|dc.description||The advantages of Integrated Pest Management (IPM) for farmers in developing countries have been clear for many years. It cuts production costs by reducing reliance on expensive agrochemicals, reduces hazards to both humans and the environment and, at the same time, stabilizes yields by ensuring the survival of natural enemies of major pests. Yet its use is not understood. Why is this so?
The idea of integrated pest management is not new. The original definition envisaged 'applied pest control which integrates biological and chemical control. Chemical control is used in a manner which is least disruptive to biological control'. Many scientists would now regard such a definition as too restrictive and would view chemical control as a last resort when other methods fail to keep a pest in check. More recently, the concept of IPM has been applied to the integrated control of different types of pests (for example, insects, diseases and weeds) in the same crop. Although originally distinct, the two terms integrated control and IPM are now used almost interchangeably to imply both concepts.
The essential features of IPM are straightforward, although their application in specific cases requires considerable understanding of the biology of the pests involved and their natural enemies. They include three important principles. Firstly, wherever feasible, control should rely on natural enemies of pests; methods that seriously disrupt natural regulation of pest populations should never be used. Secondly, natural enemies should be enhanced, either directly or indirectly, and plant resistance used to reduce the necessity for expensive chemical control. The third principle is that chemical control should be selective and should only be used as and when pest populations increase to a level above which serious economic loss will result.
The last of these principles needs further explanation. In most cases, pest populations will be low following planting but, in the absence of adequate natural controls, will increase rapidly as the crop develops. However, up until a certain level, there will be little or no damage of economic significance to the crop. The population level at which significant economic damage is observed is known as the economic threshold for a given pest and only above this threshold is chemical control employed. In this way, natural enemies are allowed to build up and expensive chemicals are not wasted. The economic threshold is obviously a moving target and differs between crops, pest species and, since it is dependent on the market value of a given crop, between seasons. Nevertheless, use of economic thresholds has proved its value in reducing crop production costs and is now very widely used in pest control schemes.
The components of integrated pest control
The weapons available to the farmer are of four types; biological control, cultural control, selective pesticide use and plant resistance.
Biological control using natural enemies of pests can be achieved through introduction of predators, parasites or diseases. Introduction (in some cases re-introduction) of natural enemies is now a familiar concept. Although there have been some spectacular successes, for example the control of prickly pear by the moth Cactoblastis cactorium in Australia and of cottony cushion scale on citrus in California, it should be said that less than half of all attempts at biological control by introduction have been successful. While the reasons for this are complex, it does emphasize the importance of retaining existing control agents through the rational use of pesticides. This is particularly true in the tropics where many crops face dozens of different pest species. Although the effect of any one member of this pest complex may be relatively minor, in combination they can cause drastic losses. To introduce specific predators or parasites for each pest would be totally uneconomic, even if they could be found; enhancement of existing natural enemies, many of which feed non-selectively, is a more sensible alternative.
Cultural control, which involves manipulation of the crop and its surrounding habitat aims to increase the chance of survival of natural enemies or reduce that of the pests. One obvious technique is to practice mixed cropping rather than monoculture. This can both reduce the size of pest populations by limiting the food resource available and provide alternative refuges for natural enemies. Intercropping has similar aims but on a finer scale. Other techniques include the provision of refuges (not necessarily a crop species) or food sources for natural enemies. In some situations, complete weeding of the crop may reduce yields since 'weeds' can provide both refuges and food sources for natural enemies. Destruction of crop residues by removal or burning can sometimes be successfully used to interrupt the development of pests that have a resting stage in such residues.
Selective pesticide use is another possible approach. The most important facet of selective pesticide use is employing economic thresholds to limit the amount of chemicals applied. However, modern developments in both pesticides and their application can also reduce the need to spray the crop. Several insecticides now available have some degree of inherent selectivity. An example is Endosulfan (Thiodan), which is effective against a wide variety of crop pests but which, at recommended application rates, has little effect on the minute parasitic wasps that are important natural enemies of many pests. When used in ultra-low volume formulation for control of tsetse flies, this insecticide was found to have virtually no detectable effect on other insect populations. Insecticides that are based on hormonal growth regulators or on microbial agents also show some degree of selectivity in their action.
Even more selectivity can be achieved by careful formulation and application of pesticides. Because systemic insecticides are taken up through the foliage or roots of the plant they are only available to the chewing or sucking insects that attack it. The same principle applies to insecticides that are encapsulated, wrapped in a coating of gelatine or similar inert substance. The tiny capsules adhere to the surface of the plant and the insecticide can only be taken up by biting insects that feed on the plant. This avoids risk to predators and parasites.
New application techniques can reduce the amounts of pesticide required to achieve effective control. Ultra low-volume (ULV) spraying is now widely used, particularly for aerial application. A rotating nozzle breaks the spray up into a haze of minute droplets. The droplets are sufficiently small that they can pass directly into the insect's body through the spiracles, the pores through which insects, breathe, killing them more rapidly. In this way, the amounts of insecticide required for effective control can be drastically reduced, often by as much as one hundredfold. One disadvantage of ULV spraying is that the droplets are so small that they are blown away by even the lightest breeze, reducing effectiveness. An ingenious technique called electrostatic deposition avoids this problem by giving each droplet a minute positive electric charge. The surface of the plant has a small negative charge, attracting droplets to the plant cuticle, where they are deposited. The pesticide is effectively 'glued' to the plant by electric forces! A cheap, portable sprayer, using this principle has now been marketed and has proved extremely effective in the Third World, particularly for the control of cotton pests.
Plant resistance is a further option in IPM. Plant breeding has played a crucial role in the prevention of crop losses due to diseases throughout the 20th century, and disease-resistance is a major objective in the development of new crop varieties. The concept of breeding for insect pest resistance is a more recent one but has had considerable success and is being actively pursued, particularly for tropical crops. A good example is the range of rice varieties developed at the International Rice Research Institute (IRRI) in the Philippines. IRRI has bred varieties resistant to four of the major insect pests of rice which are now very widely grown in S.E. Asia. Several varieties are available which combine resistance to three of the four major pests. Insect resistant varieties of cowpea, an important tropical pulse, have been developed at the International Institute of Tropical Agriculture (IITA) in Nigeria using advanced biotechnologies.
Successes in integrated control
One of the earliest examples of successful IPM comes from the Canete Valley in Peru, where from the 1940s, cotton was grown as a monocrop with extremely heavy use of insecticides. By 1955, yields plummeted following blanket application of organochlorine and organophosphorus insecticides which resulted in breakdown of natural biological control and the development of pest resistance. In 1956, the government introduced compulsory crop rotation and mixed cropping and enforced a return to older insecticides. One of these, lead arsenate, is a stomach poison which only kills insects that chew the plant and is ineffective against natural insect enemies. These measures were combined with regular inspection of the crop for pests so that insecticides were only used when pest populations exceeded the economic threshold. Cotton yields rapidly recovered to levels close to those before the crisis and the Canete Valley remains today, over 40 years later, a major cotton producing area of Peru.
Integrated control of pests has not been widely used in Africa but there have been some successes. The common coffee mealybug was a major pest of coffee in Kenya where its populations had exploded due to resistance to persistent organochlorine insecticides and to loss of natural enemies. Restrictions on the use of these insecticides, combined with the introduction and dissemination of a minute parasitic wasp (Anagyrus sp. ) brought the pest back under control. Similarly, in Tanzania a major pest of sugar cane, the sugar cane scale insect, has been successfully controlled by carefully timed insecticide application in conjunction with the release of a ladybird beetle which preys specifically on the pest.
Cassava, an important staple crop for some 200 million people in Africa, is seriously threatened by the cassava mealybug, accidentally introduced from South America 20 years ago. In this case, classical biological control is proving highly successful. A parasite of the mealybug, itself from South America, is released from the air and has successfully controlled the bug over large areas of West and Central Africa. In Asia, Indonesia experienced a severe reduction in rice yields in recent years due to the ravages of the Brown Planthopper. This has resulted from excessive use of insecticides, to many of which the planthopper now shows resistance. By 1986,100,000 hectares of rice had been destroyed by this pest alone, despite 4 to 5 applications of insecticide in each growing season. In 1988, the Indonesian government took the decision to ban the use of 57 different types of insecticides and to reduce severely the use of the recommended Carbamate insecticides. This has allowed populations of natural enemies to recover and to provide adequate control of the planthopper. Between 1986 and 1988, average yields of rice in Indonesia increased from 6.1 tonnes per hectare to 7.4 tonnes per hectare, a gain of 21% in two years. Considering this was achieved simply by reducing the amount of insecticide applied, and, thus, production costs, the advantages of rational insecticide use had been confirmed. As a result, FAO is now planning experimental programmes of integrated rice pest control in 6 other countries in the region.
IPM is not a panacea and, as with biological control, there have been failures as well as successes. Clearly, IPM requires that farmers have a better knowledge of both the pests and their natural enemies than is the case with simple chemical control. It is also difficult for farmers to 'do nothing' until the pest reaches an economic threshold, even though it is clearly present in the crop. This is particularly true if, previously, they have been advised to spray at the first signs of the presence of pests. Neither can IPM be used against all types of pests. It has proved impossible to devise schemes for highly migratory pests, such as locusts, and soil nests. such as termites.
The future of integrated control
IPM is used on only a very small proportion of the world's crops. Apart from the problems mentioned above, the rapid development of synthetic pyrethroid insecticides since the 1970s has largely overcome the problems of pest resistance to earlier insecticides. Nevertheless, the need for more rational pest management will grow. The problem of pest resistance will not simply go away (some insects already show resistance to pyrethroids) and the cost of agrochemicals, which are largely oil-based, is likely to continue to rise sharply.
Several of the components of IPM are now widely used in crop protection in the industrialized countries. Crop surveillance, combined with the use of an economic threshold for chemical control, is now widely used in such crops as cotton, alfalfa, tobacco and many fruit crops. And biological control has proved particularly successful for glasshouse crops, where chemical control poses particular problems and where there is good environmental control.
What, then, is the likely future of IPM in the tropics ? The need for food security has lead to greater emphasis on sustainable agriculture, within which IPM can play a crucial role. It has proved most successful where farmers have sufficient resources and education to exploit the possibilities of longer term control strategies, but there are many developing countries, particularly in Africa, where this does not yet apply. In the medium term, it is likely that more rational use of insecticides, based on crop surveillance, is likely to increase, if only for economic reasons. Whether this will develop into fully integrated control in the longer term will probably depend more on economic and political factors than on purely technical considerations. Above all, successful introduction of IPM depends critically on reasonably stable and fair prices for agricultural produce, both staple foods for home consumption and cash crops for export.||