Insecticide knockdown resistance

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... 

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... 

Soderlund, D.M. and Knipple, D.C., The molecular biology of knockdown resistance to pyrethroid insecticides, Insect Biochem. Molec. Biol., 33,563, 2003. 

This term was originally used to describe the incapacitation of insects such as mosquitoes by insecticides (Asher, K.R., Preferential knockdown action of cetyl bromoacetate for certain laboratory-reared resistant stains of houseflies. Bull. World Health Organ. 18, 615-611, 1958 Cohan, F.M. and Hoffmann A.A., Genetic divergence under uniform selection. II. Different responses to selection for knockdown resistance to ethanol among Drosophila melanogaster populations and their replicate lines. Genetics 114,145-164, 1986 Bloomquist, J.R. and Miller, TA., Sodium channel... 

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... 

Ingles, RJ., Adams, RM., BCnipple, D.C., and Soderlund, D.M. 1996. Characterization of voltage-sensitive sodium channel gene coding sequences from insecticide-susceptible and knockdown-resistant house fly strains. Insect Biochem. Mol. Biol, 26, 319. 

Sawicki and Elliott (31) re-examined the insecticidal activity of pyrethrin extracts and its four insecticidal constituents against four strains of houseflies, and checked the relative toxicity of pyrethrins I and II. This work confirmed the earlier results, showing that pyrethrin II was 1.3 to 1.6 times more toxic than pyrethrin I, but that the relative toxicities of pyrethrins I and II against the four strains of flies differed little. Resistance to knockdown but not to killing was associated in these strains with resistance to organophos-phorus and chlorinated insecticides. 

The products are based on inert materials such as silica gel or diatomaceous earth (DE) and contain no insecticides or knockdown agents. They are effective against species resistant to pesticides, and are stable at high and low temperatures (McLaughlin, 1994). In contrast to chemical insecticides, which induce rapid immobilisation and kill, the action of inert dusts is slow, and extended exposure periods of 20 days or more may be required to eliminate an insect population. Most products, at the appropriate concentration, provide protection for at least 12 months (McLaughlin, 1994). 

The knockdown and residual aphicidal activity of an insecticide depends on one or more of these biological properties (a) contact advity, (b) translaminar systemicity, (c) downward or phloem systemicity, (d) upward or xylem systemicity, and (e) inherent efficacy against insecticide-resistant aphids. We have compared RH7988 with several important commerdal aphicidal compounds for these properties through laboratory tests. 

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