Hence, the significance of chemical pesticides in controlling mosquitoes, termites, houseflies, cockroaches, house crickets, weevils, locusts and grasshoppers, borers, snails, rats, rabbits, and a multitudes of other animals has been great, and it is difficult to imagine modern disease control and agricultural programmes without some forms of chemical control. Like pesticides, herbicides are specifically designed chemicals for the control of weed pests and unwanted plant growth. Our dependence on pesticides and herbicides have increased up to the extent that if modern agriculture will attempt to operate without chemical control of any sort, crop production will probably decline in many areas, food prices will soar far higher and food shortage will become even more severe. Ecologically, however, pesticides and herbicides have created two major serious problems which were not previously anticipated. In the first place many of them have persisted and accumulated in the environment and have harmed or contaminated numerous animals or plants not intended to be targets. Secondly, many of them have directly or indirectly affected human health.
Most pesticides such as DDT, DDE, DDD, dihedron and heptachlor epoxide and most herbicides such as 2, 4-D (2, 4 dichlorophenoxy acetic acid), 2,4, 5-T (2,4,5-trichlorophenoxy acetic acid) and dioxin have been extensively used for the control of disease carrying and crop-destroying insects, rodents and weeds. These chemicals are non-degradable pollutants that either do not degrade or degrade very slowly, so, they have seriously contaminated the global ecosystem and entered food chains. Since they become more concentrated at higher trophic levels (biomagnifications), these toxicants affect the predacious animals most adversely. Fish eating birds such as grebes, ring billed gulls, barring gulls, robins, etc.
, are endangered because chlorinated hydrocarbons interfere with their reproductive ability. Certain pesticides are found to have mutational effects on human DNA molecules (Wurster, 974). The small doses of DDT in human tissues are reported to have lethal effects. DDT is suspected to be a carcinogenic in human tissue. Even certain herbicides, such as dioxin is found cause congenital birth defects in offspring of experimental female who have ingested or inhaled certain concentrations of dioxin at critical periods.
Herbicides are also reported to have certain most serious ecological effects during defoliation programmes of USA during Vietnam War: (1) Destruction of 50 per cent upland forest; (2) 90 per cent destruction of mangrove forest; (3) Accidental damage to rubber plantations rice crops; (4) Loss of insectivorous birds; (5) Increased mosquito populations and increased malaria: (6) increased soil erosion; (7) Invasion of forests by grasses and bamboo; (8) Increase birth defects in both the people and livestock of North Vietnam (Galston. 1974). Visualizing the damaging ecological impacts of pesticide and herbicide pollution, recently biologists became interested in integrated pest control (see Odum, 1971). The concept of integrated pest control involves coordinated use of a mixed bag of weapons, including old fashioned but common sense cultural practices, judicious of degradable or “short-lived” chemical pesticides, and greater use and stimulation of nature’s own control methods, i.e. biological control.