Resistance to DDT and pyrethroids

Mechanism of action can be an important factor determining selectivity. In the extreme case, one group of organisms has a site of action that is not present in another group. Thus, most of the insecticides that are neurotoxic have very little phytotoxicity indeed, some of them (e.g., the OPs dimethoate, disyston, and demeton-5 -methyl) are good systemic insecticides. Most herbicides that act upon photosynthesis (e.g., triaz-ines and substituted ureas) have very low toxicity to animals (Table 2.7). The resistance of certain strains of insects to insecticides is due to their possessing a mutant form of the site of action, which is insensitive to the pesticide. Examples include certain strains of housefly with knockdown resistance (mutant form of Na+ channel that is insensitive to DDT and pyrethroids) and strains of several species of insects that are resistant to OPs because they have mutant forms of acetylcholinesterase. These... 

Pyrethroids are widely used to control many agriculturally and medically important insect pests. Due to intensive use of pyrethroids in pest control, many pest populations have developed resistance to these compounds. One major mechanism of pyrethroid resistance, conferred by the knock down resistance gene (Mr), is reduced target site (sodium channel) sensitivity to DDT and pyrethroids. Studies on the molecular basis of Mr and Mr-type resistance in various insects are enhancing our understanding of the structure and function of insect sodium channels and the molecular interaction between insect sodium channels and pyrethroids. In this chapter, I will review recent advances in... 

Houseflies on Danish farms have become resistant to almost every new insecticide introduced for their control since the 1950 s (M.). In the mid 1970 s, scientists showed that resistance to DDT and the new pyrethroid insecticides is due to a common resistance factor (the kdr and super-kdr genes), and they predicted that rapid resistance development to the more persistent SP compounds would rapidly evolve in the field. In 1978-79, surveys for SP resistance indicated that resistance to long-residual compounds was beginning to develop in the field. Overall, the survey data indicated that resistance would soon be widespread. Several steps were immediately taken to evaluate the resistance potential of... 

Characteristically, DDT resistance in flies does not extend to prolan however, strains that are resistant due to receptor-site modification are also resistant to prolan and pyrethroids. This was observed early in houseflies and stable flies (Busvine, 1953 Stenersen, 1966). The DDT resistance in stable flies did not depend on metabolism (Stenersen, 1965). 

It is said that the action site of pyrethroids in flies is on the neuroaxonal excitatory membrane, similarly to that of DDT. Moreover, DDT-resistant M. domestica is known to show high cross-resistance to synthetic pyrethroids and the kdr gene is involved in the onset of the resistance. It has also been shown that such resistant flies exhibit high cross-resistance to many synthetic pyrethroids developed to date. 

Only countries with political clout and political sense about DDT, notably South Africa, China, and India, still use it. South Africa had stopped using DDT in 1996 under pressure to join the world s Green community and switched to the next best alternative, the synthetic pyrethroids, which are three times the price and are effective over a shorter time span. Four years later, in 2000, South Africa decided to resume DDT spraying after malaria cases jumped by 1,000 percent because of mosquito resistance to the synthetic pyrethroids. 

Resistance genes can be dominant, recessive, incompletely dominant, or incompletely recessive. Resistance to carbamates and organophosphates is usually dominant or incompletely dominant. Resistance to DDT, Bt, and spinosyns is usually recessive. Resistance to dieldrin is usually incompletely dominant. Resistance to pyrethroids is usually incompletely recessive. As shown in Figure 10.1 and 10.2, diamondback moth resistance to per-methrin was inherited as an incompletely recessive, autosomal factor, whereas resistance to methomyl was inherited as an incompletely dominant, autosomal factor. In cases of monofactorial inheritance of resistance to insecticides, the degree of dominance (D) in the progeny can be calculated, as described by Stone (1968), as follows ... 

Fore recently a comparable enhanced inhibition in resistant strains has been observed with aryloxadiazolone anticholinesterases (38). A second promising example is the discovery that some natural and synthetic isobutylamides are selectively toxic against houseflies that carry the super-kdr resistance trait (39). This gene causes an alteration in the sensitivity of the site of action for DDT and pyrethroids and is a major threat to the continued efficacy of synthetic pyrethroids in many of their applications. 

Enhanced detoxification has also been shown to be a contributing factor in resistance of the tobacco budworm to DDT Ql), pyrethroids (32.33). and most likely the carbamates. 

It is well established that the modes of action of DDT and pyrethroids are somewhat alike, and that genes for resistance to DDT also confer cross-resistance/tolerance to pyrethroids in a number of arthropod species (25). This probably is true for the tobacco budworm as well. With its past history of DDT resistance and the likelihood that genes for resistance to DDT still exist within the population, the tobacco budworm would be expected to have a head start in developing resistance to pyrethroids. It is interesting that H. zea. which also demonstrated high levels of DDT resistance... 

Insects have acquired resistance to organochlorine compounds, such as DDT and BHC, developed as agricultural and hygienic insecticides after World War II. This insect resistance was also acquired to subsequent organophosphorus compounds and carbamate insecticides. Photostable pyrethroids have been developed for outdoor use because pyrethroids were found to be effective against these resistant pests. As a matter of course, these pyrethroids are also effective against sanitary pests however, problems associated with safety and chemical residues indoors must be resolved. 

Resistance to insecticides has drawn global attention since the Korean War in 1950 when the mass use of organic synthetic insecticides, such as DDT and BHC, against agricultural pests and sanitary pests became common. Organophosphorus compounds and carbamates were used thereafter, but invited problems of safety concerns and insect resistance. Synthetic pyrethroids were watched with keen interest as alternatives and have become used widely not only for sanitary pests but also agricultural pests. The development of resistance to synthetic pyrethroids is also not a rare phenomenon and has spread all over the world. 

Cross-resistance refers to a situation in which a strain that becomes resistant to one insecticide automatically develops resistance to other insecticides to which it has not been exposed. For example, selection of a strain of Spodoptera littoralis with fenvalerate resulted in a 33-fold increase in tolerance to fenvalerate. The resistant strain also showed resistance to other pyrethroids (11- to 36-fold) and DDT (lower than for the pyrethroids). Exposure of Cidex qninquefasciatus to fenitrothion resulted in the development of resistance to the carbamate insecticide propoxur. Similarly, selection of a housefly strain with permethrin resulted in a 600-fold increase in resistance to permethrin. The resistant strain also showed resistance to methomyl, DDT, dichlorvos, and naled (Hassall, 1990). 

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