Cover Image
close this bookCERES No. 074 (FAO Ceres, 1980, 50 p.)
close this folderCerescope
View the documentNew credit systems for smallholders begin to show promise
View the documentFirst move made to tap geothermal source of energy project in Kenya
View the documentThe protein gap persists as mark of income disparity
View the documentPyrethroids win high rating for potency, safety
View the documentUse of true seeds may help spread potato culture
View the documentNew techniques for small-scale coconut growers
View the documentCost factors limit potential value of vegetable ''factories''
View the documentSome countries are avoiding the banana trap
View the documentTanzania to weigh impact on environment

Pyrethroids win high rating for potency, safety

costly compounds

A range of insecticides perhaps 100 times more potent than DDT but without the long-term ill effects associated with chemical control is undergoing final laboratory checks before being released for widespread use in public health projects and agriculture.

The new synthetic compounds, copying the molecular structure of natural pesticides derived from pyrethrum, an East African flower, hold out a promise for controlling insects that transmit such diseases as malaria, yellow fever, megacardia (the enlargement of the heart), bubonic plague and sleeping sickness which affects humans as well as cattle. The insecticides are also useful against caterpillars that attack fruit, vegetables, tobacco, olive, vine and cotton crops and against insects damaging stored food stocks. Almost a third of the world's grain crops is lost annually to pests while in storage. A large proportion of this immense wealth may now be saved.

Since more insecticide is used on cotton than on any other crop, the new compounds may well account for a quarter of the world's foliar insecticides by 1982, according to specialist estimates. They are photostable, which means that they remain lethal even after long exposure to sunlight; and their use in the cotton fields could thus start a boom in that lucrative crop.

Since the introduction of DDT over 30 years ago, resistance by insects to that chemical and to other pesticides groups has developed in populations of no less than 108 vector species of public health importance; and some of them, in limited areas, have become resistant to almost all insecticides hitherto available. When a species develops a high level of resistance to an insecticide, one solution is to replace the insecticide with another belonging to a different chemical group. This often means the replacement of a safe and cheap compound with something more expensive or toxic requiring costly and complicated safety measures.

Insecticides have thus played an important role in reducing diseases, but they are no longer regarded as the most effective public health tool. Malaria, for example, has appeared in many areas once cleared of the disease and the vectors-anopheline mosquitos-have grown resistant to all the inexpensive insecticides that have been used to control them. While insecticides are introduced in nature to protect human health and welfare, they are powerful environmental pollutants. The assessment of the risks involved requires elaborate studies and screening programmes, which add heavily to the cost of developing new insecticides.

The new compounds have been developed by the world's oldest agricultural research institution at Rothamsted in England. Industry in Western Europe, North America and Japan is ready to go ahead with large-scale production pending final approval by the United Nations World Health Organization (WHO).

A spokesman for the WHO expert committee responsible for the final testing series observes that these compounds are "among the most active insecticides known. Sixteen synthetic pyrethroids have so far been tested in the WHO programme. Their unit cost is high, but their cost-effectiveness may be acceptable. This and their effects on non-target organisms will be critical for their acceptance for vector control."

Dr. Charles Potter, a leader of the research team that has produced the new compounds, was already employed at the Rothamsted Experimental Station when DDT was first widely introduced after the Second World War. He and his associates realized that the new agricultural chemicals were likely to cause long-term environmental damage. They set out to find biodegradable alternatives.

They turned to the daisy-like pyrethrum flower which had been used for a very long time as a source of potent, yet non-poisonous insecticides, the pyrethrins. But these natural insect killers are expensive to produce and they also break down chemically too quickly when they are exposed to air and light.

Late in the 1940s, the scientists launched a long-term project to investigate the relationship between the molecular structure and the insect-killing activity in pyrethrin compounds. This led to the synthesis of the new chemicals from other substances. The compounds are comparable to pyrethrins in their safety to man-but they are far more resistant to the effects of air and light and far more deadly to insects. Yet they are readily metabolized when swallowed by mammals and they break down in the soil, avoiding the pollution of the environment.

However, the new pesticides have some drawbacks. They kill beneficial insects such as bees and ladybirds and they are toxic to fish. But if they are carefully handled, these difficulties can be circumvented. Insects can also develop resistance to them, as to just about all other insecticides. In the meantime, the new compounds may win a breathing space for man to study the very mechanism of insect resistance.