When a pest becomes resistant to a pesticide, the resistant pest can become a serious threat to our livelihoods, especially when the pesticide it is resistant to is the most widely one used for control. One example is glyphosate.
Glyphosate (Roundup) was first marketed in the 1970s as a nonselective herbicide. With little herbicide resistance linked to glyphosate several years after its launch, scientists engineered glyphosate resistant crops such as corn, soybean, cotton, and canola, with the first glyphosate resistant crop coming to market in 1996. This development made it possible to apply glyphosate in crops that were once susceptible to the herbicide.Soon thereafter, glyphosate became the most widely used herbicide.
In 2022 there are 38 weed species that are known to have developed resistance to glyphosate.
What is pesticide resistance?
Pesticide resistance refers to a change in the sensitivity of a pest population to a pesticide, resulting in the failure of the pesticide to control the pest. Today, a variety of pests, including weeds and insects, have been shown to be resistant to pesticide products.
Why does pesticide resistance occur?
- It is generally accepted that within a pest population, there is a range of susceptibilities to a pesticide; from highly susceptible to tolerant or resistant.
- When pesticides are applied, they are a form of selection pressure with whichsusceptible pests are eventually eliminatedwhile tolerant and resistant pests become more prevalent.
- Our livelihood is closely linked to our ability to control pests and we often rely, almost exclusively, on pesticides for their control. This increases the selection pressure for the pest which drives it towards becoming resistant, i.e., the more tolerant pests prevail.
- In response to resistance, managers may increase pesticide quantities or frequency of application, which exacerbates the problem. In addition, some pesticides are toxic toward species that feed on or compete with pests. This can paradoxically allow the pest population to expand, requiring more pesticides.
Pesticide resistance is one of many battles that farm managers must fight every year. At the heart of that battle is herbicide resistant weeds.
What is a herbicide resistant weed?
A herbicide-resistant weed is a weed species that has developed the ability to survive application of a herbicide which previously controlled it. The intensive and continuous use of the same herbicide(s) over the last few decades has resulted in the evolution of herbicide-resistant weeds.
Herbicide resistance weed management
The prime strategy for managing herbicide resistance in weeds is to reduce the selection pressure for resistance evolution by any selecting agent, while maintaining adequate weed control. Selection pressure has the greatest impact on herbicide-resistance evolution and is a factor that farmers can control.
Herbicides do not cause resistance but instead select for naturally occurring resistant individuals within a population.
Tactics and practices for managing herbicide resistant weeds
Herbicide resistant weeds exist in any given population. When herbicides with the same mode of action are used in the same field year after year, the repeated applications act as selection pressure for resistant weeds. Eventually, the entire population of weeds within the field maybecome predominantly resistant to the herbicide that originally could control the population.
When resistant weeds are left to grow in a field, they can produce hundreds of thousands, and sometimes even millions of seeds, making the resistance problem even bigger.
The following management practicescan help slow down or prevent the establishment of herbicide resistant weeds:
- Diversifyingthe herbicide plan.
- Use herbicides with multiple modes of action.
- Mode of action is the way in which a herbicide controls a susceptible plant. It usually describes a biological process or enzyme in the plant that the herbicide interrupts to affect normal plant growth and development. For example, glyphosate is a protein synthesis inhibitor.
- There are eight herbicide modes of action.
- Rotating herbicide modes of action helps prevent the same product from being used year after year in the same field.
- Avoid using herbicides that rapidly lead to the development of resistant weeds.
- Use herbicides with multiple modes of action.
- Follow herbicide label instructions and use adjuvants and the recommended application rates.
- Provides the most effective method to control weeds in fields.
- Adjuvants help the spray solution stick, cover, and spread on target plants for enhanced penetration.
- Spray weeds at the recommended size in order to get the best control.
- Practice crop rotation to help reduce selection pressure and minimize the potential for developing resistant weeds.
- Scout fields after application of herbicides to detect weed that escape control. If weeds that escape control are suspected to be resistant, treat them with a herbicide of a different mode of action. Include an adjuvant that helps stick, spread, cover, and penetrate target plants.
Resistance management helps extend the life of the arsenal of pesticides that have are carefully developed, tested, and regulated at high cost by the US Environmental Protection Agency. Losing the effectiveness of any one of these pesticides due to the development of resistance should not be allowed to happen.
- Herbicide Resistance Action Committee (HRAC). Guideline to the Management of Herbicide Resistance. https://hracglobal.com/files/Management-of-Herbicide-Resistance.pdf#:~:text=not%20present%20%20%20%20%20%20Risk,Many%20times%20%20%204%20more%20rows%20 [Accessed, June 27, 2022].
- United States Environmental Protection Agency (US EPA). 2017. Guidance on Managing Pesticide Resistance. https://www.epa.gov/pesticides/epa-guidance-managing-pesticide-resistance. [Accessed, June 27, 2022].
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