The battles against locusts: new insecticides sought
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CTA. 1990. The battles against locusts: new insecticides sought. Spore 26. CTA, Wageningen, The Netherlands.
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Last year the Sahelian countries again fell victim to attacks by locusts. This time the desert locust populations (Schistocerca gregaria) - devastated by massive doses of insecticide and by cold and drought - posed little threat, but the various...
Last year the Sahelian countries again fell victim to attacks by locusts. This time the desert locust populations (Schistocerca gregaria) - devastated by massive doses of insecticide and by cold and drought - posed little threat, but the various grasshoppers, in particular the Senegal grasshopper (Oedalus senegalensis) - which benefited from the plentiful rains - soon took its place. They arrived just at harvest time and caused ten times more damage than their near relatives did in the previous year. To control these pests thousands of litres of insecticides (700,000 litres of concentrate over 14.5 million hectares in 1988) were applied over the affected regions. As a result, locust specialists and environmental lobbyists often found themselves in conflict over the type of product which should be used. To limit the damage to the environment, it is essential that the most efficient, yet least harmful, chemical should be used and that the area of use should be strictly controlled. Use of dieldrin prohibited For nearly 30 years dieldrin has been used successfully to prevent locust swarms developing in their breeding areas. As a persistent chemical, it can be used to treat strips of land where the locusts can be poisoned. Dieldrin has other advantages, not least that it is cheap and can be stored easily, even in hot climates. But two years ago, when locusts swarms reached worrying proportions, ecologists began to protest strongly against the use of dieldrin on the grounds that it was dangerous to men and animals. For more selective treatment in a desert terrain, where the larvae hatch prior to flight, spraying with a less persistent chemical may be considered preferable. But such chemicals are only effective for a few days, and the chemical must be reapplied if it has not totally eliminated the danger of infestation. The results of this are higher costs, more work and complete destruction of all insect life in the treated areas. However, in agricultural and pasture land these organo-phosphorus chemicals have the great advantage that they leave a residue that is less toxic to the human population. They are efficient: they kill the locusts in 24 hours and can be the swift and effective answer to a swarm of locusts, but because they lack the persistence of dieldrin, they cannot be used as a preventative treatment. The same applies to pyrethroids, which are the least harmful of all insecticides, and which are used in built-up areas. Growth regulator brings hope Scientists who have been searching for a new form of chemical control which will combine the advantages of dieldrin and organophosphorus - persistent yet safe to the environment - have developed a new product which seems to bring some hope. The new product is a growth regulator which disrupts the development of the larvae when they eat treated vegetation. It is a persistent chemical which poses no danger to vertebrates and can be used for barrier spraying. It is effective against both desert locusts and grasshoppers and can be applied without risk of damage to agricultural land. However, more field trials are necessary before this material can be mass produced and marketed. Keep a watchful eye on the environment While new products are being developed, the environmental effects of massive and repeated insecticidal treatment of currently available chemicals have been the subject of close scrutiny in recent years. Several studies have been undertaken, by the Norwegians in Mali, and in Senegal by a Dutch French multi-disciplinary team, who are trying to gauge exactly the effects on fauna and human beings. One of the findings so far is that Fenitrothion is toxic to birds. But to evaluate the long-term effects of the less residual chemicals used today is extremely difficult. Their effects are subtle and hard to detect, especially in the early stages of experimentation. One thing is certain, however: a stricter use of insecticides would allow doses to be reduced and damage to the environment to be limited. Another danger is that although pesticide containers are highly dangerous they are often reused for various domestic and other purposes and the users rarely take any safety measures or wash out the drums to get rid of all the poisonous residue. Finally, there are skill large stocks of diel drin stored in the Sahel. These cannot be used since the chemical is now banned in most Sahelian countries. What will happen to them? A register has been compiled of these stocks but funding agencies (especially the Americans) are still seeking ways of getting rid of them. For more details, contact: CIRAD/PRIFAS - BP 5053 34032 Montpellier cedex - FRANCE
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