Keeping pace with resistant pests

Press, 8 September 1982, Page 24

Keeping pace with resistant pests

From the “Economist,” London

This northern autumn the United Nations will try to ensure that pesticides are used more intelligently. The Food and Agricultural Organisation (F.A.0.) hopes to persuade countries to restrict the use of “dangerous" pesticides. Restrictions alone will not end abuse of pesticides. Farmers will also have to be offered better ways of protecting their crops than spraying ever more deadly chemicals each vear.

Farmers are under pressure to use more toxic pesticides because pests develop resistance to chemicals. Evolution is speedy: the less resistant pests are killed off. leaving the hardier ones to breed. Pesticides often make the problem worse, by destroying their target's natural enemies. The unwanted insect (now pes-ticide-resistant) then breeds unchallenged. Even if the target pest is killed, another nasty bug resistant to that chemical may take over. Graphic examples of this poisonous spiral abound. Cot-ton-growing was wiped out in north-east Mexico and cut by two thirds in Texas in the 1960 s by a burgeoning population of tobacco budworms. The budworm had been a minor pest until farmers started to use large doses of pesticide against the boll weevil. The weevil suffered; the budworm flourished.

Sudanese cotton production is one of the worst cases. In 1980, yield per acre was half its level in 1970. The whitefly, which secretes a.'sticky substance on to the buds and makes them unusable, had become resistant to all known * pesticides. Rice production all over Asia has been hit by the brown planthopper — a relatively minor pest before intensive crop spraying destroyed rival pests. Bugs seem to be able to beat anything. Chemical companies last year introduced their newest pesticide family, synthetic pyrethroids. Already resistance is being recorded. There is a way out of the spiral, clumsily named integrated pest control. With this method, farmers must expect to lose about 20 per cent of their crops, but they can maintain that figure year after year. The system works by combining traditional chemical pesticides with various other techniques. Such techniques include:

® Natural enemies. Parasites can be bred and let loose on the pest in question. The approach has been taken up com-

mercially: parasites of the whitefly and the spider-mite are already sold. Field trials of a fungus that attacks the brown planthopper are under way. © Pheromones. Insects naturally use these substances to communicate with each other, to give a mating call for example. When introduced artificially into the fields, pheromones can disrupt normal behaviour, and help kill off the pests.

© Resistant strains. A lot of the problems with rice were caused by high-yield varieties developed for the "green revolution." They turned out to be much more susceptible to pests. Plant breeders are nowconcentrating on resistance rather than yield. ® Planting timetables. Crops can be planted so that they miss the high season of pests. This is traditional wisdom — the pigeon pea in India, for example, has always been planted to flower in December and miss the November pests. @ Specific pesticides. These are pesticides which attack only a particular target and preserve that pest’s natural enemies. Scientists are enthusiastic, but the chemical companies doubt that such pesticides would sell in large enough quantities to cover the costs of development and regulatory approval. The F.A.O. has been advocating combinations of these approaches for nigh-on 10 years. Endless demonstration projects have shown that crop losses can be much lower than with chemicals alone, yet big American farmers are the only ones to have taken up the idea on a large scale. Cost is a problem. Because of the planning required for an integrated pest-management system, the initial outlay is generally greater than the price of the chemicals alone. After a couple of years, however, when stronger, more expensive pesticides would be needed in larger doses in traditional systems, the relative costs tend to even out.

Farmers have to be sophisticated to use integrated systems. They require detailed knowledge about a crop and its particular pests and a highly developed information and distribution service to tell them what to use when.

In America, companies sell-, ing pest-control plans are sprouting. Others (which produce, for example, maggots for fishermen) are starting up sidelines in breeding pests’ natural enemies. The big commercial farmers, producing cotton, soya or alfalfa, are their main customers.

Much of the impetus for these developments came from the Carter Administration. As a peanut-farmer, the President

had a certain interest in the field. Most European countries have adopted the idea' — though pest problems in

Europe are much smaller than they are in America and the Third World. Governments in the Third World are on the

whole enthusiastic, but only a few have managed to get integrated pest-control systems working. . j,