Resistance development organophosphates

A newer class of insecticides is the pyrethroids. These are synthetic derivatives of pyrethrins, which are natural extracts from chrysanthemums. Pyrethroids have been developed to be more stable (and thus more effective as insecticides) than the pyrethrins, which are particularly instable in light. Pyrethroids are frequently used as broad-spectrum insecticides. They have high insect toxicity, but lower mammalian toxicity than their organophosphate or carbamate counterparts. Pyrethroids are still limited in effectiveness due to their environmental lability, their high cost, and their potential for resistance development. 

More recently, Mota-Sanchez et al. (2002) found a similar trend using database analysis. Table 10.3 shows that the majority of reported cases of resistance are arthropod resistance to organophosphate (44%) and organochlorine (32%) insecticides. It is understandable, because these two classes of insecticides have been used in pest control for over half a century. As shown in Table 10.4, twelve species of insects have developed resistance to various strains of Bt and Bt toxins in the laboratory and/or the field. 

In 1916, calcium arsenate [7778-44-1] dusted by airplane was used to control the boU weevil however, throughout many developments in effective insecticides, such as organophosphates, the boU weevils became resistant to poisons that were formerly effective (see Insectcontroltechnology). 

Currently there are few insecticides registered as surface treatments to control stored-product insects. For years the organophosphate insecticide malathion was used as a surface treatment for structural facilities, but stored-product insects throughout the world have developed extensive resistance to malathion (Subramanyam and Hagstrum, 1996). Most of the resistance reports were generated from studies with bulk grains, but in the United States, resistance has been documented for field populations of the red flour beetle, T. castaneum (Herbst), and the confused flour beetle, T. confusum (DuVal), collected from flour mills (Arthur and Zettler, 1991, 1992 Zettler, 1991). Populations of the Indianmeal moth, the almond moth, and the red flour beetle collected from bulk peanuts and empty warehouses were also highly resistant to malathion (Arthur et al., 1988 Halliday et al., 1988). 

Carboxylesterases are well-represented in insects and are sometimes important in the development of resistance to insecticides. Thus, a well-characterized carboxylesterase E4 is responsible for resistance to organophosphorus insecticides in the aphid (Myzuspersicae) [107]. In the California Culex mosquito, the esterase B1 is 500-fold more abundant in organophosphate-resistant than in susceptible insects. The increase of esterase levels is the result of gene amplification, i.e., the resistant animals have an increased number of copies of the structural esterase gene [108],... 

Another important problem is the development of insects resistant to insecticides. This often arises as a result of increased levels of carboxylesterases which hydrolyze both organophosphates and car-baryl.h/1 A mutation that changed a single active site glycine to aspartate in a carboxylesterase of a blowfly changed the esterase to an organophosphorus hydrolase which protected the fly against insecticides.)... 

In this case, a rather serendipitous pattern of resistance has developed among species to certain organophosphate (OP) insecticides. This resistance or lack thereof in some species has been exploited, providing an example of IPM, as well as resistance management. 

Because of these problems, and also the development of DDT-resistant strains of insects, DDT was banned in the United States in 1973 and has since been banned in other countries in the developed world. Alternative insecticides are available, such as organophosphates, but they cost at least three times as much as DDT. The developed world has the luxury of choosing to use these, as it can pay more for the food which they help protect and for a pleasant environment. However, in the Third World the nations resources are so small that even DDT can barely be afforded. It is therefore still used on a large scale in these countries. 

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