Unexpected Effects Of Pest Control

Press, Volume C, Issue 29609, 4 September 1961, Page 18

Unexpected Effects Of Pest Control

[By D. S. MILNE, Science Correspondent of the •'New Zealand Herald”!

HONOLULU, Aug. 25. Man has always struggled to keep other forms of life from interfering with his comfort, his health and his means of securing a livelihood. But it is really over the last 20 years only that he has developed effective measures for dealing with many of his pests. The organic insecticides, the new herbicides, and modern antibiotics have put him ahead in the struggle to increase agricultural production and to reduce disease. But many of these powerful new materials have brought unexpected effects in their train—the possibility of harm to both man and to wildlife other than that being attacked. This is such an important problem that several sessions at the Tenth Pacific Science Congress in Hawaii are being devoted to it, and related topics, such as the effect of man on native plants and anima Is. Necessary Decisions The picture that has emerged is one of necessity. If hundreds of millions of human lives can be saved by the use of DDT in malarial areas, but only at the cost of, say, a few hundred native birds, the decision can only be made one way. This is fully realised by practically everyone at the congress, including the most rabid conservationists. But most of the sessions have agreed that the cost in hazards to wild life and man can be avoided, if only the proper precautions are taken.

For the first few years after the new chemicals were developed few people took much notice of the possible side effect's of their use. But it gradually became apparent that man’s health might be adverseley affected by the consumption of food from plants or animals treated with some of these new compounds. Legislation was introduced in many countries, and restrictions such as that on the amount of D.D.T. in meat imported into America soon came into use.

One point that has come out is that the American standards are not bosed on the minimum health hazard, but rather on necessity, for the production of a food. Standard Levels If the United States Public Health Service considers that only a certain level of pesticide is necessary for the production of a crop or food, this level is made the standard. In practically all cases it will be well below the amount that could constitute a hazard. At present nearly 50 pesticidal compound's are known to have produced poisoning in man. The death rate is not high—about one in every million of population in the United States. But little is known yet of the possible effect of the long-term ingestion of small amounts of pesticides, too small to cause any immediate effect. "The results of our studies contradict the assumption that pesticides constitute a serious hazard to health,” reported two scientists from the United States Public Health Service. “But poisonings 'do occur and could be reduced by extensive education on the proper use of these new chemicals.” Both men considered that few or no measureable effects had been produced in people exposed to minimal amounts of pesticides, such as might be ingested through sprayed foods. But much more work needed to be done. “Unsatisfactory” The position over wildlife is not so satisfactory. It is only recently that scientists and the public generally have become concerned over the effects of pesticides on birds and fish, as well as other types of wildlife; but some disasters of the past have shown what can happen with indiscriminate use. "Some reported ecological reactions to pesticides have been both abrupt and unfortunate,” said Dr. I. N. Gabrielsen. president of the American Wildlife Management Institute. “Aquatic and terrestrial animal life has been destroyed, with severe population depressions in both the target animal and in species

which occupy the same habitat "Livestock, poultry. pets and in some cases humans who have aod dentally or unwittingly come into contact with the poisonous materials have lost their lives.” “The unremitting drive to develop more powerful chemicals and to rush them into commercial use has completely overshadowed efforts to gain an understanding of the relationship of pesticides to man and animal life.” Salmon Lost Dr. Gabrielson described the application of DDT to an area in New Brunswick, Canada, which was followed fey up to 91 per cent, loss of young Atlantic salmon through the run-off of the insecticide into streams. All efforts to restore the salmon in the area were upset by additional spraying in following years. In Wisconsin, the population of songbirds in areas where ornamental trees had been sprayed for insect control was reduced by from 31 to 90 per cent. . Sprays used in forests were affecting rabbits, racoons, opossums, mice, rats and armadillos, said Dr. Gabrielson.

“Even in well-regulated use pesticides sometimes have a serious effect,” he said. “The 'problem now facing us is to make sure that pesticides are applied only under known conditions and when the result of their use can be reasonably determined in advance. Alternatives

The alternatives to the use of pesticides, put forward by many scientists, are either cultural controls—such as land management, rotation of crops, and development of resistant varieties—or biological controls, through the utilisation of parasites, predators, antibiotics, viruses and other disease organisms. These, too, can have their hazards; but not as great as the chemical pesticides. But cultural and biological controls are long-term processes aimed at keeping pest numbers at a relatively low level. They Cannot be relied on for epidemic outbreaks, as can the chemical pesticides. The ultimate solution, and the one most likely to be used, would seem to be to rely on cultural or biological control for routine control, and to use chemicals only as an emergency measure, to be undertaken as a last resort to avert immediate major losses.