Plot the data in the table on graph paper.
Which of these are features of a good chemical pesticide?
Specific to the pest.
Kestral numbers declined as a result of the use of organochlorines pesticides. This was because of;
Find out more about the effect of pesticides on the environment by looking at the Friends of the Earth web site.
Chemical Control of Pests
Competition between crop plants and pests.
In North America flax is grown as a crop. Wild oats compete with it, its seeds ripen earlier than flax an drop to the ground. It's effect is shown in the data in the table from an experiments carried out at Fargo.
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wild oat density (no./m2)
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fertilised plots, yield of flax
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unfertilised plots, yield of flax
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average reduction %
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0
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19.5
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17.9
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-
|
|
10
|
13.4
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14.3
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26
|
|
40
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6.7
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8.0
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60
|
|
70
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4.3
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6.3
|
72
|
|
100
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3.5
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4.2
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80
|
|
130
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3.4
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3.4
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82
|
|
160
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2.9
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2.3
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86
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Weed species compete with the crops for light, space water and nutrients.
Animal pests (mainly insects) mostly directly destroy the crops by feeding on them.
Aphids (greenfly) feed by inserting their mouthparts into the phloem and sucking out the contents. This can cause the leaves to curl over, reducing the surface area for photosynthesis to take place.
Use of chemical pesticides.
There are four main ways of classifying insecticides:
CONTACT INSECTICIDES-These chemicals are applied in such a way that an insect cannot avoid contact. The insect is either sprayed directly or walks through deposited spray.
SYSTEMIC INSECTICIDES-Sap feeding insects are the particular target for these chemicals. Sprayed material is absorbed by the plant, entering the circulatory system. When the insect pierces the leaf cuticle and penetrates the sap stream, it withdraws poisoned fluid. Thus an aphid feeding on protected crops will inevitably suck up some insecticide .
STOMACH INGESTION INSECTICIDES-These compounds are sprayed over the crop foliage so that those pests with biting mouth parts, like flea beetles, caterpillars and weevils, eat a poisoned meal. These pests may also be affected by other insecticides if they consume sprayed plant material.
FUMIGANT INSECTICIDES-Vapour given off by the insecticide is inhaled by the insect. Once the chemical has gained entrance it may exert its lethal action in a variety of ways. Fumigants are mainly used for soil treatment or in grain stores, where the chemical vapour can give optimum penetration.
Problems with chemical pesticides.
Some pesticides are persistent, they do not break down in the environment or within the tissues of living organisms. This give rise to two potential problems.
Bioaccumulation is the accumulation of a substance in a biological tissue. Organisms at any trophic level may be capable of bioaccumulation.
Biomagnification is the increasing concentration of a substance up a food chain - i.e. from one trophic level to the next . Animals at the higher trophic levels will be most affected.
Example
Some of the earliest insecticides were organochlorines or chlorinated hydrocarbons. These insecticides kill by both contact and stomach ingestion. Dichlorodiphenyltrichloroethane (DDT) was the first known contact insecticide, synthesized in 1874. This organochlorine compound has saved more human lives than any other invention; by killing mosquitoes it has helped to eradicate malaria in many parts of the world. Unfortunately, DDT persists in fatty tissues (bioaccumulation). Larger, long lived predators at the end of a food chain may accumulate a lethal quantity of DDT as a result of eating large numbers of smaller species (biomagnification).
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Sparrow hawks are long lived and eat large numbers of smaller species in their lifetime. |
This dangerous effect coupled with the appearance of immune insects has led to a considerable decline in the use of DDT. Other halogenated hydrocarbon insecticides are also in decline because of possible implications with ecological problems.