Pesticides

There are 725 pesticides listed in the Pesticides Manual, and of these 450 are approved for use in the UK by the Ministry of Agriculture, Fisheries and Food (MAFF). They are used to control weeds, insects and fungi on crops, animals and fish. In 1993, the amounts of different pesticides used were as follows:

10,700 tonnes of herbicides 6,850 tonnes of fungicides 2,670 tonnes of growth regulators 1,140 tonnes of insecticides 635 tonnes of other pesticides.3

Pesticides are usually popularly thought of as being 'bad' chemicals but they protect our food, our homes and the environment (for example, by controlling invasive plants such as bracken and rhododendron). It is estimated that the yield of crops would decline by 40 per cent if they weren't protected by various pesticides. Another statistic is that, world-wide, the loss of crops from pests is estimated at 35 per cent of the total amount which are grown.

The main problem with pesticides is that they can also poison nontarget organisms. If pesticides are washed into a stream during rainfall or if the spraying machine accidentally passes over a water course, the pesticides can affect the aquatic life, whether that be the water plants or fish. Fortunately, the problems of environmental pollution from pesticides have decreased markedly in recent years with the introduction of less harmful but effective pesticides and advances in the design of spraying equipment so that less pesticide is used but it still reaches the target pest.

Despite these improvements, there are still concerns about the effects of pesticide spraying on farmland birds. The treatment of cereal crops with pesticides can result in a reduction in the number of invertebrates in and on the soil which are food to songbirds. A survey carried out in the UK showed that in the past 20-30 years the population of certain species of farmland birds had declined significantly, as shown in Table 9. This decline is correlated with the increased use of pesticides: for example, in 1970, 5 per cent of farmland was treated with insecticides, but by 1996, this had

Table 9. Decline in bird numbers on UK farmland in the past 20-30 years

Species

% decline

Tree sparrow

89

Grey partridge

82

Turtle dove

77

Bullfinch

76

Song thrush

73

Lapwing

62

Reed bunting

61

Skylark

58

Linnet

52

Swallow

43

Blackbird

42

Starling

23

Source: The Indirect Effect of Pesticides on Birds, report by the UK Joint Nature Conservation

Committee, Natural History Book Service, Totnes, Devon

Source: The Indirect Effect of Pesticides on Birds, report by the UK Joint Nature Conservation

Committee, Natural History Book Service, Totnes, Devon increased to 90 per cent. The reduction in the bird population has been caused, not by direct poisoning, but by the loss of food for the birds and their young. Some of the birds feed on the insects which are a pest to the crop, whilst for other birds the use of herbicides kills off the weeds and their seeds that they eat. Other factors accounting for the loss are the decline in spring-grown cereals, the decreasing number of mixed farms and the reduction in hedges and ponds in farmland.

In the past, many pesticides were based on organo-chlorine molecules such as DDT (dichlorodiphenyl trichloroethane), Dieldrin and Lindane (see Figure 14).

DDT's insecticidal properties were not discovered until 1939 and thereafter it was used very successfully to control malaria by killing off the mosquitoes which carried the disease. DDT was also used to protect many crops from insect damage. To be effective, pesticides should have the following properties:

Cl Cl Cl

30,000 tonnes manufactured in 1945 200,000 tonnes manufactured in the late 1950s 1,600,000 tonnes manufactured in 1964

Banned in 1972 in the USA, and in 1984 in the UK

Cl Cl

Figure 14. Structure of DDT and selected organo-chlorine pesticide molecules

Cl Cl

Figure 14. Structure of DDT and selected organo-chlorine pesticide molecules

• high toxicity to pests

• low toxicity to other organisms, especially humans and aquatic organisms

• adequate stability so that they complete the task of killing the pest before they degrade, and

• ability to degrade so that after completing their task they disappear into the environment without causing harm.

The organo-chlorine pesticides met most of these criteria but they did not degrade easily. For example, DDT takes between 4 and 30 years to degrade by 95 per cent, and Lindane takes 3-10 years. Another problem was that they bioaccumulated, in other words they were preferentially absorbed into the bodies of birds, animals and fish at much greater concentrations than in the environment because they were more soluble in fats and oils than in water. An example of bioaccumulation is shown in Figure 15.

Bioaccumulation Ddt
Figure 15. Bioaccumulation of an organo-chlorine pesticide in the food chain

The pesticide is washed by rain into the water where it is adsorbed on to aquatic plants. These are grazed by snails which bioaccumulate the DDT because it is preferentially absorbed from the plant into the fatty tissue of the snail. Further bioaccumulation takes place as the snail is eaten by the fish and the fish by the cormorant. If the fish was eaten by a person instead of the bird, then the DDT would be absorbed into the fat of that person. That is why each of us still has traces of DDT and other organo-chlorine compounds in our bodies. In the 1960s, samples of human fatty tissue from different parts of the world were analysed for DDT and the results can be seen in Table 10. The variation depended on how much DDT was used in the country and the food that was eaten.

Table 10. Concentration of DDT in human fat

Country

Concentration (pg/kg)

England

0.7

Hungary

S.7

Italy

10.B

India

1B.0

Source: C.H. Edwards (ed.), Environmental Pollution by Pesticides, Plenum Press, London, 1973

It eventually became clear that these organo-chlorine compounds were causing harm to the environment and the organisms that lived in it. It was noticed that birds were dying or failing to breed properly, that fish and insects were killed when the pesticide drained into streams, and that crop-sprayers and farm-workers were suffering adverse effects after using the substances. The issues were clearly described in Silent Spring, the famous book by Rachel Carson, and this had a major impact on pesticide use. DDT and other organo-chlorine compounds were either banned from use or restricted. The USA banned DDT in 1972 and the UK banned it in 1984.

Since then, chemists have devised new and more effective pesticides -in particular, they are much less harmful to the environment because they degrade more rapidly after use. They can also destroy the pest at lower concentrations so less of the substance is used. Some examples of pesticide changes resulting in reduced rates of application are shown in Table 11. Many of the new pesticides are either organo-phosphorus or

Table 11. Changes in pesticide application rates for selected crops, 1983-93

19B3

1993

Crop Pesticide

Application Pesticide Application rate (kg/ha) rate (kg/ha)

Oilseed rape TCA Apples captan

1.45 fluazifop-P-butyl 0.09 2.75 myclobutanil 0.05

POLLUTION FROM FARMING 49

organo-nitrogen compounds. The absence of the chlorine atoms in their structures makes them more amenable to biological breakdown.

A further new class of pesticides, synthetic pyrethroids, are now used in many applications. They are based on the natural insecticide pyrethrum that can be extracted from the chrysanthemum flower. Scientists have identified the chemical structure of pyrethrum and have made synthetic derivatives of it which are more effective than the natural product. The first synthetic pyrethroid was developed in 1973 and now, over 25 years later, this class of pesticide is one of the most commonly used. Typical uses and crops that are now protected by pyrethroid pesticides are shown in Table 12.

Table 12. Typical uses

of pyrethroid pesticides

Use

Pests controlled

Protection of Cotton

Boll worms

Cereals

Aphids

Carpets and pets

Fleas

Timber

Wood-boring beetles

Use in Hospitals

Cockroaches

Aerosols

Flies and mosquitoes

If you look at the labels on any bottles or aerosols you may have in your house or garden shed for killing garden or house pests you will find that most of them contain synthetic pyrethroids.

One side-effect to the success of the pyrethroid pesticides is that they are very toxic to aquatic life - more toxic in fact that some of the pesticides they have replaced. This can be seen in the use of pesticides to protect sheep from parasites. Sheep are prone to attack from various parasites, such as ticks, mites, keds and blow-fly larvae. A badly infected animal suffers from itchy skin which can make it feel very ill and lose its appetite.

Each year shepherds protect their flocks by 'dipping', i.e. plunging the animals into a bath full of insecticide so that it soaks the coat of wool and kills the insects on the skin. In the early days of dipping, the insecticide would be an organo-chlorine-based compound, such as Lindane, but these were phased out by the 1980s in favour of organo-phosphorus (OP) compounds. However, even these are now falling out of use because it is alleged that farmers exposed to OP pesticides suffer from side-effects. (The same products have also been implicated in causing the so-called Gulf War Syndrome because the forces serving in the war against Iraq were protected from sandflies and other biting insects by spraying tents and bedding with OP insecticides.) The symptoms range from exhaustion and memory loss, to muscle weakness and depression. Farmers are now switching to sheep dips based on synthetic pyrethroids but extra care has to be taken in their use if the dip bath is situated close to a river or stream because of their extra toxicity for aquatic life. In 1995 and 1996, in the UK, there were some serious pollution incidents caused by the use of these new pesticides. For example, in July 1995, in the River Teviot in the Scottish Borders, about a thousand fish were killed by sheep dip containing pyrethroids. Similarly, in April 1996, almost all the mayfly larvae and shrimps in a 30 km stretch of the River Caldew in Cumbria were wiped out, whilst in Perthshire, the invertebrates in 25 km of the River Earn were killed by the same type of pesticide. The environment agencies are now contacting farming organizations to warn them of the environmental dangers of the new sheep dip compounds.

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