Pesticides resistance development

Another important result of the appearance of resistant target species is the increase in the number of pesticides to which pests acquire resistance. Resistance develops to all the pesticide groups used (Table 5.2). 

Almost all target species that are monitored and controlled develop resistance to the pesticides used. As early as 1984, the US National Academy of Science gave an international conference on the issues of resistance it was admitted that, both in theory and practice, there are no satisfactory and universal methods of fighting pesticide resistance. The situation has not changed in the decades since. 

I. Denholm, A. L. Devonshire, andD. W. Holloman, eds., Resistance 91 Achievements and Developments in Combating Pesticide Resistance, Elsevier Applied Science, London, 1992. 

The development of strains resistant to insecticides is an extremely widespread phenomenon that is known to have occurred in more than 200 species of insects and mites, and resistance of up to several 100-fold has been noted. The different biochemical and genetic factors involved have been studied extensively and well characterized. Relatively few vertebrate species are known to have developed pesticide resistance and the level of resistance in vertebrates is low compared to that often found in insects. Susceptible and resistant strains of pine voles exhibit a 7.4-fold difference in endrin toxicity. Similarly pine mice of a strain resistant to endrin were reported to be 12-fold more tolerant than a susceptible strain. Other examples include the occurrence of organochlorine insecticide-resistant and susceptible strains of mosquito fish, and resistance to Belladonna in certain rabbit strains. 

Side by side with this growth of knowledge there has been increasing concern that the implications of the large-scale utilization of synthetic chemicals be fully understood. Chemical methods of pest control have conferred such spectacular benefits on agriculture and the health of mankind that it has become difficult to conceive that these benefits could be offset or outweighed by serious disadvantages. Some of these effects are extremely subtle others, such as the development of pesticide resistance, rapidly become obvious because no further economic benefit is obtained by continued pesticide use. 

Reducing the risk of resistance development, for example, for pesticides... 

Herbicides comprise 60-70 percent of total usage of pesticides in developed countries. Plants develop resistance to herbicides which in turn demands altered management strategies through improved herbicides and/or herbicide mixtures. This may involve different modes of action, for short-term efficacy and long-term control to be effective. 

Considering the limited number of pesticides developed in recent years and the need to extend the useful life of the available compounds, WHO, in collaboration with other relevant institutions, should strengthen pesticide resistance monitoring in vectors and pests of public health importance and promote effective and practical resistance management strategies. 

Figure 10.6 Relationship between generations per year and development of resistance in species selected by soil application of aldrin/dieldrin. 1 and II, root maggots III, southern potato wireworm IV, northern corn rootworm V, European chafer VI, Japanese beetle VII, sugarcane wireworm. (From Georghiou, G.P. and Taylor, C.E., in Pesticide Resistance Strategies ami Tactics for Management, National Academy Press, Washington, D.C., 1986,157 With permission.)...

Population mobility is a very important behavioral factor for resistance development. The influx of migrants tends to dilute the frequency of resistance among survivors of treatments, especially for highly mobile insects such as the fall army worm and velvetbean caterpillar. Computer simulation indicated that a moderate rate of immigration of susceptible individuals could ensure the containment of resistance if the initial population was of low density and if a short-lived pesticide was used in regular treatments. 

The use of selected insecticide mixtures should retard resistance development because it should be more difficult for an insect to develop several adaptations simultaneously. The concept of joint use of insecticides assumes that the mechanisms of resistance to each member chemical exist in such low frequencies that they do not occur together in any single individual in the population. Thus, insects that may survive one of the chemicals are killed by the other. This approach delays resistance in laboratory experiments and has been, at least temporarily, successful in a few field cases, particularly with certain organo-phosphate combinations (Hopkins and Moore, 1980). The use of insecticide mixtures is not without problems. Resistance to both compounds used in mixtures has sometimes developed rapidly. Cross-resistance also occurs among some of the pesticides. 

Exceptionally comprehensive, thoughtful coverage of the history of resistance may be found in a paper of Georghiou and Mellon ("Pesticide Resistance in Time and Space", 37) and in a more recent update by Georghiou (33) This section offers a much abbreviated summary, followed by speculations on why resistance developed as it did and observations on the response of industry. 

Two factors are most commonly associated with the development of pesticide resistance in arthropods enhanced metabolic detoxification and/or decreased target site sensitivity. Other factors that sometimes come into play are reduction in the rate at which a toxicant is absorbed into the body, or in the rate at which it is translocated to the site of action. In rare cases, there may be behavioral adaptations which allow the arthropod to minimize contact with the toxicant (25-22.) ... 

OP pesticides still provide adequate control of key pests, while not upsetting biological control of certain secondary pests by their resistant predators and parasites (39). In those areas where the program is successful, the keys to success are monitoring for resistance in the species complex (e.g., among leafrollers, 40), rapid response to early signs of resistance (e.g., 41), maximum use of IPM and alternative control tactics other than pesticides (42), and the lack of resistance development in key pests such as the codling moth. 

If it were possible to develop two pesticides such that increased resistance to one of them led to increased susceptibility to the other, resistance development would at least be delayed. There is one example of this principle. The systemic fungicide diethofencarb is particularly effective against Botrytis spp., which are resistant against benzimidazole. Benzimidazoles like car-bendazim and thiophanate bind to a site on the tubulin protein and inhibit mitosis. Resistant Botrytis has a tubulin that does not bind benzimidazoles, but may bind diethofencarb better. 

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