Pyrethroid insecticides resistance mechanisms

Resistance mechanisms associated with changes in toxicokinetics are predominately cases of enhanced metabolic detoxication. With readily biodegradable insecticides such as pyrethroids and carbamates, enhanced detoxication by P450-based monooxygenase is a common resistance mechanism (see Table 4.3). 

In some resistant strains, both types of resistance mechanism have been shown to operate against the same insecticide. Thus, the PEG87 strain of the tobacco bud worm (Heliothis virescens) is resistant to pyrethroids on account of both a highly active form of cytochrome P450 and an insensitive form of the sodium channel (Table 4.3 and McCaffery 1998). 

Yasutomi K, Takahashi M (1989) Insecticidal resistance of Culex tritaeniorhynchus in Chinen, Okinawa Prefecture, with special reference to the mechanism of pyrethroid-resistance. Jpn JSanitZool 40 315-321... 

The mechanisms of resistance fall into two main categories. Many insects produce an increased level of detoxifying enzymes, such as esterases, that modify the insecticides to inactive metabolites very rapidly. Such a system is seen in aphids that are resistant to OP insecticides. In other cases it is the target site that is modified such that the insecticide (the enzyme inhibitor) no longer binds to the target and is, therefore, ineffective. This has recently been shown to occur in some aphids that are resistant to OP insecticides but the classical example is knockdown resistance (kdr) and super-kdr to pyrethroid insecticides shown by many insects but particularly house flies Musca domes tied). This resistance is thought to result from a modification of... 

Sequestration in insects This is another resistance mechanism in which GSTs were found to be involved in pyrethroid resistance. Kostaropoulos et al. (2001) have reported that GST confers protection against deltamethrin by binding to the insecticide in the yellow mealworm ijenebrio monitor). 

A case in point is the unraveling of resistance to methyl parathion in the tobacco budworm, Heliothis virescens, which is a major pest of cotton as well as tobacco. In South Carolina, there is very severe, stable resistance. Although pyrethroid insecticides are very effective and there is no resistance to them in South Carolina at this time, it would be very useful to understand the genetic basis of methyl parathion resistance in case resistance to pyrethroids should arise in the future or spread eastward from Texas where it has been detected. Recent investigations with this pest will be described to illustrate certain mechanisms. 

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... 

Results used here to illustrate this approach refer to two insecticides widely used for housefly control, the pyrethroid permethrin and the OP trichlorphon (Figure 1), and to two mechanisms implicated in resistance to these compounds, kdr and AChE-R respectively. 

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... 

In this article, the level of susceptibility to various insecticides in different human head louse populations in the US was investigated to assess the distribution of resistance. We also reported on the molecular mechanisms of head louse resistance to pyrethroid mediated by sodium channel mutations. Also discussed were the genotyping techniques for resistance monitoring and the... 

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