Research led by: Muhammad Alyan

Syed Saim Ali, Ansa Ismail, Saman Shahid

‍Introduction to Pesticides

Pesticides, chemical or biological agents designed to prevent, destroy, or control pests and diseases affecting crops, have significantly boosted yields of major fruits and vegetables over the past five decades. Pesticides are classified on many bases of their mode of action, systemic or not and their persistence.

Trends of Pesticides

Since their introduction in the 18th century, pesticides have evolved substantially. Initially, natural substances like neem (margosa) leaf powder and plant extracts were utilized. Later, in the 19th century, sulfur, heavy metals, and salt were employed as pest control measures. However, the toxicity of sulfur compounds led to the development of organic chlorinated pesticides like DDT, BHC, and toxaphene, which were initially considered safer alternatives. Unfortunately, the bio accumulative and toxic effects of these chlorinated organic compounds soon became apparent, prompting a shift towards herbicides due to their relatively lower dietary exposure. Although herbicides are also non-biodegradable and lead to many diseases after prolonged use. Contemporary agriculture often combines herbicides, insecticides, and fungicides, with popular choices including Roundup (Glyphosate) and atrazine. In Pakistan, the most commonly used are insecticides namely: Aldrin, BHC, Dieldrin followed by herbicides and fungicides. With prolonged use these pesticides are causing many environmental as well as health issues which include chronic heart diseases, cancer Parkinson’s, ADHD. Regardless of all these repercussions, the use of pesticides has increased to 2.5 million tons annually and is still increasing by time.

How pesticides work

The issue of weeds in agriculture is often resolved by the use of herbicides. These herbicides alter the biological processes of the weeds such as damaging the leaf cells causing weeds to dry up, interfering with photosynthesis or altering the uptake of nutrients all of which lead to the same result. For herbicides to work as desired it is necessary they are absorbed into the plant and are delivered to the site of action without being deactivated where they can accumulate and alter the biological processes of the plant. An example of herbicides 2,4-D which eliminates broadleaf weeds but not grass. It works by endlessly dividing the cells in the plant that carry water and nutrients. Some characteristics of a plant affect the performance of pesticide on that plant. Herbicides tend to work better in young, rapidly growing plants compared to older plants. Furthermore, the plant structure or shape also affects herbicide performance, for example, a thick waxy cuticle slows down the absorption of an herbicide into the leaf cells. Some plants are also immune to specific herbicides depending on the products in it.

Fungicides are used to get rid of fungi and they exist in a variety of different categories. Protective fungicides are applied as a protective measure as they only work when they are present on the plant as they prevent penetration but provide little to no effect when applied after infection. Curative or eradicate fungicides stop the development of the infection after it has begun. Some fungicides do this by restricting the reproductive abilities of fungi which can completely stop diseases when applied in the early stages of the disease. Uni-site fungicides target a specific process of fungal development, like respiration and cell division, which makes it prone to resistance development. Even one mutation could affect the effectiveness of the fungicide. In correlation, multi-site fungicides target multiple processes of fungal development which makes them less prone to resistance development compared to Uni-site fungicides as resistance must develop in all sites for the fungus to be completely immune to the fungicide.

Most insecticides act at specific sites of insect nervous systems causing immobilization leading them to die by starvation or dehydration. These types of pesticides are called Cholinesterase inhibitors who interfere with impulse transmissions in nerves in the synaptic gap. Depending on the pest the effect could take place by contact with residue of the pesticide or ingestion of the sprayed plant. Insect growth regulators (IGRs) are chemicals based on hormones that regulate arthropod development meaning they aren’t as effective on adults as they are on immature stages of insects.

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Negative effects of pesticides on the Environment

 There's a worldwide growing concern over the hazardous impacts of pesticides to humans, plants and other organisms present in the environment of developing countries, especially the long-term exposure of such chemicals. There has been an increased use of agrochemicals due to an increase in world population and increased demand for raw material, industrial goods, water, energy and living space. The use of these chemicals will continue as long as alternatives are unavailable. The risks linked to pesticide use now outweigh their benefits, severely impacting non target plant and animal biodiversity, aquatic as well as terrestrial food webs and ecosystems.

Pesticides make their way to undesirable parts by either dissolving in water or enter mammal bodies and get absorbed by the fatty tissues of animals; hence resulting in the persistence of pesticides in food chains for extended periods of time. These mammals unintentionally consume these chemicals by direct absorption through skin, uptake through gills during breathing and through drinking contaminated water. The higher the trophic level the greater will be the pesticide concentration which is known as bio amplification. This causes a huge disruption in the food web as it leads to the decrease in one population and increase in the other population causing an imbalance, which can eventually lead to extinction

Threats to biodiversity

Accumulation of pesticides directly target predators and raptors but indirectly they can reduce the quantity of weeds, shrubs and insects on where higher orders feed. Spraying insecticides, herbicides and pesticides have been linked to the decrease in populations of rare species of animals and birds. Long term and frequent usage lead to bioaccumulation and reduction in biodiversity.

Risks towards Aquatic biodiversity

Runoff of pesticides into water bodies influence the aquatic life. They affect aquatic plants which leads to decrease in the dissolved oxygen in the water. This is highly concerning as around 80% of dissolved oxygen is provided by these plants which is necessary for the sustenance of other aquatic organisms. Lack of oxygen can also cause changes in physiological and behavioral changes in fish populations Such as hypoxia. Atrazine is toxic to some fish species and indirectly affects the immune system of some amphibians. Carbaryl herbicide has been found toxic for several amphibian species whereas herbicide glyphosate is known to cause high mortality of tadpoles and javelined frogs. Small concentrations of malathion have been shown to change the abundance and composition of plankton and periphyton population which directly affects the growth of frog tadpoles.

Such chemicals have caused mutations, for instance Dr. Hayes discovered that 10% of male frogs raised in atrazine contaminated water become hermaphrodite. This showed us that the frogs that were genetically males, phenotypically developed ovaries within their testes. This disrupted the reproductive potential as these frogs developed the tendency to mate with other males and lay sustainable eggs. These frogs also had compromised immune systems leading to increased mortality from bacterial disease.  

Hazardous impacts on Terrestrial biodiversity

Pesticide exposure can cause sub lethal effects on terrestrial plants as well as killing non target plants. It is known that pesticides evaporate into the atmosphere after a few days of being sprayed. The volatilization of phenoxy herbicide can injure nearby trees and shrubs. These agro chemicals have disrupted populations of beneficial insects such as bees and beetles leading to significant decline as well as negatively affecting bee foraging behavior. This was proved when in the start of 21st century there was a sudden disappearance of bees. Although other factors were also a cause, neonicotinoids were one of themes it directly affected the bee’s ability to navigate, forage and reproduce. This was a major threat to the food industry as ⅓ of food production depended upon the pollination of these honeybees. The honey and wax obtained in commercial hives were reported to have been contaminated with a significant number of neonicotinoids. Since 2006 there has been a drop in their population each year.

Another concerning factor of the overuse of pesticides is their leaching into soil affecting microbes residing in it. These microbes are beneficial in many ways such as increasing soil fertility, breakdown of organic matter as well as nutrient uptake. In the future, if this overuse continues, we may eventually loose these organisms forever and the soil may degrade. Some fungicides have been shown to disrupt denitrification, nitrification and other bacteria dependent processes. For example, the glyphosate herbicide restricted growth of nitrogen fixing bacteria in the soil.

Influence on Human Health

According to world health organization, around 3 million cases of pesticide poisoning, and 220,000 deaths were reported in developing countries. Some people are more prone to the toxic effects than others such as babies and agricultural workers. The human bodies retain them through absorption in the circulatory system. The toxic effects are produced when the concentrations of pesticides in the body increases more than its initial concentration in the environment. The effects are variable, they may be short term or long term. For instance, short term includes headaches, stinging of eyes, irritation of nose throat and skin, rashes, blisters etc. It may be years for the long-term effects to appear and by then it has already caused damage to other organs. Some effects include neurological effects such as loss of coordination and memory, reduced visual ability and reduced motor signaling. Pesticide exposure may also lead to damages in the immune system and cause hypersensitivity, asthma and allergies. They have also proved to cause damage to the liver, lungs kidneys and may cause blood diseases.  

Alternatives to pesticides

Due to the health risks of basic pesticide use, the use of alternate methods is essential for crops to be consumed. Some physical methods like mechanical barriers and companion planting help to reduce weed growth and insect population. Using barriers, like mulch, between rows of crops helps to reduce the growth of weeds and barriers of eggshells and other similar obstacles helps to stop slugs another such organisms like snails from entering the crop area.

Biopesticides can also be used as an alternative to conventional pesticides. The US environmental protection agency defines biopesticides as “... naturally occurring substances that control pests...” most of which don't have the same health risks as conventional pesticides. Biopesticides include microbial pesticides and Bio-derived chemicals. Microbial pesticides consist of bacteria, viruses or fungi that attack insects while ignoring plants. Bioderived chemicals are extracted from plants of which 4 are in commercial use including neem oil and other essential oils that control pests and other diseases.

Parasitic insecticides play a big part in natural pest control. Parasites attack and infect the bodies of insects while the plants remain unaffected. An example of a Parasitic insecticide are Parasitic wasps; Parasitic wasps lay their eggs inside hosts killing it when eggs hatch and produce larvae resulting in less harmful insects and more beneficial Parasitic wasps.

Natural and non-harmful products can be used to create fungicides to the point that homemade fungicides, that consist of milk and baking soda, are also a viable option for an alternative to conventional fungicides.

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Conclusion

Pesticides are classified in to many groups according to their mode of action such as:  insecticides target insects that disrupt crops by means such as ; IGRs an hormones which prevents insects development and reproduction, neurotoxins which disrupt the nervous system of insects causing paralysis or death .Herbicides are responsible for controlling weed growth as they can block photosynthesis in plant starving weed of energy furthermore it also damages plants cell membrane preventing it to uptake nutrients from soil. Fungicides combat fungal diseases in soil by preventing fungal cell wall growth, disrupting its membrane as well as blocking essential fungal enzymes.

Pesticides along with benefits has many detrimental effects on the environment and living things. Pesticides can cause soil contamination which can further cause water pollution as well as eutrophication in streams and rivers. Overuse of fertilizers also leads to degradation of soil quality and structure. The use of fertilizers has also said to alter the balance and functions of ecosystem. For us humans prolonged use of fertilizers can cause many neurological disorders such as Alzheimer’s, Parkinson’s. It also causes reproductive problems such as infertility, birth defects and development delays. Furthermore, it can also lead to endocrine and respiratory problems.

The huge number of adverse effects of pesticides cannot be ignored which is why there is need to develop the pesticides mechanism in such a way that potential risks are decreased by developing a safer and more sustainable solution balancing agricultural productivity with environmental and human health. Biopesticides can be used which ecologically manipulates habitat to promote beneficial organism and reduce pest population. Robotic weeding and pruning can be used to avoid the use of herbicides. Furthermore, precision agriculture can help monitor crop health so that pesticides can be used only where necessary. Alternatives of pesticides include installing physical barriers to prevent Pest access and crop rotation to disrupt pest life cycle. Moreover, organic pesticides such as neem oil, pyrethrin and garlic sprays can be used to get rid of pesticides naturally.

The utilization of Pesticides in agriculture has no doubt yielded significant benefits, as by increasing crop productivity, however the health and environmental risks brought up by it can never be overlooked. The global community is striving to adopt an integrated pest management strategy that balances efficiency with environmental stewardship and human well-being. This way we can extenuate the unfavorable effects of pesticides as well as advance towards a healthier and more sustainable food system for the future generation

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