Insect resistant crops, problems

Provides a coordinated crop protection industry response to the development of resistance in insect and mite pests. During the last decade, IRAC has formed several international working gi oups to provide practical solutions to mite and insect resistance problems within major crops and pesticide groups. 

The levels of antifeedants can be raised safely only in plants such as forest trees or fiber crops, not in food plants for humans or livestock. Such problems have arisen inadvertently. For instance, a new insect-resistant potato cultivarhad to be withdrawn from the market because it contained high levels of the carcinogens solanine and chaconine (Renwick etal, 1984). In another example, an insect-resistant celery had 10 times the usual concentration of the carcinogen 8-methoxypsoralen (and related psoralens), which caused dermatitis in produce handlers (Seligman etal, 1987). 

There are a number of possibilities for using plant secondary chemistry to control herbivory in crop plants. One possibility is to select for insect resistant lines and though it has been done in only a few cases, select for specific allomones. There are, however, some potential problems with this approach. There is a cost for the production of the secondary compounds which may be useful for defense ( ). Insect resistant soybean cultivars produce lower yields of seeds and accumulate nitrogen at a slower rate than insect susceptible varieties in the absence of herbivores 3 ). Conversely, varieties of crop plants selected for high yield are often more susceptible to insects, pathogens, and weeds (35). 

The use of GM organisms in the environment poses certain potential problems. For example, genes for herbicide or insect resistance may spread from crop plants to wild plants, with possible serious consequences tor both agriculmre and namral ecosystems. Farmers may be faced with new superweeds , while insect populations could decline. Moreover, the products of GM crops have to be fully evaluated to ensure that they are safe to eat. Genetic modification of animals often has unforeseen side-effects and raises ethical issues about such treatments. 

The time-honored method of controlling insect-borne virus diseases is by breeding resistant varieties. This has been practical in annual crops, but is hopelessly slow in tree crops, where it may take 20 years or more to test a new variety. What is needed desperately is some sort of treatment which will control the virus, probably a systemic treatment, as the virus works within the plant cells. This is not a new idea and work has been done along this line by many workers. A sense of urgency is inevitable, however, when 500 to 600 acres of citrus can be wiped out completely in 3 to 5 years time, followed by an expensive replanting job and a wait of 5 to 6 years to get back into production. This is the outstanding problem at the present time and may need years to answer. 

The recent advances in identifying and utilizing allelochemlcs Involved in host-plant resistance has drawn the attention of the pesticide Industry. A potential problem that may not be recognized, is the effect on insects if analogs of plant protective chemicals are sprayed on agricultural crops. Insects treated with such analogs, could rapidly become tolerant not only to the analog, but also to the natural allelochemlc. 

Rotating to a cover crop also helps reduce insects and nematode pests, weeds, and plant disease. When a grower doesn t rotate, he is likely to be faced with one or more of these problems. Conventional growers can use pesticides to control these pests rather than rotate. So there is a choice between crop yield (while cover crops are planted) and pesticide use (that can maintain yield but may have toxic effects on the environment), at least in the short run. In the longer run, pests become resistant to pesticides and farmers may be forced to rotate because the crop can no longer be grown in that area. 

Abundant use of one of the most commonly used insecticides in crop protection, pyrethroids, has led to the development of resistance in many insect species.One of the most important mechanisms is that of knockdown resistance (kdr), caused by several mutations in the gene (L1014F and M918T), which confers crossresistance to the entire class of pyrethroids. Another problem is that most insecticides cause toxicity in organisms other than insects because of the general conservation of the voltage-gated Na" " channel structure... 

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