Insecticidal resistance mechanisms

Knowledge of the underlying molecular details of insecticide resistance mechanisms is, at present, at a very preliminary stage. To date there is only one case, i.e., that of elevated carboxylesterase levels in Culex quinquefasciatus resulting from gene amplification (H ), in which a specific variation of a cloned gene has... 

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). 

Apart from the importance of OP resistance in pest control, ecotoxicologists have become interested in the development of resistance as an indication of the environmental impact of insecticides. Thus, the development of esteratic resistance mechanisms by aquatic invertebrates may provide a measure of the enviromnental impact of OPs in freshwater (Parker and Callaghan 1997). 

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... 

Extracts of plants have been used as insecticides by humans since before the time of the Romans. Some of these extracts have yielded compounds useful as sources (e.g., pyrethrins, rotenoids, alkaloids), others as models (e.g., pyrethrins, physostigmine) of commercial insecticides. Recent technological advances which facilitate the isolation and identification of the bioactive constituents of plants should ensure the continued usefulness of plant compounds in commercial insect control, both as sources and models of new insect control agents and also as components in host plant resistance mechanisms. The focus in this paper will be on several classes of compounds, including limonoids, chromenes, ellagitannins, and methyl ketones, which were found to be components of the natural defenses of both wild and cultivated plants and which may be useful in commercial insect control. 

The Annonaceous acetogenins offer a unique mode of action (ATP depletion) against MDR tumors and against insecticide-resistant pests and are predicted to become important future means of thwarting ATP-depleting-resistance mechanisms. Their SARs in several systems have been determined (Landolt et al., 1995 Alfonso et al., 1996 He et al., 1997 Oberlies et al., 1997 Miyoshi et al., 1998) and optimum structural features generally point to the bis-adjacent THF compounds such as bullatacin (1) and asimicin (2). 

It should be possible to develop either biochemical- or molecular-based tests for resistance in flatworms, as has been successfully used to detect insecticide resistance in insects. However, the necessary information is not available on the mechanism of resistance to be able to design tests of this type for use with trematodes or cestodes. When developed they will hopefully be based on real-time polymerase chain reaction (PCR) or pyrosequencing. 

The Progression of Resistance Mechanisms Developed against Insecticides... 

C. A. Mullin C. H. Mason J.-C. Chou J. R. Linderman, Phytochemical Antagonism of 7-Aminobutyric Acid Based Resistances in DiabroticaAn Molecular Mechanisms of Insecticide Resistance Diversity among Insects C. A. Mullin, J. G. Scott, Eds. ACS Symposium Series No. 505 American Chemical Society Washington, DC, 1992 pp 288-308. 

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. 

Resistance to insecticides in insects and mites is preadaptive i.e., the genes controlling the resistance mechanism are already present in very low frequencies in the population prior to any use of insecticides. Therefore, the resistant genes are mutants from some earlier era in the insect s history, or were selected by other toxicants in its earlier environment, and... 

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). 

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