Mechanisms pyrethroids

Many pesticides are not as novel as they may seem. Some, such as the pyre-throid and neonicotinoid insecticides, are modeled on natural insecticides. Synthetic pyrethroids are related to the natural pyrethrins (see Chapter 12), whereas the neo-nicotinoids share structural features with nicotine. In both cases, the synthetic compounds have the same mode of action as the natural products they resemble. Also, the synthetic pyrethroids are subject to similar mechanisms of metabolic detoxication as natural pyrethrins (Chapter 12). More widely, many detoxication mechanisms are relatively nonspecific, operating against a wide range of compounds that... 

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. 

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 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). 

The other major mechanism of pyrethroid resistance found in some field strains of Heliothis virescens was enhanced detoxication due to a high rate of oxidative detoxication, mediated by a form of cytochrome P450 (McCaffery 1998). Some strains, such as PEG 87, which was subjected to a high level of field and laboratory selection, possessed both mechanisms. Other example of pyrethroid resistance due to enhanced detoxication may be found in the literature on pesticides. 

Bromley-Challenor, K.C.A. (1992). Synergistic Mechanisms of Synthetic Pyrethroids and Fungicides in Apis Mellifera. M.Sc. Thesis, University of Reading, UK. 

McCaffery, A.R., GladweU, R.T., and El-Nayir, H. et al. (1991). Mechanisms of resistance to pyrethroids in laboratory and field strains of Heliothis virescens. Southwestern Entomologist Supplement 15, 143-158. 

Pilling, E.D., Bromley-ChaUenor, K.A.C., and Walker, C.H. et al. (1995). Mechanism of synergism between the pyrethroid insecticide lambda cyhalothrin and the imidazole fungicide prochloraz in the honeybee. Pesticide Biochemistry and Physiology 51, 1-11. 

Clark, J.M. and M.W. Brooks. 1989. Neurotoxicology of pyrethroids single or multiple mechanisms of action ... 

Salgado, V.L., M.D. Herman, and T. Narahashi. 1989. Interactions of the pyrethroid fenvalerate with nerve membrane sodium channels temperature dependence and mechanism of depolarization. Neurotoxicology 10 1-14. 

Taylor, K.S., G.D. Waller, and L.A. Crowder. 1987. Impairment of a classical conditioned response of the honey bee (Apis mellifera L.) by sublethal doses of synthetic pyrethroid insecticides. Apidologie 18 243-252. Theophilidis, G., M. Benaki, and E. Papadopoulu-Mourkidou. 1997. Neurotoxic action of six pyrethroid insecticides on the isolated sciatic nerve of a frog (Rana ridibunda). Comp. Biochem. Physiol. 118C 97-103. Tippe, A. 1987. Evidence for different mechanisms of action of the three pyrethroids, deltamethrin, cypermethrin, and fenvalerate, on the excitation threshold of myelinated nerve. Pestic. Biochem. Physiol. 28 67-74. 

While the mechanism of resistance to various synthetic pyrethroids in flies has been elucidated in terms of physiology, biochemistry, and genetics, it seems that the resistance mechanism is mostly common to mosquitoes. 

A recently commercialized U.L.V. (ultra-low volume) -type aerosol sprays a fixed volume of ultra-fine particles into the room and retains effectiveness for 12 h with one spray. The mechanism of the effectiveness was considered to be the contact of pyrethroid particles with mosquitoes in the air at the initial stage followed by re-volatilization into the air of pyrethroids attached to walls and floors however, according to experiments by the present author et al., it was confirmed that mosquitoes are knocked down by contact with pyrethroid particles in the air during the first 1 h and then the lethal effect on mosquitoes is achieved by their contact with pyrethroids adhering to walls. 

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... 

Kasai S, Komagata O, Itokawa K, Kobayashi M, Tomita T (2010) Mechanisms of pyrethroid resistance in adult Aedes aegypti. Med Entomol Zool 61(Suppl) 46... 

---