Today, pesticides are widely used as a way of maximising the output of crops and animals in agriculture. By growing or maintaining large populations of one species of animal or crop plant in one particular area, a large food supply is created for other organisms such as insects. These other organism’s populations are then allowed to become much bigger than they would normally be. These organisms rapidly become pests diminishing the yields of the cultivation.
As world hunger is a problem that is prevalent today, to be able to maximise world food production is an important issue that needs close attention, and any possible solution to this problem should be considered. Estimates indicate that without the problem of insect pests alone, world food production would increase by about a third; as this represents the loss with the current control measures, it would indisputably be disastrous if control of insect pests alone was not attempted.
One way of striving to do this is by the use of pesticides. Pesticides are poisonous substances that are used in order to make use of their toxic properties by eliminating a particular pest. However they can become pollutants when they reach the wrong targets, and overuse can result in a pest becoming resistant to the chemical pesticide used – thus rendering that chemical useless there, and creating a superior chemical resistant pest. The main groups of pesticides are herbicides, used to kill weeds; fungicides, used to kill species of fungi; nematocides, used to control eelworms; and insecticides, used to kill insects.
Normally used against a specific organism, ideally it should poison it, but otherwise be harmless. Yet although it is possible for a chemical to be selective (the chosen pest is destroyed) complete selectivity is virtually impossible; there’s always a risk that pesticides will cause damage to man, or other non-target organisms.
Some pesticides are acutely poisonous and unstable substances; they can cause severe damage over a restricted area, but not long-term pollution. Others maybe less acutely poisonous, but be much more persistent, continuing to have ecological effects for a very long time after they were used. Such persistent chemicals can reappear when least expected and also at long distances from the original site; although originally diluted down to a fairly harmless level bioaccumulation can mean that it can eventually reconstitute to a toxic level within biological systems.
Herbicides kill weeds that are in competition with crop plants; that house pests, e.g. aphids; that release chemicals into the soil that cause the retardation of other species and that contaminate the yields with weed seeds. Furthermore weeds can prove toxic to animals that graze on the land.
Herbicides make up the majority of pesticides used today, and also seldom cause serious environmental damage. They are important for the control of weeds and furthermore the farmer no longer has to turn the soil or till it before use. This reduces the risk of soil erosion, and saves time, money and effort
There is a range of herbicides with different actions on weeds:
* Pre-emergence herbicides – used before the plant grows. For example, Paraquat is very toxic and will kill any plant above the ground that it comes into contact with. Yet it is broken down before any crop plants emerge from the soil.
* Post emergence herbicides – sprayed over the crop plants, killing only weeds (being selective). The best known ones are the phenoxy compounds, e.g. 2,4-D (2,4-dichlorophenoxyethanoic acid), 2,4,5-T (2,4,5-trichlorophenoxyethanoic acid) and MCPA (2-methyl-4-chlorophenoxyethanoic acid). These are used against broad-leaved plants (dicotyledons), not those such as grasses and cereals (monocotyledons), and use natural plant growth regulators (synthetic auxins) to promote abnormal growth of the weeds. As the plant cannot support the atypical growth they die.
* Systematic herbicides – sprayed onto fields after the crop has been harvested, in order to kill any weeds for the new crops. For example, Glyphosate.
Considerable crop losses result from fungi diseases, which act as parasites on plants. Some herbicides contain compounds of heavy metal, (e.g. Calomel – HgCl), such as mercury and copper, and are toxic to humans and animals. Various organic fungicides (e.g. captan) are very effective, but are not used very often, as they are usually expensive – and so used with care; and as a result no serious environmental damage has been done. Others are systematic herbicides, which are not highly toxic and do not present pollution problems.
Insects can reduce crop yields by eating the plant, and acting as hosts for plant diseases, bacteria, viruses and fungal spores. Insecticides can be divided into two groups: broad spectrum insecticides, which kill a wide range of insects and narrow spectrum insecticides, which target a particular insect pest. The actions of various insecticides are
– Stomach poisons that are sprayed onto the leaves to be ingested by insects in order to kill them. E.g. to kill caterpillars on cabbage.
– Contact poisons are absorbed through the insect’s exoskeleton, e.g. DDT.
– Systematic poisons are sprayed onto a plant’s leaves or the soil. They are then absorbed and transported through the phloem. Particularly effective against insects that feed on sap, such as aphids.
– Fumigants only used in enclosed areas, such as greenhouses, as it is distributed in the form of a toxic gas or smoke. This enters the insect’s breathing system to kill them.
Many plants produce their own chemicals to ward off insects, and extracts from these plants have been used as naturally occurring insecticides. These include pyrethrins extracted from the flowers of the African daisy Pyrethrum cinereafolium, and nicotine from the tobacco plant Nicotiana tabacum. Substances have been synthesised to act as insecticides. These fall into four categories: pyrethroids, organochlorines, organophosphates and carbamates.
There are many problems with using pesticides. The over-use of pesticides has led to a number of ecological problems. Insects have developed that are resistant to pesticides, the toxicity to non-target species, pest resurgence, pesticide persistence and mobility within the environment and bioaccumulation within an ecosystem; as discussed previously.
When strains of insects appear which are resistant to a pesticide they have a superior advantage over the non-resistant insects (as long the particular pesticide they are resistant to is in use). While the non-resistant organisms are killed, the resistant insects will increase in numbers as competition is reduced.
This is very common; as in the 1940s DDT was used extensively to success, in the 1950s DDT was used in the fight against insect-bourne diseases, only to discover that many insects had evolved strains resistant to DDT. By 1987, over 500 species of insect had been identified as resistant to one insecticide or another.