Resistance dimethoate

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... 

In addition to ester bonds with P (Section 10.2.1, Figures 10.1 and 10.2), some OPs have other ester bonds not involving P, which are readily broken by esteratic hydrolysis to bring about a loss of toxicity. Examples include the two carboxylester bonds of malathion, and the amido bond of dimethoate (Figure 10.2). The two carboxylester bonds of malathion can be cleaved by B-esterase attack, a conversion that provides the basis for the marked selectivity of this compound. Most insects lack an effective carboxylesterase, and for them malathion is highly toxic. Mammals and certain resistant insects, however, possess forms of carboxylesterase that rapidly hydrolyze these bonds, and are accordingly insensitive to malathion toxicity. 

Aphid strain Resistance factor to dimethoate Paraoxon hydrolyzed (pmol/mg aphid/hr)... 

Kusic et al. (1991) have tested the oxime HI-6 in OP pesticide poisoning in 60 patients. HI-6 was administered four times a day as a single i.m. injection of 500 mg with atropine and diazepam treatment. Oxime therapy was started on admission and continued for 2 to 7 days. Most patients were treated with HI-6 and nine patients severely poisoned with quinalphos were treated 2-PAM Cl (1,000 mg four times per day). HI-6 rapidly reactivated human red blood cell AChE inhibited by diethoxy OPs (phorate, pyr-idaphenthion, quinalphos) as well as that inhibited by dichlorvos (a dimethoxy OP). AChE inhibited with other dimethoxy OPs (dimethoate and phosphamidon) was reported to be resistant to HI-6 treatment, whereas reactivation with malathion was slow (reactivation half-time 10 h). Both HI-6 and 2-PAM successfully reactivated AChE in quinalphos-poisoned patients, with HI-6 acting as a faster AChE reactivator than 2-PAM. 

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,... 

The so called external insecticides coat the surface of the plant and have more or less penetration power (e.g., azinphos-methyl and -ethyl, dichlorvos, diazinon, malathion, parathion-methyl, and -ethyl). Others, called systemic insecticides, penetrate the plant tissue, are transported by the sap, and diffuse within the plant (e.g., dimethoate, mevinphos, omethoate, vamidothion). Mites have developed a resistance to OPPs, most of which are no longer marketed as acaricides. OPPs are now found in products developed to treat plants, soil, buildings, and foods. 

AChE inhibited by several OP pesticides, including dimethoate, demethon, triamiphos, ethoprophos, pro-fenofos, fenamiphos, and p5u-idafenthion, was resistant to any attempt of reactivation with any oxime, probably because of the variations in phosphoryl moiety and distribution of electron charge (Bismuth et al., 1992 Jokanovic and Maksimovic, 1995 Jokanovid, 2009 Jokanovic et al., 2010 Jokanovic, 2012). 

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