Housefly resistance mechanisms

CYP6D1 of the housefly (Musca domestica) has been found to hydroxylate cyper-methrin and thereby provide a resistance mechanism to this compound and other pyrethroids in this species (Scott et al. 1998 see also Chapter 12). Also, this insect P450 can metabolize plant toxins such as the linear furanocoumarins xanthotoxin and bergapten (Ma et al. 1994). This metabolic capability has been found in the lepi-dopteran Papilio polyxenes (black swallowtail), a species that feeds almost exclusively on plants containing furanocoumarins. 

We now know that a single gene on chromosome IV is responsible for dieldrin resistance in houseflies. The mechanism is still obscure, although recent work has shown that houseflies do metabolise small amounts of dieldrin (28). 

By the utilization of marker strains bearing visible mutants, which are available for the housefly and their position on the 6 chromosomes in that species (17) it became possible to locate the genes responsible for the various DDT-resistance mechanisms already described (Fig. 4). The resistance due to dehydrochlor-... 

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). 

With this comprehensive range of resistance mechanisms and monitoring techniques to hand, the major objectives of the housefly work are as follows ... 

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... 

Table IX. Mechanisms of DDT-Resistance in the Housefly and other Insects.

Results used here to illustrate this approach refer to two insecticides widely used for housefly control, the pyrethroid permethrin and the OP trichlorphon (Figure 1), and to two mechanisms implicated in resistance to these compounds, kdr and AChE-R respectively. 

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