It is important that plants have the right conditions to grow successfully and produce a good harvest in. These conditions include space, good sources of light and water, an appropriate temperature, and a supply of inorganic ions. With crop plants this is especially important since the greatest possible yield needs to be achieved. However, there are factors which affect the yield of a crop produced, such as competition, disease, and pests.

Pests are unwanted organisms that reduce the quality and growth rate of a plant, also likely to affect the amount of seed it produces. Most pests only cause significant damage to the environment when their population reaches a specific level. This is called the ‘economic injury level.’ Controlling all of these factors, so that this level is not reached can be done so using either biological or chemical control. I shall be investigating into these two methods to distinguish which is the better out of the two.

Biological control relies on the introduction of other living organisms or biological products to control pests. A predator or a parasite of the pest is usually used in this case. There are many different ways in which the predator attacks its prey. For example, a small insect the whitefly causes damage to greenhouse crops such as tomatoes and cucumbers. To control this, a parasitic wasp can be introduced into the greenhouse. It lays its eggs in the fly larvae, which is then eaten up by the wasp larvae. Ladybirds on the other hand use their prey as a food source, for example the scale insect which damage and kill citrus fruit trees.

Fungal pathogens, which are used to target against weeds and insects, use a completely different method to those mentioned above. Entomopathogenic fungi induce a lethal quantity of a particular disease into the scale insects it attacks. The disease breaks out within the scale insect population. However, the population is not wiped out completely, and fungi can only work efficiently in humid conditions, such as rainforests or tea-leave plantations like those in Sri-Lanka.

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The Larvae of many butterflies and moths eat away at the foliage of many important plants. An effective form of biological control is by introducing the HD-1 strain of the bacterium bacillus thuringiensis, which parasitizes the larvae, preventing them from doing any harm.

Another example of a biological control agent is the Hydra, which control mosquitoes. These are blood-sucking insects that act as a vector of the malarial parasite plasmodium. In doing so, they increase the chances of the organisms they feed on developing malaria, and due to the transfer of blood from one organism to another, they increase the likelihood of AIDS. Hydra is a predator of mosquito larvae and is an effective way of lowering mosquito numbers, decreasing these risks.

There are three main types of this kind of biological control; classical, inoculative and inundative bio-control. With classical bio-control, the pest has been accidentally introduced into an environment in which it does not normally live. As a result there are no predators of its kind so the pest triumphs. Biologists then research the pest to find a predator. Once it is found they screen it for diseases and release it into the new surroundings. Unlike classical bio-control, inoculative control is seasonal, and occurs when pest populations are low, so that they have time to establish themselves as well as the pest has. Inundative bio-control relies on large quantity release of the predator to immediately restrict growth. This type of control is similar to that of chemical control, because the pest has no time to establish.

Another increasingly used form of bio-control nowadays is gene control. With genetic modification, plants that can protect themselves against interference of pests are grown. This is done by selecting seeds of the ones that survive pest attacks and the ones that are least affected by the pest. New varieties of them are bred and used in next year’s crop. Another technique is the genetic modification of the pest itself. A disadvantageous characteristic of the pest is engineered and then after its modification, the pest is released into the appropriate environment. An example of this approach is making the male insects sterile so that when they mate with the female, the chances of offspring are unlikely.

Gene transformation or genetic engineering, where resistant genes are transferred into plants, is another biological method used to control pests. An example of this is the insecticide-producing Bt gene in cotton. The gene from a bacterium is inserted into a plants genetic material, allowing the plant to become resistant to insect attack. Similarly, potato plants have been genetically modified to increase their resistance to the potato leaf roll virus.

The alternative to biological control is chemical control. This relies on the use of a poisonous substance called a pesticide to destroy and prevent reproduction of any pest, so that it cannot harm other important organisms such as crop plants. This type of agricultural chemical comes in many forms, for instance, herbicides are used to kill weeds, fungicides kill mould, and insecticides kill insects such as aphids. Other pesticides include contact pesticides, which kill pests without being consumed by them, systemic pesticides, which are absorbed by a plant, and enter the pest when it consumes the plant, and, residual pesticides, which are sprayed onto soil and kill pests like insect eggs and larvae as they germinate.

Aphids are greenflies that feed by inserting their mouthparts into the phloem of a plant and deriving it of its contents. This can cause the leaves of the plant to curl over, reducing the surface area and therefore its ability to photosynthesise. Also, the plants quality is ruined and eventually it may die, so insecticide is used. An example of a process used to kill aphids is the use of light traps. They are placed around a country to collect samples of flying insects. It estimates when the population of aphids is increasing, so that farmers know when to spray their crops with insecticide.

One advantage biological control has over pesticides is that it is specific; meaning it only affects the target pest in a harmful way. The predator is only going to kill the pest, nothing else. However, this is not the case with pesticides. They are the cause of many ecological problems. Not only do they kill the pest, but they also may harm other organisms including the predators of the pest. Pesticides also have dangerous affects on the atmosphere. Bromomethane is used to fumigate pests. This substance is one reason for depletion of the ozone. This can have repercussions such as increased cases of human skin cancer because more damaging ultraviolet radiation can pass through the atmosphere and reach Earth.

Ideally, a chemical pesticide destroys the pest population completely so that no further damage can take place and the nuisance of the pest is rid of and will not resurface. Bio-control only reduces the levels of pests, so there is a risk of the quality of the plant or crop to still be slightly poor.

Applying biological methods into pest control can be a very lengthy process due to the amount of time, effort, research and money that needs to go into setting the programme up. There is no guarantee of success. Also, it takes a long time to see any noticeable change in the levels of pests. Producing and applying pesticides is a much quicker way of dealing with the problem as they act faster and take less time to develop. However, re-application of the pesticide is needed because their effect is short-lived. The reason as to why most pesticides ultimately lose their effect is because either the pest has genetic factors to allow them to survive the attacks or it simply acquires a resistance to it after some time. This can prove to be expensive and also time-consuming because new pesticides will have to be continually developed. Whereas in bio-control, once the predator is introduced and establishes itself it does not need to be re-introduced.

Most chemical pesticides are stable and persistent, meaning they do not fade over time. This gives rise to two potential problems; bioaccumulation and biomagnification. Bioaccumulation is the accumulation of the pesticide in the fatty tissue of an organism. Biomagnification is the increasing concentration of the pesticide up a food chain i.e. from one trophic level to the next. Organisms at the highest trophic levels will be affected the most.

Dichlorodiphenyltrichloroethane (DDT) is a pesticide that illustrated these factors. It was used in the 1940’s to kill pest insects of fruit trees, and by killing mosquitoes it has helped to get rid of malaria in many parts of the world. It still has an on-going persistence in the environment today. However, it has since been banned due to its devastating effects. The chemical stability of DDT and its fat solubility was the foundation of the problem. DDT is not broken down very rapidly by animals; instead, it is deposited and stored in the fatty tissues. The biological half-life of DDT is about eight years meaning it will take about eight years for an organism to break down half of the amount it has in its body.

Despite having been contaminated with the DDT some of the insects that feed on the crops sprayed with it, survive. Birds, predators of the insects, consume the insects further concentrating the chemical in their bodies. As the food chain continues, the DDT accumulates in greater concentrations, until eventually lethal quantities may be concerned. As a result the bird’s behaviour is altered and infertility may occur. Any eggs that are produced may have a tendency to break due to the lack of calcium being deposited by the mother bird.

Integrated Pest Management (IPM) is another procedure for pest control. This form of control makes use of both biological and chemical control by including the use of natural predators or parasites, and pesticides. Pest populations are monitored, and pesticides are only used when and if numbers of the pest exceeds the economic damage threshold. The aim is not to exterminate pests totally, but more to keep pests under control. An example of integrated pest management is biologically-based pesticides. Some insects produce pheromones, chemical substances that attract the opposite sex. These pheromones are synthesised and used to attract pest insects into traps, which contain insecticides.

Biological control has proven to have long-lasting or permanent affects on suppressing pests, making it an effective method of pest control. Unlike chemical control, it is a specific process and has no harmful effects on organisms other than the pest itself, so the ecosystem as a whole is not disturbed. Although chemical control increases crop yield, it has both harmful effects on other organisms and the environment. If used incorrectly, chemicals may enter the soils or surrounding waterways and this can disturb the ecosystem with fatal consequences. With biological control pests cannot become resistant to their predators, unlike pesticides, which pests can adapt or become resistant to. So in this way pest populations can be controlled for longer periods of time by biological control. This also means that biological control is a cheaper form of control, because new chemical pesticides would have to be researched costing more money and time.

However, biological control does have its weaknesses. The time taken to see any reduction in the number of pests can be very extensive due to the length of time the predator needs to establish itself in its new surroundings. Also, the pests are only suppressed, meaning the pest population is only reduced, never completely destroyed. Using a chemical pesticide on the other hand would exterminate the pest in most cases. Also the issue of genetic engineering of organisms is controversial. On the down-side the toxins produced by the process of gene transfer may have harmful effects on beneficial organisms or on human health, but on the up-side the transferred gene might ‘escape’ into related species of the organism.

I think that neither biological nor chemical control used alone is ideal for pest control. I feel the way forward with pest control is the method of Integrated Pest Management (IPM). Although not fully successful, this form of pest control generally has more advantages than the conventional methods, and in the long run is more likely to be cost effective. With further research into new methods of integrated pest management, the environment is likely to be safer from damage, and pests will be under sustainable control.


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