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Pesticides

Have you ever helped your parents apply some weed killer in the garden when nasty weeds take up too much space? This is an example of a pesticide. These chemicals are extremely useful in the agricultural world because they help deter pests and allow crops to grow and reproduce without being disturbed. However, the inappropriate use of pesticides can have dangerous effects on the surrounding environment and consequences on human health. Today we shall look at some examples of pesticides, their benefits, and their negative environmental impact.

The meaning of pesticides

A pesticide is an artificial or natural substance that aims to deter or exterminate a 'pest'. These pests can be anything from caterpillars and aphids eating farmers' crops, magpies that eat the berries in your garden or even weeds that overtake the cultivated plant. Pesticides came to prominence in the late 1800s during the Agricultural Revolution. Their use has only improved over the last 150 years as scientific research has allowed us to understand the effects of pesticides on the target organism, non-target organisms, and the environment.

A pesticide is a substance that destroys pests that are harmful to cultivated plants or animals.

A pest is any organism that can cause destruction or attack crops, food or livestock.

There is still not enough research about many pesticides, especially concerning the long-term effects on the gene pools of wild populations.

The importance of pesticides

Let's have a look at why pesticides are important for agricultural workers:

  • Consumption: pesticides deter or exterminate pests that consume agricultural crops and reduce yield.

  • Space: herbicides exterminate weeds that take up space and outcompete crops.

  • All-year-round foods: pesticides can deter pests even in their most active months (such as larvae in the spring), so farms can provide food throughout the year.

  • Contamination: pesticides decrease exposure to dangerous microorganisms.

Examples of pesticides

Many types of pesticides intend to deter pests, such as insects, fungi, weeds, rodents, and larvae. Let's have a look at some examples:

Herbicides

Herbicides are synthetic chemicals that aim to exterminate weeds or herbs that grow between desired crop species. These chemicals are not harmful to humans and animals but will negatively impact non-target producers and have consequences for the surrounding ecosystem. Herbicides inhibit photosynthesis within plants or stop them from synthesising vital molecules like starch and cellulose.

Starch is an essential storage molecule that contains the energy-release molecule, while cellulose is necessary for building cell walls.

Two types of herbicides differ depending on their application:

  • Selective herbicides: these will be manufactured to affect only the target organism on application and have limited effect on other species.

  • Non-selective herbicides: these chemicals are applied over a large area and affect various species.

Selective herbicides are more environmentally friendly than non-selective ones and will have a much smaller effect on the surrounding ecosystem.

Fungicides

Fungicides are poisonous chemicals that target fungi and fungal spores growing on or near crops. Fungi can inhibit the growth of stems and leaves and take vital nutrients from the plant while also releasing harmful toxins that damage the host and nearby plants. Poisonous fungi on the farm can also endanger the health of livestock and workers.

There are two types of fungicides you should know:

  • Systematic fungicides: these often spray and are applied to the whole plant to kill any spores and inhibit the future of spores. The effects of these chemicals must be well researched because they are applied to the entirety of the plant, so any side effects will most likely result in death.

  • Seed fungicides: these chemicals are applied to the plant prior to germination, causing them to become immune to certain fungi in the future.

Fungicides bind to specific carrier proteins and enzymes inside the target organism, which will stop or inhibit a variety of biochemical processes.

Natural fungicides can be produced by processing plants like rosemary and oregano. This can limit any contamination of the soil.

Insecticides

Insecticides are synthetic and sometimes organic chemicals that are applied to crop fields to deter insects or directly to the insect. The chemicals are toxic to the pest and can cause poisoning by inhalation (fumigants) or after consumption of the protected crop.

There are two types of insecticides that farmers use:

  • Repellents are chemicals that are sprayed on the protected plant and release unpalatable smells and pheromones to deter the pest.

  • Non-repellents are insecticides that are applied over a large area and will poison and kill any potential pests.

Fumigant insecticides are most effective against flying pests that do not stay on the crops for long periods of time. In comparison, stomach poisons are effective against larvae that spend time-consuming the crop and are more likely to ingest the poison.

Pesticide applicationFigure 1: Pesticide application for control of nematodes in Turkey, via Wikimedia Commons.

Rodenticides and larvicides

Rodenticides are poisonous chemicals that aim to exterminate rodents, whereas larvicides target invasive organisms in their larval stage.

Rodents include rats, mice, squirrels, and groundhogs and will feed on edible crops and spread disease.

Benefits of pesticides

Pesticides benefit agricultural workers and help maintain an economically productive and efficient farm. Here's how:

Benefit

How does the pesticide prove to be useful?

Disease prevention

Pesticide use can help reduce diseases transmitted from flying pests to crops and livestock or from rodents to produce. This will protect agricultural workers, their products, and those purchasing their produce.

Improves crop yields

The use of pesticides will protect crops from dangerous insects, fungi, and weeds that may hamper the growth and germination of crops.

Improved soil quality

Pesticides can maintain soil fertility by protecting producer populations and ensuring that nutrients continue circulating.

Time-saving

Using herbicides will save farmers lots of time because they will not have to remove each invasive weed physically.

Reduces tillage

Maintaining arable land with pesticides means farmers do not have to re-prepare the ground as often throughout the year.

Tillage is the preparation of land for crop growth by physical methods (such as ploughing and sowing).

Effects of pesticides on the environment and human health

We have covered the benefits of pesticides, but the overuse of these substances can have devastating effects on soils, nearby waters, surrounding ecosystems, and human health. Let's have a look at some of these in more detail:

Soils

When pesticides are applied on large scales, many poisonous chemicals will seep into the soil. These chemicals will reduce the diversity of decomposing microorganisms in the ground. These bacteria are essential in building up organic material and making nutrients available to plants, so without them, the soil becomes less fertile.

Organic matter also goes a long way in retaining a water supply in the earth, which is essential for the hydration of plants and the movement of ions.

Water

The contents of soils overloaded with pesticides will run off into nearby waters and pollute aquatic ecosystems. As these chemicals are often toxic and aim to exterminate wild species, they have the potential to do the same in aquatic ecosystems. Toxic chemicals can undergo biomagnification in ecosystems, leading to widespread genetic defects and population declines.

Biomagnification involves pesticides increasing at higher trophic levels, as consumers eat many poisoned organisms at lower trophic levels.

Air

Pesticides can be carried to new locations by wind currents in a phenomenon called 'pesticide drift'. This means that they are taken to other ecosystems with crops that may be badly affected by the poisonous chemicals. Some pesticides contain volatile organic chemicals (VOCs) that contribute to forming ground-level ozone, a dangerous greenhouse gas.

Ecosystems

The overuse of pesticides can reduce soil fertility, meaning that growth and eventual crop yields will be limited. The total effect of pesticides on non-target organisms is not fully understood for many pesticides, so other crops and wildlife may be affected. This will have consequences for any individuals in the ecosystem involved in interspecies relationships with the affected organisms (such as pollinators and predators). This may lead to bioaccumulation and biomagnification, which can disrupt the food web.

Bioaccumulation is the process whereby a toxic substance persists in an ecosystem and is present in individuals across generations.

Biomagnification in a terrestrial ecosystemFigure 2: Biomagnification in the food chain, dots represent organic molecules, while crosses represent toxic chemicals, via Wikimedia Commons.

The concentration of harmful substances increases up the food chain, as consumers will eat many poisoned organisms at lower trophic levels.

Human health

There are direct impacts of pesticides on agricultural workers, such as skin irritation or accidental inhalation, but there are many more indirect impacts.

  • Produce delivered from the farm to stores may contain dangerous levels of pesticides. Human contact with these can result in nausea, eye irritation, respiratory problems, and diarrhoea.
  • Biomagnification in aquatic ecosystems can lead to heavily caught fish (such as trout) containing large amounts of toxic chemicals that will make their way into fisheries.

The effects of pesticides on the environment and human health: how are pesticides controlled?

There are a few methods by which the distribution, type, and use of pesticides are controlled in the UK:

  • Authorisation: a wide range of independent research units are available to test pesticides' effectiveness and potentially damaging impacts before they are distributed to farmers.

    The Plants Protection Products Regulations of 2011 are referred to in this process.

  • Where and how: measures are taken to protect surrounding terrestrial and aquatic habitats from pesticide use and to decide the training required to apply pesticides.

  • Residues: pesticides can remain in food in residues. Therefore, maximum residue levels depend on the pesticide and product.

Organochlorine pesticides are highly restricted because of their lipophilic qualities (they quickly accumulate in fatty foods such as meat and milk).

Organophosphates inhibit neuromuscular enzymes in pests and humans, so they are highly restricted too.

Pesticides - Key takeaways

  • Pesticides are artificial and organic chemicals used in agriculture to deter and exterminate pests such as locusts, aphids, and caterpillars.

  • Pests can harm agricultural productivity by slowing crop growth, reducing yield, and spreading disease among livestock.

  • Examples of pesticides include fungicides (aim to kill fungi and fungal spores), insecticides (seek to deter and kill insects), and herbicides (aim to eradicate invasive weeds).

  • Pesticides can reduce microorganism biodiversity in soils and make them less fertile, they can leach into aquatic ecosystems and poison vulnerable species, and winds can also carry them to neighbouring ecosystems.

  • The effects of pesticides on non-target organisms are not always well understood, so many organisms in the ecosystem are affected too.

Frequently Asked Questions about Pesticides

Pesticides are artificial or natural substances that deter or exterminate pests.

To improve crop yields by reducing the spread of disease, consumption, and invasive weeds.

Pesticides can poison organisms at lower trophic levels that will then be consumed poisoning organisms at higher trophic levels.

Pesticides have the potential to contaminate produce and drinking water. They will also damage ecosystems by reducing species populations and affecting interspecific relationships.

Pesticides cause declines in pest populations, which have consequences for the entire ecosystem (interspecific relationships, competition, and dependencies). They may also leach into nearby waters and poison aquatic organisms.

Final Pesticides Quiz

Question

What is a pesticide?

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Answer

An artificial or organic substance that aims to deter or exterminate pests.

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What is biomagnification?

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Answer

The process by which poisonous chemicals are present in larger quantities at higher trophic levels.

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What are some examples of agricultural pests?

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Answer

Locusts, aphids, caterpillars, and weeds.

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How do herbicides work?

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Answer

They inhibit photosynthesis in weeds or stop them from manufacturing essential molecules like starch, cellulose, and proteins.

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What are the two types of fungicide?

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Answer

Systematic fungicides and seed fungicides.

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How do seed fungicides work?

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Answer

They are applied to the plant prior to germination and protect them against future infection by the fungi.

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What is a fumigant insecticide?

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Answer

A chemical that is inhaled through the spiracles.

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What are larvicides?

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Answer

Types of pesticides that aim to exterminate insects in their larval stage.

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Why are larvae so dangerous for agricultural crops?

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Because they are sedentary organisms that spend long periods of time feeding on the same crop.

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How do pesticides benefit farmers?

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Answer

They maintain soil fertility (when used appropriately), reduce the spread of disease, and save time that would be spent physically removing pests.

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What are the effects of overuse of pesticides on soils?

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Answer

Microorganism biodiversity will decrease so fewer nutrients will be available to the crops.

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What are the effects of pesticides on nearby waters?

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Answer

When pesticides leach into waters they will poison vulnerable organisms and bioaccumulate in the ecosystem.

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How do pesticides cause global warming?

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Answer

Some pesticides release volatile organic compounds (VOCs) which initiate the formation of ground-level ozone.

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How do pesticides affect ecosystems?

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Answer

They may harm non-target organisms and have consequences for the food web.

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How can contact with pesticides affect humans?

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Answer

They can cause nausea, eye irritation, respiratory problems, and diarrhoea.

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