Pesticides may be divided into 3 categories. Insecticides, which are used to combat insect pests. Fungicides, which target the fungi that cause many plant diseases. And finally, herbicides which are used to kill weeds.
Each type of pesticide can work in 3 different ways. Contact pesticides are pesticides that are sprayed directly onto the crop. Contact insecticides are usually absorbed by the insect through tiny gas exchange pores, the spiracles, along its body. Contact herbicides and fungicides are absorbed directly through the surface. They have the advantage of being inexpensive but they often have to be reapplied. This is because their effect is usually short lived and, in addition, there are always some pests that avoid the pesticide.
Systemic insecticides are sprayed onto the crop. They are absorbed by the leaves and transported around the plant. A sap-sucking insect such as the aphid feeding on the crop plants takes in the pesticide and is poisoned. Systematic herbicides are absorbed by the leaves and, because they are transported through the weed, kill all of its tissue, including underground parts such as the root.
Residual pesticides are sprayed onto the soil or used to treat seeds before the crop is planted. They remain active in the soil and will kill fungal spores, insect eggs and larvae, and weed seedlings as they germinate.
For the application of pesticides in everyday life they must have certain specifications. They must be specific and therefore have and immediate and lethal effect on the pest concerned but should be harmless to humans so they can be applied safely. In addition, they should not affect mammals, birds, predators of the pest, earthworms or plant processes such as photosynthesis. However, as stated below there are certain people who can be particularly at risk if exposed to pesticides for a period of time.
The most vulnerable populations are children, the developing foetus, the elderly and ill. Most worrisome from a public health perspective are chronic health affects such as cancer, infertility, miscarriage, birth defects and effects on the brain and nervous system.
Pesticides must be chemically stable but biodegradable. This means it will have a long shelf life but once applied, will be broken down rapidly in the soil to a harmless substance. In most cases this is true, however, the more deadly pesticides can have drastic effects on the body if exposure is great enough. One of these incidents is explained below.
Some of the most toxic insecticides include nerve gases. These chemicals block the action of an enzyme which breaks down the neurotransmitter acetylcholine after it has caused synapses to fire signals to a variety of muscles such as those controlling breathing. Without the enzyme the nervous system continues to stimulate these muscles. Dan Charles, New Scientist (March 1994)
Pesticides must be cost effective as a new pesticide undergoes extensive tests before going on sale which may make the development costs very high. The manufacturer must recover these along with the production costs from sales. If certain pesticides were to be banned this would have drastic effects for the consumer as expressed below.
If these chemicals are banned, farmers may not be able to grow some crops, such as tomatoes, all year round. For consumers this means that fresh fruit will be harder to find during much of the year and will be more expensive. Charles, New Scientist.
This is further complicated as a new pesticides only remains useful for a limited time. When it is used repeatedly, the pest may develop genetic resistance so the pesticide becomes ineffective as shown below.
The more pesticides farmers apply, the more likely they are to create strains of insects that can tolerate large doses of chemicals. Insects show an astonishing ability to adapt to pesticides. Charles, New Scientist.
When pests develop a resistance to the pesticide it will be useless and another pesticide must be introduced. This is due to the passing on of genetic resistance to off spring as explained in the following extract.
A resistant strain of insects is created, paradoxically, when pesticides are too effective. The chemicals may kill off all insects except a tiny minority that are genetically immune to their lethal action. These survivors will breed only with each other, and all their offspring carry the genetic ability to survive high doses of the chemicals. Charles, New Scientist.
Although many aspects of pesticide use are beneficial to today’s world there are certain moral and ethical issues concerning their use. Firstly some insecticides are persistent meaning that they are only broken down very slowly so they remain in the bodies of insects for a long time after they are applied. If these insects are eaten by animals further up the food chain, they pass into the animal concerned. With each step in the food chain, they become more concentrated and more likely to build up to a lethal concentration. However there are many contradictory views and some people believe the risk of poisoning from pesticides is very low as the statistics below state.
A person may consume 1.5g of natural toxins per day in foods such as coffee, potatoes, tomatoes, whole wheat and brown rice. This is about 10000 times the average amount of pesticide residue consumed. Charles, New Scientist.
It is argued that pesticides are a contributor to some diseases if people have enough exposure to them. However, there is a definite conflict of interest between whether pesticides are more harmful or more useful to people. This is conveyed in the following extract.
Widespread use of organochlorine compounds such as DDT caused serious ecological effects, particularly in animals at the top of food chains. However, these same chemicals saved millions of people from contracting and possibly dying from malaria when they were used for large scale spraying of the breeding grounds of the mosquitoes that transmitted the disease. Biological Sciences (Nov 1999)
Although the use of pesticides is integral to the farming industry this usage may be damaging our health. In my opinion pesticides are extremely important in our world as it revolves around the price and quality of such indispensable goods such as crops. Therefore pesticides in my opinion are vital in the production of crops as they increase efficiency and quality. Also, there is no conclusive evidence to say that limited exposure to pesticides will cause harm to humans.
BIBLIOGRAPHY
While compiling my Biology Coursework I referred to varies sources as a guide. These references came from the internet, magazines and text books:
