herbivorous species, regardless of their resistance
- If possible, cultivating primarily resistant plant varieties – the breeding method
- Avoiding the use of all chemicals to stimulate the development of certain groups of pests
- Using integrated pest control, which is a rational long-term action that minimizes the pace of selection, since this method makes it possible to reduce the number of treatments as well as the volume of pesticides used, and thus reduces the selection pressure
The fight against the emergence of resistance to pesticides’ active ingredients is first and foremost a matter of prevention. To reduce the risk of infestation, care should be taken to limit the overwintering forms of the pests. To this end, a combination of agronomic measures and cultural practices will be used to ensure good prevention against pest-resistant breeds:
- Crop rotation to limit the frequency of crops and weed hosts of the insect
- Destruction of crop residues such as corn canes, pruning wood, or others
- Working the soil with occasional plowing
The reasoning of pesticide protection is the second part of the fight against resistance phenomena. Valid recommendations for fungicide and herbicide protection can be found here:
- Alternate over the season active substances with modes of action that do not present cross-resistance
- Adjust the positioning of treatments according to their mode of action and the intervention threshold
- Respect the recommended doses as the fractionation of applications favors the appearance of resistance
Challenges for today and tomorrow
The challenge in the fight against pesticide resistance is to preserve over the long term the effectiveness and diversity of the crop protection solutions offered to farmers.
However, regulatory pressures are one of the many obstacles to renewing the portfolio. Good phytosanitary practice measures are essential to maintain the effectiveness of existing solutions.
In order not to reach a deadlock, it is essential that the different actors in the agricultural sector put in place crop protection strategies. Those should be integrating prevention and/or resistance management in a concerted manner.
If you’re looking for the best natural pesticide or the best organic pesticide, then you’ve come to the right place! Check out our selection of organic botanical pesticides here.