Farming practices: chemical and biological control of agricultural pests, and the use of natural and artificial fertilisers and the environmental consequences of using nitrogen fertilisers

By Sara B 13B

Chemical and biological control of agricultural pests

Pests are destructive insects or other organisms that attack crops, food or livestock. Pest control is vital in the agricultural industry as if the pests are not treated properly, they can invade our food sources, corrupting large amounts of food. There are two main types of pest control:

1) Biological Control: This type of control uses predators, parasites or pathogens to kill and therefore control the level of damage carried out by the pests.

2) Chemical control: This type of control uses pesticides, which are chemicals that either kill pests or interrupt their development.

Explain the effect of chemical pesticides on local wildlifeAs mentioned above, pesticides are chemicals that are used to kill organisms. In order to do this, they contain small, weak forms of poison. Even though the pesticides do their job killing pests, they also have an effect on the wider ecosystem and wildlife in an area. Firstly, although only small amounts of fertiliser are sprayed on plants, the effect of the poison increases as you travel up the food

chain. This is known as bio-magnification. An example of this can be seen in a wheat field. A farmer may spray 0.000 003 ppm toxins from a fertiliser on to one strand of wheat, which is completely harmless to the majority of organisms. However, when a mouse starts to eat the wheat, it

does not just eat one strand of wheat, it eats many, which means that the amount of toxins in its body rises to 0.04 ppm of pesticide. Then, a weasel, which is a secondary consumer, eats the mice and the level of toxins in its body rises to 0.05 ppm as it accumulates in its body. When the food chain reaches its fourth trophic level, where an eagle is preying on the weasels, the concentration of the fertiliser poisons in its body becomes so high that it is life threatening for the organism. This example shows how fertiliser can still pose a huge threat to larger organisms due to the cumulative effect it has as it travels up the food chain. The idea of the concentration of a

Figure 1: www.pakagrifarming.blogspot.co.uk/pollutant from the environment increasing from the first organism in a food chain to the last is called bioaccumulation, and is the effect seen by the movement and amassing of fertiliser toxins. Figure 1 displays a similar example to the one described, beginning from an aquatic ecosystem. Furthermore, although most fertilisers are used for a particular pest, few are specific and therefore kill many beneficial and harmless organisms. In addiction, the majority of the pesticide does not even go on the crop. Over 90% is transferred to the surrounding eco-systems and has lasting effects. One prominent example is that of the honeybees, which are one of the most important organisms in natural ecosystems due to their roles as pollinators. A recent (June 2014) study, which looked at 800 reports, stated that pesticides have been linked to the decline of honeybees. Bees are particularly vulnerable to pesticides because chemicals hurt their ability to navigate and learn. Another example is that of worms, which aerate soil and are fundamental in the role of decomposition as the break down decaying matter so that bacteria can return useful compounds (nitrogen and carbon) back to the soil. The chemicals in pesticides affect the worm’s ability to tunnel through the soil. Furthermore, the decline in the population of worms could also affect the tropic levels of the food chain above them. Birds in particular are very vulnerable to this.

Compare chemical pesticides with biological control and identify the advantages and disadvantages of bothFarmers use both chemical and biological controls in order to ensure maximum crop yield and both have distinct advantages and disadvantages. One of the largest advantages of biological controls is that the farmer does not have to worry about poisoning himself, his family or the surrounding environment. This is extremely advantageous to the ecosystem, as there will be no effects of bio magnification or bioaccumulation to cause danger to large organisms supported by producers that have been treated with pesticides. Furthermore, there is no risk that compounds from the pesticides will run into water bodies, causing detrimental effects such as eutrophication. Another key advantage of biological control is that they are usually specific: the predator or parasite will only control the population of the pest and will not affect any other aspects of the wildlife or ecosystem. In addition, once a biological control is introduced, it is usually self-sustaining. This means that usually, the farmer will not need to put in any extra effort after introducing the biological control to manage the pest problem. This also means that biological controls can still be functioning after long periods of time, unlike chemical controls. Additionally, biological controls can also be very cost effective in the long run. Although the initial cost of introducing a new species may be high, the self-regulating nature of biological controls and the fact that they last for

extended periods of time generally make them good investments. Finally, biological controls are usually very effective. As the species that is introduced will naturally gravitate towards the pest, it is extremely likely that a reduced pest population will be seen. Although there are many benefits of biological controls, there are also some disadvantages. Firstly, one must consider the dangers of introducing a non-native species to control pests. Essentially biological control is somewhat gamble: one cannot control the biological control as soon as it is released into the eco-system. It is very hard to ascertain whether it will do the job it is meant to do. There is always the possibility that, whilst the control is supposed to manage one pest, it attacks the crop instead and destroys harmless parts of the ecosystem. The control could even become more of a pest than the original pest it was brought to control. An example of this is the introduction of a tree-climbing ladybird in the United States in the 1970’s to deal with the growing problem of aphids destroying pecan orchards in the Southern states. However, although the ladybirds did devour the aphids, they had such an enormous appetite that they also starting feeding on the native ladybird species which led to its eventual extinction. Furthermore, introducing a non-native species may disrupt the natural food chains already in the ecosystem. Biological controls also take a long time to produce a reduction in the pest population. Other pesticides have much quicker effects. Additionally, biological controls will never completely eradicate a pest. In order for the biological control species to survive, there must always be a population of pest to feed on. Lastly, although in the long term, biological controls are reasonably cost effective, the initial cost of introducing the non-native species is very expensive. There is also a lot of planning that must go in before hand, which is also time consuming. The chemical control of pests is one that farmers tend to prefer. It has many benefits, such as the fact that it works very quickly at eradicating the pest. They can also work by prevention, so that the pest control problem never has to arise in the first place. Chemical controls are also a lot cheaper to buy than biological controls. They are widely available, so there is little trouble in purchasing good quality pesticide. Furthermore, if a farmer had more than one pest, there are many types of pesticide that can be used on various types of pest where as biological controls are very specific to one species. A farmer can also decide on how much pesticide to use and where to use it, so he can specifically target problem areas compared to biological controls that are very general. There is also no uncertainty with chemical pesticides. The farmer knows exactly what he is getting and the likely effects. However, there are some large disadvantages to chemical pesticides. Firstly, chemical pesticides are very damaging to the environment. Chemicals are rarely specific to one species of pest, and so many other harmless and useful organisms, such as bees,

worms and types of plants get destroyed as collateral damage. Furthermore, the bioaccumulation of toxins from fertilisers can lead to illness and death in larger organisms due to bio magnification (explained above). The leaching of these chemicals into bodies of water can also have huge effects on aquatic organisms and ultimately lead to eutrophication (explained below). Moreover, there is the risk to the farmer and his family if they are constantly eating food that contains poison. The build up of toxins in their bodies could lead to severe illness. In addition, the cumulative effect of buying chemical pesticides every year is very expensive. Unlike biological controls, they are not long lasted and therefore treatment of crop by pesticides must be repeated periodically in pest rich seasons. In addition, unlike biological controls, chemical controls must be physically distributed amounts the crop. This means the farmer either has to pay someone to regularly spray pesticide on his crops, which is expensive or do it himself, which is time consuming. As well as this, the constant use of pesticide could over time, lead to resistance in the pest species. This could then lead to an increase in the pest population and the farmer having to invest in a new pesticide to kill off the pest population again.

List the features of an effective chemical pesticide and biological control method.There are several key points that make an effective chemical pesticide:

1) It should be biodegradable. A pesticide should be able to be broken down into smaller, non toxic materials that will to cause problems for organisms in the future

2) It should also be cost effective. Even though the cost of developing pesticides is high, a pesticide can only be used in one ecosystem for a limited amount of time before the pest becomes resistant. Furthermore, as pesticide must be regularly applied, a single bottle of pesticide should not been too expensive.

3) Non-accumulative. The pesticide should not be able to bring toxins in greater concentration up the food chain.

4) Non-mobile. A pesticide should stay where it is applied so that other organisms are not damaged unnecessarily and the full force of the pesticide is on the pest.

An effective biological control should be the following: 1) Specific. A biological control should specifically target the pest

organism rather than other, non-aggressive species. It should also be of no harm to the farmer’s crop.

2) Good coloniser. A biological control should be able to quickly colonise an area and maintain a population that causes the highest decrease in pests that the control can survive on.

3) Sustainable. The non-native species should be able to sustain its population and therefore the pest control for extended

amounts of time, particularly due to the high initial costs of introducing foreign species into a new ecosystem.

Explain how different methods of pest control are integrated (integrated pest management) to minimise damage on the environment.The Integrated pest management scheme is pitched as an effective yet environmentally sensitive type of pest control that uses a combination of up-to-date information about pests (their life expectancy and life cycle) and available pest control methods. IPM manages pests in an economical way with the minimum damage to people, property and the environment. IPM works by having a series of different evaluations, controls and decisions, some of which are stated below:

1) Set action thresholds. Before taking action against the pests, identify the level of harm the pest population must be reaching before it is advisable to take action.

2) Monitor and identify pests. As not all insects and weeds are pests, farmers must monitor the organisms that seem to be harming the crops before making a decision on whether it would be economical and acceptable to take action.

3) Prevention. In order to minimise the necessity of pesticides in an environment, prevention measures can be put in place to manage the pest population to a level where they are not out of control

4) Once all three above points indicate that pest control is needed, IPM must identify the type of control needed. Effective but less risky controls are used first, for example using less harmful chemicals such as pheromones or by simply trapping or weeding pests. If these methods do not work, then harsher measures can be used. Non-specific pesticides are a last resort.

Therefore, IPM helps to minimise the environmental effects of pesticides in a number of ways. Firstly, IPM encourages farmers to not use harmful pesticides except as a last resort. It is extremely likely that most pest problems can be solved using effective measures and prevention schemes. By doing this, IPM also helps keep a balanced ecosystem: by introducing chemicals into an environment, food chains and habitats are disrupted and can cause long term effects and damage. By reducing the number of chemicals that go into an eco-system, even if pesticides are needed in certain incidences, the overall negative effect on the environment is minimised. In addition, the IPM outlines guidelines for farmers who, perhaps do not know of any other pest control methods other than pesticides to control their pests. It gives detailed information on the steps to take so that farmers can easily follow the guidelines.

The use of natural and artificial fertilisers

Compare organic (natural) and inorganic (artificial) fertilisersFertilisers are compounds that are added to the soil in order to enrich it with mineral ions such as nitrogen, phosphate and potassium. There are two types of fertiliser:Natural Fertiliser: This fertiliser consists of dead and decaying remains of plants and animals as well as animal waste such as manure and bone material. Artificial fertilisers: This type of fertiliser is mined from rocks and deposits which are the converted into different forms and blended together to create a mix of minerals that are uniquely suited to a particular crop. Due to the importance of the elements of nitrogen, oxygen, phosphorus, they are almost always present in fertilises.

Explaining the advantages and disadvantages of both One of the main benefits of organic fertilisers is that all the food made is non-toxic as they contain no chemicals. This is of benefit to both humans and other organisms as there is less of a chance of illness and disease due to the consumption of foods containing fertiliser toxins. Moreover the farmer himself can prepare the majority of organic fertilisers, using products and waste found on his farm. This helps to limit cost and save the farmer money. Additionally, organic fertilisers also improve the quality of the soil by enriching them with natural compounds. They also do not upset the soil’s natural chemical balance, as they are adding no artificial compounds. Organic fertilisers also correct imbalance to soil pH and deliver nutrients at a slow, but sustained rate so they plants are not shocked by the sudden increase in nutrients. There are, however some disadvantages to organic fertilisers. If there are plants that are in a dying and need a quick dose of nutrients fast, organic fertiliser cannot provide this. In addition, you cannot tailor the amount of nutrients released into the soil to suit one particular crop. For example, more nitrogen could be released when the plant needs phosphorus. This can also lead to a nutrient imbalance in the plants and deformities. In addition, if a farmer has a large amount of land he needs to fertilise organically, he cannot provide all the waste needed to produce this compost. In addition, it takes a while to actually from the organic fertilisers, so there may be issues with supply and demand. Inorganic fertilisers are also popular with farmers for a number of reasons. Firstly, inorganic fertiliser can be used immediately and has an immediate effect. They can revive nearly dead plants and be easily bought by farmers in conjunction with the size of their land. Additionally, these contain all the nutrients that plants need to grow successfully and produce a large yield for the farmer. Nutrients can also be tailored to specific crops for optimum growth. Inorganic fertiliser is also extremely convenient: it is readily available to buy in shops and is sold at a reasonable, affordable price. There is no need to wait a long time to produce this fertiliser.

Environmental consequences of using nitrogen fertilisers, explain the process of eutrophication, explain how inorganic fertilisers and

sewage, resulting in the death of fish and invertebrates in rivers and lakes, accelerate it

Inorganic fertilisers also have some disadvantages, and with this kind of fertiliser, it is usually environmental. Firstly, use of inorganic fertiliser can lead to

leaching, which is a process by which nutrients are removed from the soil and carried beyond the reach of plant roots. This usually occurs when soluble nutrients dissolve in the rain. These nutrients eventually get transported into bodies of water, such as rivers and streams that may drain into lakes. The process that occurs due to this is called eutrophication. Eutrophication is a natural event that occurs in mostly freshwater lakes and is caused by the leeching of nutrients. Eutrophication itself is not a major environmental hazard, but accelerated eutrophication, which is caused by the use of inorganic fertilisers, is very harmful to aquatic ecosystems. Firstly, in most rivers and lakes, there is a very small concentration of nitrate ions and so this is a limiting factor for plant and algal growth. As the nitric concentration increases due to the run-off of nutrients into the lake, it stops being a limiting factor and plants and algae growth both increase hugely. Due to the fact that most algae grow on the surface of a water body, the upper layers of water become densely populated with algae, leading to a phenomenon called algal bloom. This surface layer of algae absorb the light coming into the water body, thus preventing it from penetrating to lower depths. As a result, light then becomes a limiting factor for the growth of plants and algae at the lower depth of the water body. This causes their eventually death. Saprobiotic bacteria that live on the floor of the water body start to feed on these dead and decaying plants andFigure 2: Lake Taihu, China (www.lakescientist.com)algae and the number of saprobiotic bacteria grow exponentially as they use the dead organisms as their food. However, these bacteria also have to respire and this creates an increased demand for oxygen due to the increasing population of bacteria. The concentration of oxygen continues to decrease and nitrate ions are released from the decaying organisms. Oxygen then becomes a limiting factor for the remaining aquatic organisms.

These organisms ultimately die as the concentration of oxygen becomes too low for their survival. Without the aerobic organisms, there is less competition for anaerobic organisms whose population increases rapidly. As the anaerobic organisms further decompose dead and decaying matter, they release toxic chemicals, including hydrogen sulphide. The aquatic ecosystem is now referred to as a ‘dead zone’. Even if the oxygen concentration of the water were to increase, the toxic water environment is no longer sustainable for life. Figure 2 shows an example of a eutrophic lake in China caused by the nitrogen and phosphorus run off from resulting from its fast economic growth. Nitrogen fertilisers as a general also lead to a reduced species diversity. Because nitrogen-rich soil favours the growth of rapidly growing organisms, grasses, nettles and other such plants quickly outcompete other species, which then die out as a result.

Explain why fertilisers are necessary to maximise crop yield. Explain how they are necessary if we are to feed an ever-growing human population

If we examine the limiting factors of photosynthesis, they are light, carbon dioxide, water and nutrients. In the open, carbon dioxide, light and water are often readily available. This leaves nitrogen and other nutrients as the limiting factor for plant growth and thus maximum crop yield. The main nutrients that fertilisers provide are nitrogen, phosphorus, and potassium but nitrogen is often deemed the most important of these nutrients due to the fact that both Figure 3: Population growth (http://www.bbc.co.uk/schools/gcsebitesize/geography)

amino acid and protein synthesis and synthesis of DNA require it. When plants have a plentiful amount of nitrogen and other nutrients, they can focus the energy gained through photosynthesis for increased growth, which as a consequence increases the yield of crops for farmers.When Fritz Haber invented the Haber process as a way to derive ammonia, with fertiliser being one of the leading products following this discovery, it was a huge advancement in the agricultural

production. In a world where the population is still growing, even though not as exponentially as in previous years, increasing crop yield is a main priority to provide as much food from an area of land as possible. Figure 3 shows the projected population of the world in ten years time.

Explain how modern farming practices, including the manufacture of inorganic fertilisers contributes to global warmingThe main producer of the nitrogen used in inorganic fertilisers is the Haber process. The equation is as follows: N2 + 3 H2 → 2 NH3

This reaction is carried out at temperatures of 400 - 550C and pressures of 200 – 300 kPa and as a result, approximately 4% of the world’s natural gas supply is used to maintain the reaction conditions. Although natural gas is one of the more plentiful fossil fuels, the rate of consumption if it by the Haber process is not sustainable for an extended period in the future. Furthermore, nitrogen oxides produced from the Haber process created fertilisers destroy the UV-blocking ozone layer.