Discussion

The control that can be exerted over pest insect species by their natural enemies needs to be harnessed and used to its maximum potential in any insect pest management programme. The way in which the natural enemies are used, i.e. introductions, inundation, augmentation, inoculation or conservation, will depend on the characteristics of the pest and the cropping system in which it causes damage or yield loss. As has been described, more stable systems lend themselves best to use of natural enemies, but augmentations can be used in less stable systems and even in highly intensive systems such as glasshouses, provided that the approach is economically feasible. Techniques that conserve natural enemies are already part of traditional mixed subsistence farming methods but more needs to be done to promote natural enemy conservation in more intensive cropping systems. This would require a greater interest by scientists and investment of funds for research in this subject than is currently apparent. Research into techniques for conservation of natural enemies has its dedicated advocates but as a subject it lacks the glamour and prestige that might be associated with control through introductions and augmentations. The problems associated with implementation and adoption of conserva tion techniques can also be formidable, hardly an incentive to budding applied scientists. However, a greater emphasis needs to be placed on the conservation of natural enemies and work on this given a higher priority, especially because of its relevance to insect pest management in developing countries.

Biological control was superseded earlier this century by the development of and increased use of chemical insecticides so that research into biological control decreased markedly. Although biological control has once more gained its due recognition as an extremely valuable form of control, especially within the context of the developing philosophy of IPM, on the whole chemical and biological control are incompatible techniques, mainly because so many insecticides are detrimental to natural enemies. There have been moves to evaluate the effects of insecticides on beneficial insects and a general commitment to recommend use of insecticides less toxic to natural enemies or to apply insecticides at times when natural enemies are less at risk. However, in practice, natural enemy populations are consistently being annihilated by insecticide use.

An increasingly appropriate alternative to chemicals is now being provided by a whole gamut of biological control agents formulated as biopesticides. A greater emphasis on rational, more selective chemical insecticide use, combined with tions and gaining maximum impact from biopesticides, should provide an effective biocontrol agents in integrated programmes means of preserving natural enemy popula- of pest management.

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