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Pest control Mode of Action: An overview

Theme: Basics of biocontrol

Theme: Integrated pest management

Theme: Biocontrol agents

Overview
Mode of Action 
Understanding Control of Weeds and Pests 
Biopesticides 
Chemical Pesticides 
Challenges and Limitations of Different Approaches 
Summary

Weeds, microbes, insects, and other animal pests cause significant damage to global agriculture. In the US alone, data suggests that pests deal billions of dollars worth of damage annually. Fortunately, there are many control methods for dealing with these pests, including chemical and biological approaches. Both these approaches have widely varying modes of action tailored to the type of pest being targeted. Having multiple modes is advantageous because it means more pests can be targeted and makes combination strategies more viable, and additionally, it enables a slowing down of pest resistance.

In this article, we explore different pest control methods, their mode of action, and the challenges and limitations of the different approaches.

Mode of Action

In crop protection, mode of action is a term used to describe how a substance or treatment causes a change in a pest or plant. Chemical herbicides have modes of action that refer to the enzymes (specific proteins that are important for the pest to live) they target, while modes of action for biopesticides include competition and hyperparasitism, which we explore in this article. 

Understanding the mode of action of different control methods is important because it allows the optimal solution for a specific pest to be selected. Furthermore, switching between modes of action can help prevent pests from developing resistance – which has become a growing issue in agriculture. 

Understanding control of weeds and pests

Pests that target crops come in a range of different species. This variety means that we need solutions for different pests. Solutions to pests include biological solutions like biopesticides, chemical pesticide products, preventative, or cultural methods. The choice of method depends upon many factors including the pest being targeted and environmental considerations. Fortunately, in many cases, there are multiple solutions available for a given pest.

Biopesticides

Biopesticides are microbes, natural substances and semiochemicals originating from nature or natural processes that are used to control pests that attack crops. Microbes include bacteria, fungi, and viruses, though oomycetes and algae are also used. Within these groups, there are many species and strains with specific activities against different pests.

Natural substances are often derived from plants, but they also come from animals, fungi, other microbes, and minerals. These include neem oil which is effective against a range of pests like mites and whiteflies. Other natural products include chitosan which is derived from the shells of crustaceans and is effective against fungal diseases, and orange oil which targets both fungi and insects.

Close of image of Neem (Azadirachta indica) with fruit.
Neem leaves and fruit. Credit: CABI

Biopesticides are sometimes broken down into two groups based on whether they attack pests or diseases. Biopesticides that attack plant diseases are called antagonists (e.g. Bacillus subtilis), while ones that attack insects are called entomopathogenic (e.g. Bacillus thuringiensis).

A close-up image of Bacillus thuringiensis under a microscope, showing small purple coloured rods which are the spores
Microscopic image of stained, rod-shaped Bacillus thuringiensis spores – Credits: Ansel Oommen via Bugwood.org

Biopesticides are diverse by nature and possess different modes of action.

Entomopathogenic biopesticides kill insects, directly or indirectly, by:

  • Toxin production, in which the substance directly kills the pest, and interferes with physiological activities such as inhibiting pest growth and preventing egg-laying.
  • Desiccation, which is the drying out of the pest.
  • Repellence, which prevents pests from feeding on the plant.
  • Biopesticides can be consumed by the pest or gain access by penetrating the insect.

Antagonistic biopesticides work through several modes of action including:

  • Hyperparasitism, where the biopesticide’s active ingredient consumes the plant disease (i.e. a parasite feeding on a parasite)
  • Direct antagonism and antibiosis, where the biopesticide’s active ingredient produces compounds that kill the disease
  • Competition, where the biopesticide’s active ingredient outcompetes the disease for resources and space
  • Plant resistance, where the biopesticide’s active ingredient releases compounds that activate resistance to the disease.

Chemical pesticides

Pesticides are chemical substances not originating from nature that are used to control pests. As such, pesticide is a broad umbrella term that applies to various methods of pest control. They are categorized based on which pests they control, and types of pesticides include insecticides, herbicides, rodenticides, virucides, and slimicides (fungi and molds).

Landscape image of a farmer spraying crops with chemical pesticides
Spraying pesticides in a field. Credit: Sundaram via Pixahive.com

Chemical herbicides are used to kill plants and are typically used for weed control. Chemical herbicides belong to different chemical families and can be used to target specific weeds or multiple species of plants. The use of chemical herbicides raises several safety and effectiveness questions due to the impact they can have on the environment and human health. Chemical herbicides act through different mechanisms, including inhibition of amino acid synthesis, and are usually grouped into chemical families.

Challenges and limitations of different approaches

Resistance

Many growers have used chemical pesticides with the same mode of action for years. This is problematic because pests can develop resistance against a specific mode, making the product ineffective. For example, the Colorado potato beetle, a significant pest in the USA and Canada, developed resistance to common chemical pesticides over time. A combination of products, using different modes of action, is likely to be more effective than using only one type and also offers a solution to dealing with resistant pests.

A close up image of two adult Colorado potato beetles on a leaf.
Colorado potato beetle (Leptinotarsa decemlineata) Credit: Howard F. Schwartz, Colorado State University, via Bugwood.org

At CABI, we recommend biological control methods for their efficacy, safety, and sustainability. We further suggest rotating between products with different modes of action to help prevent pests and weeds from developing resistance. While biopesticides have more complex modes of action that are less likely to trigger resistance than chemical pesticides, it is worth noting that resistance to biopesticides can sometimes occur. Therefore, we recommend rotating between different modes of action, even with biopesticides.

Side effects

Another challenge when controlling pests in agriculture is the potentially negative side effects that chemical pesticides can have on the environment and human health. Chemical pesticides are popular because they are relatively cheap and easily scalable. However, they can contaminate water, air, and soil and lead to a significant loss of biodiversity. This is in addition to the severe negative effects on human health.

Biological solutions largely mitigate these negative effects by providing a more sustainable and healthier alternative. Regulations from the Environmental Protection Agency (EPA), an independent government agency for environmental matters in the US, aim to reduce the use of harmful chemicals while promoting the use of sustainable, biological methods. Additionally, using biological plant protection products can allow growers to sell their products in more markets and reduce pesticide resistance.

Summary

There are many available strategies for controlling pests. These methods work through distinct modes of action. Understanding the mode of action of different control methods is important for determining which to choose, as different pests are susceptible to different modes. Biological control methods offer advantages in safety, effectiveness, and sustainability over chemical methods, which harm the environment and damage human health. Ultimately, it is important to consider the mode of action and which would be the most effective to use when deciding how to deal with your pest problem.

For more information on different types of pests and control strategies, you can consult the CABI BioProtection Portal resources and product database. To help find the best solution for your specific problem, visit our products page.

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