Resistance to DDT and

Several species and populations from other Anopheles complexes have been discriminated based on CHC patterns. Examples include all five species of the An. quad-rimaculus complex (Carlson et al., 1997), some species of the An. maculipennis complex (Phillips et al., 1990a), malaria-vector and non-vector forms of the An. maculates complex (Kittayapong et al., 1990, 1993), and An. Stephensi strains susceptible or resistant to DDT and malathion (Anyanwu et al., 1993, 1997). CHCs have been used in combination with isoenzyme analysis to successfully differentiate populations of An. darlingi (Rosa-Freitas et al., 1992). All these findings demonstrate that hydrocarbon analysis is a powerful tool for distinguishing mosquito species and populations. This is particularly important for disease vectors, since it can facilitate interpretation of epidemiological data and assist implementation of control measures. 

Biologists have now observed hundreds of cases of natural selection, beginning with the well-known examples of bacterial resistance to antibiotics, insect resistance to DDT and HIV resistance to antiviral dmgs. Natural selection accounts for the resistance of fish and mice to predators by making them more camouflaged, and for the adaptation of plants to toxic minerals in the soil.32... 

Yet antibiotic resistance, resistance to DDT and resistance of HIV to drugs have all been known for decades. All of Coyne s examples were well known when Darwin s Black Box first appeared a decade ago, they all involve tiny, simple molecular changes, and none helps to explain any of the examples of the book. With all that space he still avoids engaging... 

Rates of malaria continue to increase over the years between 1970 and 1997 there was a 40% increase in malaria rates in sub-Saharan Africa alone. In the mid-f 950s, the World Health Organization launched a massive campaign to eliminate malaria from the globe. The initiative was initially successful and combined insecticide use and drug treatment malaria was conquered completely in some areas and sharply curbed in others. However, nature eventually intervened. Anopheles mosquitoes, the carriers of malaria, became resistant to DDT and other insecticides used in their elimination. 

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... 

The use of DDT increased enormously on a worldwide basis after World War II, primarily because of its effectiveness against mosquitoes that spread malaria and hce that carry typhus. The World Health Oi anization estimates that approximately 25 million fives have been saved through the use of DDT. DDT seemed to be the ideal insecticide—it is cheap and has relatively low toxicity to mammals (oral LD is 300-500 mg/kg). However, problems related to extensive use of DDT began to appear in the late 1940s. Many species of insects developed resistance to DDT, and DDT was also discovered to have a high toxicity toward fish. The chemical stability of DDT and its fat solubility compounded the problem. 

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... 

Resistance to DDT has been developed in many insect species. Although there are some cases of metabolic resistance (e.g., strains high in DDT dehydrochlorinase activity), particular interest has been focused on kdr and super kdr mechanisms based upon aberrant forms of the sodium channel—the principal target for DDT. There are many examples of insects developing resistance to dieldrin. The best-known mechanism is the production of mutant forms of the target site (GABA receptor), which are insensitive to the insecticide. 

The complexity of the new insecticidal chemicals brings many other problems. Synthetic organic chemicals are not effective against all pests. There is a marked selectiveness in action even between closely related species of insects. Some insects have already developed resistance to some of the newer materials. The idea of insects developing resistance to certain chemicals is not new. The over-all principle is well established in a few cases. The early development of flies resistant to DDT, a chemical which had been highly and universally effective for fly control, came as a surprise. Other cases of resistance to DDT are being indicated, and at least one kind of mite has developed resistance against another of the newer chemicals—parathion. 

A matter of very great interest is the increase in resistance to DDT that has been shown recently by houseflies in various parts of the world. This is often so marked that practical use of DDT is no longer feasible. An especially resistant strain, found in southern California by March and Metcalf, is called the Bellflower strain for purposes of identification (4). They reported that no residual deposit of DDT gave 100% kill. By the small vial method not over 25% kill of these female flies could be obtained with several thousand micrograms per vial. Further tests with various naturally occurring and selected resistant races are in progress. 

Malaria is one of the oldest parasitic diseases. The difficulty of malaria control is aggravated by the appearance of strains of Plasmodium falciparum resistant to antimalarials, as well as resistance of the vector mosquitoes to DDT and other insecticides. The molecule quinine isolated from the bark of the Cinchona sp. tree, represents the model for the synthesis of the majority of drugs currently used for malarial treatment [194], Davioud-Charvet et al [206] describe the screening of a library of... 

Ranson, H., Rossitter, L., Ortelli, F., Jensen, B., Wang, X., Roth, C.W., Collins, F., and Hemingway, ]., Identification of a novel class of insect glutathione S-transferases involved in resistance to DDT in the malaria vector Anopheles gambiae, Biochem. ]., 359, 295,2001. 

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 ... 

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... 

Dietary lipids play a unique role in the toxicity of chlorinated hydrocarbon pesticides. Dietary lipids may favor more absorption of these pesticides, but once these chemicals are absorbed into the body, they may be stored in the adipose tissue without manifestation of toxicity. For this reason, obesity in humans is considered protective against chronic toxicity of these chemicals. Similarly, the body fat in a well-fed animal is known to store organochlorine pesticides. Fat mammals, fish, and birds are thus more resistant to DDT poisoning than their thinner counterparts. In times of food deprivation, however, organic materials such as DDT and PCB can be mobilized from mammalian fat deposits and can reach concentrations potentially toxic to the animal. 

During the last two decades there has been continuing controversy concerning the classification of the enzyme DDT-dehydro-chlorinase as a GSH-S-transferase ( ). DDT-dehydrochlorinase is the enzyme that converts DDT to the relatively non-toxic DDE and has been intensively studied in houseflies and other insects because of its established importance in insect resistance to DDT. 

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