Housefly selectivity

Figure 10.3 Development and regression of resistance in colonies of the housefly selected by Iso-Ian or m-isopropylphenyl methylcarbamate. (From Georghiou, G.P., Annu. Rev. Ecol. Syst., 3, 133, 1972. With permission.)...

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 organophosphorons insecticides dimethoate and diazinon are mnch more toxic to insects (e.g., housefly) than they are to the rat or other mammals. A major factor responsible for this is rapid detoxication of the active oxon forms of these insecticides by A-esterases of mammals. Insects in general appear to have no A-esterase activity or, at best, low A-esterase activity (some earlier stndies confnsed A-esterase activity with B-esterase activity) (Walker 1994b). Diazinon also shows marked selectivity between birds and mammals, which has been explained on the gronnds of rapid detoxication by A-esterase in mammals, an activity that is absent from the blood of most species of birds (see Section 23.23). The related OP insecticides pirimiphos methyl and pirimiphos ethyl show similar selectivity between birds and mammals. Pyrethroid insecticides are highly selective between insects and mammals, and this has been attributed to faster metabolic detoxication by mammals and greater sensitivity of target (Na+ channel) in insects. 

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. 

Studies with susceptible and selectively bred carbofuran-resistant houseflies (Musca domestica) indicated that LD50 values for susceptible and resistant strains were 0.1 and 1.3 pg/insect, respectively (Dorough 1973). Resistant flies contained up to 34% more cholinesterase than susceptible strains and could excrete carbofuran almost twice as fast (Dorough 1973). Carbofuran resistance among pestiferous insects is not yet widely known or adequately documented. 

J. W. (1999). Size, symmetry, and sexual selection in the housefly, Musca domestica. Evolution, 53, 527-534. 

In the mid-1960s we showed firstly that the natural tolerance of houseflies to cyclodienes resulted mainly from oxidative detoxication (33 55) and secondly that another enzyme system, epoxide hydrase, converted certain dieldrin analogues into the corresponding trans-diols, (56,57) Interspecific differences in ability to attack enzymatically the unchlorinated ring systems of various analogues, either oxidatively and/or hydratively (if appropriate) can confer selective toxicity between insect species and also between insects and mammals (58) ... 

The Swiss product DDT was introduced for housefly control in neutral countries in 1944, and already by 1946 resistance had developed in northern Sweden (of all unlikely places). When housefly resistance appeared near Rome, Italy in 1947, Professor Missiroli considered that it was a different subspecies which he named Musca domestica tiberina at the very same time that Wilson and Lindquist in the USDA Orlando laboratory were producing a resistant strain from a susceptible one by laboratory selection. 

The multiresistant strains now extant also show a certain cross-tolerance, but not resistance, to the third-generation insecticides such as the juvenile-hormone mimics and other so-called insect growth regulators, as was found in strains of the housefly, flour beetle and tobacco budworm. Resistance to the JH mimic methoprene and Monsanto-585 has been induced by laboratory selection of Culex taxsalis (28) and Culex pipiens (29), and to Monsanto-585 in Culex quinquefasstatus (30). Whatever insect or IGR is chosen, the result of exposure to selective doses in successive generations is usually the development of resistance, repeating our previous experience with chemosterilants, and the... 

Insects utilize propionate and methylmalonate in the biosynthesis of ethyl branched juvenile hormones and methyl branched cuticular hydrocarbons. The sources of propionate and methylmalonate in some insects appear to differ from those in mammals. Succinate is the precursor of propionate and methylmalonate in a termite, whereas valine and probably other amino acids are the sources of propionate and methylmalonate in several other species. An unusual pathway for propionate metabolism has been shown to occur in insects and it may be related to the absence or low levels of vitamin B found in many species. Propionate is converted directly to acetate with carbon 1 of propionate lost as C02> carbon 2 of propionate becoming the methyl carbon of acetate and carbon 3 of propionate becoming the carboxyl carbon of acetate. This pathway suggested the possibility that 2-fluoropropionate might be selectively metabolized in insects to the toxic 2-fluoro-acetate. However, preliminary data indicate that 2-fluoropropionate is not toxic to the housefly or the American cockroach. 

Prestwich and coworkers (23,24) have shown that by judiciously placing fluorines on selected positions of fatty acids and sterols, insects will metabolize the fluorinated precursor to the potent toxin 2-fluoroacetate (25). Because insects convert propionate to acetate, it was possible that they might convert 2-fluoropropionate to 2-fluoroacetate by the same pathway. However, preliminary experiments using houseflies and cockroaches indicated that 2-fluoropropionate was not readily converted to 2-fluoroacetate. Houseflies injected with 2-fluoropropionate (3 pg/insect) were not affected after 1.5 hr, whereas control insects injected with 2-fluoroacetate (2 pg/insect) were all dead within 0.5 hr. Similar results were obtained with the American cockroach. 

Cross-resistance refers to a situation in which a strain that becomes resistant to one insecticide automatically develops resistance to other insecticides to which it has not been exposed. For example, selection of a strain of Spodoptera littoralis with fenvalerate resulted in a 33-fold increase in tolerance to fenvalerate. The resistant strain also showed resistance to other pyrethroids (11- to 36-fold) and DDT (lower than for the pyrethroids). Exposure of Cidex qninquefasciatus to fenitrothion resulted in the development of resistance to the carbamate insecticide propoxur. Similarly, selection of a housefly strain with permethrin resulted in a 600-fold increase in resistance to permethrin. The resistant strain also showed resistance to methomyl, DDT, dichlorvos, and naled (Hassall, 1990). 

Figure 10.7 Increase in resistance of the NAIDM strain of housefly under selection pressure from various chlorinated hydrocarbon insecticides. (From Brown, A.W.A. and Pal, R., Insecticide Resistance in Arthropods, 2nd ed., World Health Organization, Geneva, 1971, with permission. Adapted from Decker, G.C. and Bruce, W.N., Amer. ]. Trop. Med. Hyg., 1,395,1952, with permission.)...

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... 

Famham, A.W. (1973). Genetics of resistance of pyrethroid-selected houseflies. Musca domestieti L. Bestir. Sci. 4, 513-520. 

Sawicki. R.M. (1974). Genetics of resistance of a dimethoate-selected strain of houseflies iMusca domestics ..) to several insecticides and met by le ned ioxypheny synergists. J. Agric. Food Client. 22. 344-349,... 

Fore recently a comparable enhanced inhibition in resistant strains has been observed with aryloxadiazolone anticholinesterases (38). A second promising example is the discovery that some natural and synthetic isobutylamides are selectively toxic against houseflies that carry the super-kdr resistance trait (39). This gene causes an alteration in the sensitivity of the site of action for DDT and pyrethroids and is a major threat to the continued efficacy of synthetic pyrethroids in many of their applications. 

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