Target site insensitivity

Physiological resistance has been shown to involve three factors, i.e., reduced penetration, enhanced detoxification, and target site insensitivity. Generally, these resistance factors do not occur alone and are known to interact with each other, especially penetration and metabolism, to enhance the level of resistance. 

Resistance due to reduced penetration of insecticides appears to be a common phenomenon. In conjunction with other mechanisms, it confers considerable resistance to some insecticides. By itself, however, it provides only slight resistance (Plapp, 1986). We can speculate as to the possible mechanism, including a binding protein or lipid reservoir that traps the insecticide in the cuticle, a degrading enzyme located in the cuticle or, simply, a thicker or more impermeable cuticle (Terriere, 1982). 

A biochemical study indicated that the cuticle of the DDT-resistant strain of tobacco budworm contains more protein and lipid than that from the susceptible strain (Vinson and Law, 1971). Also, there was evidence of increased sclerotization of the cuticle of resistant insects. This suggests an increase in density and hardness of the cuticle that probably decrease its permeability to insecticide molecules. 

Decreased uptake as a mechanism of resistance was also observed in houseflies resistant to organochlorine, organophosphate, and carbamate insecticides. Resistant strains had higher total lipids, monoglycerides, diglycerides, fatty acids, sterols, and phospholipids in the cuticle than did the susceptible strain (Patil and Guthrie, 1979). 

Resistance to py rethroids can also be in part attributed to reduced penetration in insects. Ahmad et al. (2006) found that delayed cuticular penetration played an important role in deltamethrin resistance in Chinese and Parkinson strains of the cotton bollworm, Helicov-erpa armigera. The half-time for applied deltamethrin was 1 hr for the susceptible strain and 6 hr for both of the resistant strains. 

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This implicated target site insensitivity is more effectively demonstrated when actual amounts of endrin present in brain tissue of Ss fish are compared to Ra fish (7). In Ss fish the endrin concentration in the forebrain is 24 ng endrin equiv-alents/mg protein while in the same brain fraction of Ra fish there was 1150 ng endrin equivalents/mg protein. Further, when the amount of endrin in various brain fractions from R fish exposed to 1500 yg/1 c-endrin is monitored with time, up to 4560 ng endrin equivalents/mg protein appears in the brain fractions of Ra fish after 24 hr endrin exposure (Table VI). 

Hepatic mixed-function oxidase activities demonstrated seasonal trends, with higher specific activities in the cold weather months in both populations with few differences in enzyme activities or cytochrome levels between the two populations. Metabolism of aldrin, dieldrin and DDT was similar between the two populations. R fish have larger relative liver size and, therefore, a greater potential for xenobiotic metabolism. However, biotransformation appears to be of minor importance in chlorinated alicyclic insecticide resistance in mosquitofish barriers to penetration appear to be of greater importance and an implied target site insensitivity appears to be the most important factor in resistance. 

Disturbances of the cytoskeleton, DNA replication, and DNA topoisomerase, or DNA alkylation and intercalation usually lead to cell death by apoptosis [18] (Table 1.2). The cytotoxic properties are usually not specific for animals but also affect bacteria, fungi, other plants, and even viruses. Alkaloids thus defend plants against a wide diversity of enemies. They have the disadvantage that a producing plant could theoretically kill itself by its own poison. Compartmentation, target-site insensitivity, and other mechanisms (which are largely unknown) must have evolved to overcome such problems. 

Insecticide resistance has been shown to involve three principal mechanisms, i.e., enhanced detoxification, decreased penetration, and target site insensitivity. Usually these factors interact with one another so that the effect of each is magnified. There are two kinds of insects, those that are already tolerant and those that are susceptible at first but become resistant later. The former may be tolerant because of behavior, morphology, or detoxification capacity. The latter become resistant because of selection of individuals that possess higher levels of detoxification enzyme activity and which are thus able to survive and produce progeny. All of these aspects of resistance will be discussed at length in Chapter 10. 

Three types of target site insensitivity to insecticides are known. These are nerve insensitivity, altered acetylcholinesterase, and reduction in midgut target site binding. 

Kanga, L.H.B. and Plapp, F.W., Jr., Target-site insensitivity as the mechanism of resistance to organophosphorus, carbamate, and cyclodiene insecticides in tobacco budworm adults, j. Econ. Entomoi, 88,1150,1995. 

Metabolic resistance mechanisms are still the most widely encountered causes for OP and carbamate resistance. Yet, after the first few cases of target site insensitivity were reported in the early 1970 s in mites and ticks, many such cases have been found also in insects. Resistance can also be enhanced by a decreased rate of penetration through the integument. This resistance mechanism by itself is of minor importance, but provides an increased opportunity for detoxification. A 50-fold increase in resistance to carbaryl was seen in a house fly strain, in which a gene for reduced penetration had been combined (by selective breeding) with a gene for increased detoxification (5). 

Although implicated less frequently in arthropod pesticide resistance, diminished sensitivity of target sites in the nervous system is a major factor in many cases of resistance to different classes of insecticides. As more is learned about this phenomenon, we may find that it plays a more important role in resistance than is currently known. There are two major types of target site insensitivity. Uith insecticides such as DDT, certain other... 

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