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Use of Insect Resistant Crops as a Part of IPM
The development of insect resistant transgenic crops (particularly Bt crops) was discussed in Chapter 35 of this book. These crops should make an essential component of IPM. This is perhaps the best method and makes the biggest market for the use of biopesticides.
Refugia as part of IPM
The use of insect resistant crops or chemical pesticides leads to development of resistance against the pesticide in the insects. Multiple control measures used in IPM reduce selection for this resistance. Refugia is one such measure, recommended to reduce the development of pesticide resistance in the insects. It involves plantation of a fraction of the field by non toxin producing crop, and permitting insects to consume it. This will allow multiplication of susceptible insects, which can mate with available resistant insects to produce more susceptible hybrid insects, thus reducing the population of homozygous resistant insect (susceptibility is dominant over recessive nature).
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A total system or ecological approach of IPM
Often IPM involves replacement of chemical pesticide by an alternative approach for pest management. This may include biocontrol, host plant resistance or biopesticides. Such an approach is unlikely to provide sustainable pest control solution. Therefore, a total system approach, also called an ecological approach, has been recommended for pest management. This approach involves restructuring and managing the crop ecosystem in ways, which maximize the array of ‘built in’ preventive strengths along with the naturally occurring biological agents, so that the therapeutic measures are used only as backups to these built in devices.
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This will involve multitrophic interactions and broader ecosystem management to keep the pest within acceptable limits. Following approaches have been suggested for this purpose.
Ecosystem management. A redirection of pest management should incorporate year round practice of managing soil, weeds, crops, water, et., keeping in view the overall fauna, nutritional value and the balanced ecosystem. For instance, in USA, the problem of soil erosion was overcome by winter crop cover and conservation tillage. In addition to control of soil erosion, supply of organic manure and retention of moisture, this allows stabilization of natural enemies of pests. It has been shown that crimson clover and other legumes (hosts for enemies of pests) into which cotton can be strip tilled makes a good reservoir for predators and parasitoids of cotton pests.
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Encouraging some weeds to grow in field margins can also provide important Refugia for insects. Fleabane and horsetails are two such weeds that the insects prefer over cotton. This kind of landscape ecology may help in reducing damage due to pests.
Crop attributes and multitrophic interactions.
Plants do posses toxins and chemicals that discourage herbivores. Therefore, defense responses of plants can be customized (bred into crop varieties) for certain interactive multitrophic situations. Following are some examples : (i) When herbivores attack, plants may release from all its parts some volatile chemicals that attract predators and parasitoids, which in turn attack the herbivores. This attribute can be bred into our crop plants to protect them from pests.
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(ii) Floral and extrafloral nectaries (attributes, which can be bred into crop varieties) may attract enemies of pest, which protect crops from bollworms/budworms in crops like cotton. (iii) Injured plants may also release toxins and antifeedants, which control herbivores directly without involving predators and parasitoids of pests. Some of these attributes are induced by damage in leaves. In other cases, these attributes may express in fruit constitutively, but express in leaves only due to feeding by insects, thus providing maximum protection to fruits.
Therapeutics
Chemical pesticides and biopesticides are used to kill insects and sex pheromones are used to kill insects, and sex pheromones and other attractants are used to lure insect enemies, which in turn control pests. These therapeutics should be used only as secondary backups without disturbing the balance of the ecosystem.
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Also, a retarded development of pests may be more desirable than quick kill of the pests; the latter may though give quick results, nut destroys and limits a resource for parasitoids, permitting reduction in the population of parasitoids and, therefore, in the resurgence of the pest. Therefore, the objective should be to manage and not to eliminate the pest.
The above total system or ecological approaches have shown promise in several countries like The Netherlands, where in an area of 300 acres of land, pesticide use was reduced by 90% over a period of 15 years. Therefore, it is necessary that in addition to biopesticides, other approaches be used for keeping the population of pests within an acceptable limit.
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