ENVIRONMENTAL SCIENCE : NATURAL RESOURCES 83
(8) Damage to Crops
(i) Sometimes crops are damaged by pesticide applications, e.g. application of improper
dosage under unfavourable conditions.
(ii) Herbicides that drift from a treated crop to a nearby crop also cause serious
environmental problems. Persistent herbicides also can injure crops planted in
rotation.(9) Harm Caused to Fishery and Wildlife
Drifting or leaching pesticides drain into nearly water bodies causes fishery and wildlife
losses. Wild birds and mammals exposed to pesticides suffer by death from direct
exposure to high doses and reduced survival growth and reproduction from exposure to
subtle dosages.
(10) Harmful Effect on Decomposers
Pesticides have harmful effects on insects, earthworms, invertebrates, protozoa, and
microbes found in soils, especially the decomposers. It is reported that human pesticide
poisonings, reeducation in insects and mites, and honeybee poisonings account for about 70
per cent of the calculated socio-environmental costs for pesticides in the USA.
(11) Development of Secondary Pests
The use of pesticides kills natural enemies and creates such problems as the development
of secondary pests eg. Red spider mites. Resurgence of primary pests can also occur. To
illustrate, caterpillars of the small cabbage white butterfly in Brussels sprouts reappeared
after DDT has killed their natural enemies. Resistance to pesticides is a cause for serious
concern. Other hazards include those to the operator or worker who sprays pesticide, those
to the consumer of the crop and those to wildlife.
(12) Elimination of Birds
Some species of Eagles and top carnivors are known to be eliminated by DDT because
contaminated adults failed to lay viable eggs. Populations of peregrine falcons and some
pelicans have disappeard from some areas from some areas where excessive use of DDT
interfered with the bird’s ability to transport calcium to growing eggs, leading to marked
thinning of the eggs shells. Such weak eggs fail to reach the hatching stage. It is discovered
that in ringdoves, DDT greatly reduced the activity of carbonic anhydrase. This enzyme is
critical in providing calcium for eggshell growth. When the pesticide inhibits this enzyme,
eggshell grows thinner.
(13) Growth of New Pests
Artificial introduction of pesticides in the environment upsets natural biological controls.
This is the reason why new pests are created in this way because their natural predators,
which previously checked their populations, are eliminated. In this way mites have become
a pest as a consequence of the emergence of the pesticide industry. Indiscriminate and
excessive use of DDT killed some insect predictors of these mites, enabling the mites to
multiply to pest status.