Insecticides discussion

Chlordane (C HgClg) and Hentachlor (C gH Cly) belong to the family of bicyclic cyclodiene organochlorine insecticides discussed earlier (See under Aldrin). Both compounds are highly toxic, readily... 

In Section 17.3, rodenticidal thioureas, insecticidal thiocyanates, and fungicidal dithiocarbam-ates were discussed. Sulfur is a common constituent of other classes of insecticides. These prominently include the organophosphate insecticides discussed in Chapter 18. Mobam (Figure 17.10) is a contact insecticide of the carbamate type, closely related in structure and function to the well-known carbamate insecticide carbaryl (see Figure 15.8). Mobam has been found to have a relatively high toxicity to laboratory mammals and is considerably more toxic than carbaryl. 

This section will discuss insecticides, acaricides, and repellents having toxicological characteristics distinct from the insecticides discussed in the previous section. It discusses pyrethroids, fluorides, borates, chlordimeform, propargite, substituted haloaromatic urea compounds, chlorobenzilate, cyhexa-tin, methoprene, sulfur, diethyltoluamide, aUcyl phthalates, and benzyl benzoate. 

For more on all insecticides discussed in this Section, see Section 6.4.1. 

Organophosphates and carbamates containing a pyrazole ring, useful as insecticides as discussed earlier (Section 4.04.4.1.2), are metabolized mainly through hydrolysis of the ester function (B-80MI40406). 

We ve discussed only open-chain compounds up to this point, but most organic compounds contain rings of carbon atoms. Chcysanthemic acid, for instance, whose esters occur naturally as the active insecticidal constituents of chrysanthemum flowers, contains a three-membered (cyclopropane) ring. 

Despite the extensive screening of plants for insecticidal properties, only a vanishingly small proportion of the world s species have been examined. Apparently, even some very common kinds of plants have been overlooked Dr. Lichtenstein will discuss his research on natural insecticides in turnips, rutabaga, and other garden vegetables. 

Hurst (19) discusses the similarity in action of the pyrethrins and of DDT as indicated by a dispersant action on the lipids of insect cuticle and internal tissue. He has developed an elaborate theory of contact insecticidal action but provides no experimental data. Hurst believes that the susceptibility to insecticides depends partially on the cuticular permeability, but more fundamentally on the effects on internal tissue receptors which control oxidative metabolism or oxidative enzyme systems. The access of pyrethrins to insects, for example, is facilitated by adsorption and storage in the lipophilic layers of the epicuticle. The epicuticle is to be regarded as a lipoprotein mosaic consisting of alternating patches of lipid and protein receptors which are sites of oxidase activity. Such a condition exists in both the hydrophilic type of cuticle found in larvae of Calliphora and Phormia and in the waxy cuticle of Tenebrio larvae. Hurst explains pyrethrinization as a preliminary narcosis or knockdown phase in which oxidase action is blocked by adsorption of the insecticide on the lipoprotein tissue components, followed by death when further dispersant action of the insecticide results in an irreversible increase in the phenoloxidase activity as a result of the displacement of protective lipids. This increase in phenoloxidase activity is accompanied by the accumulation of toxic quinoid metabolites in the blood and tissues—for example, O-quinones which would block substrate access to normal enzyme systems. The varying degrees of susceptibility shown by different insect species to an insecticide may be explainable not only in terms of differences in cuticle make-up but also as internal factors associated with the stability of oxidase systems. 

Apart from the resistance of insects to insecticides, resistance has been developed by plants to herbicides, fungi to fungicides, and rodents to rodenticides. Rodenticide resistance is discussed in Chapter 11, Section 11.2.5. 

In discussing the enviromnental fate of technical DDT, the main issue is the persistence of p,p -DDT and its stable metabolites, although it should be bom in mind that certain other compounds— notably, o,p -DDT and p,p -DDD—also occur in the technical material and are released into the environment when it is used. The o,p isomer of DDT is neither very persistent nor very acutely toxic it does, however, have estrogenic properties (see Section 5.2.4). A factor favoring more rapid metabolism of the o,p isomer compared to the p,p isomer is the presence, on one of the benzene rings, of an unchlorinated para position, which is available for oxidative attack. p,p -DDD, the other major impurity of technical DDT, is the main component of technical DDD, which has been used as an insecticide in its own right (rhothane). p,p -DDD is also generated in the environment as a metabolite of p,p -DDT. In practice, the most abundant and widespread residues of DDT found in the environment have been p,p -DDE, p,p -DDT, and p,p -DDD. 

Looking at the foregoing results overall, the rates of loss in vivo are related to the rates of metabolism in vitro, measured or estimated. As with the OC insecticides, problems of persistence are associated with compounds that are not readily metabolized, for example, 2,2, 4,4, 5,5 -HCB in the foregoing examples. For further discussion of the dependence of elimination of lipophilic xenobiotics on metabolism, see Walker (1981). 

When OCs were phased out, the less persistent insecticides that replaced them were thought to be more environment friendly. However, some of the insecticides that were used as replacements also presented problems because of very high acute toxicity. The insecticides to be discussed in this chapter illustrate well the ecotoxi-cological problems that can be associated with compounds that have low persistence but high neurotoxicity. 

CBs, like OPs, can cause a variety of sublethal neurotoxic and behavioral effects. In one study with goldfish Carrasius auratus), Bretaud et al. (2002) showed effects of carbofuran on behavioral end points after prolonged exposure to 5 pg/L of the insecticide. At higher levels of exposure (50 or 500 pg/L), biochemical effects were also recorded, including increases in the levels of norepinephrine and dopamine in the brain. The behavioral endpoints related to both swimming pattern and social interactions. Effects of CBs on the behavior of fish will be discussed further in Chapter 16, Section 16.6.1. 

Example Ester (59) was needed for a photochemical synthesis of chrysanthemate ester (60), a component of the pyrethrin insecticides. The a,B disconnection (59a) gives synthon (61) and aldehyde (62). This 8,y-unsaturated compound could be made by dehydration of (63) as the double bond can appear in only the required position. On page T 149 we discussed the synthesis of (62) by the aldol dimerisation of (64), An alternative strategy is to work at the ester oxidation level (65) which means synthon (66) is needed to combine with (64). 

Electrospray ionization (ESI) and APCI are the two popular API techniques that will be discussed here. The applications to the analysis of pesticides that will be discussed include imidazolinone herbicides, phenoxy acid herbicides, and A-methyl carbamate insecticides. Matrix effects with respect to quantitation also will be discussed. Eor the... 

Figure 1 was taken from an unpublished report, DuPont Study No. AMR 4392-97, Dissipation of Dislodgeable Foliar and Soil Residues of Oxamyl Following Application of Vydate L Insecticide to Tomatoes in the USA - Season 1997-1998 . This study has been submitted to the EPA and the data were used to establish and verify re-entry intervals. Data from this study will be used to provide an example of the topics discussed throughout this article. 

Reversed-phase HPLC followed by post-column derivatization and subsequent fluorescence detection is the most common technique for quantitative determination of oxime carbamate insecticides in biological and environmental samples. However, for fast, sensitive, and specific analysis of biological and environmental samples, detection by MS and MS/MS is preferred over fluorescence detection. Thus, descriptions and recommendations for establishing and optimizing HPLC fluorescence, HPLC/ MS, and HPLC/MS/MS analyses are discussed first. This is followed by specific rationales for methods and descriptions of the recommended residue methods that are applicable to most oxime carbamates in plant, animal tissue, soil, and water matrices. 

Experimental results obtained on these products by various screening procedures are summarized in Tables I to IV. Detailed description of these tests on insects and discussion of their relative practical usefulness for evaluating insecticides will be given in a subsequent publication (5). In this screening program on seven materials and the necessary standards of reference about 38,000 houseflies and over 5000 other insects were used. 

A procedure for the determination of 7-benzene hexa-chloride and DDT in benzene hexachloride-DDT-sul-fur formulations, employing partition chromatography, is described. The procedure has also been applied to the assay of 1,1,1 -trichloro-2,2-bis(p-methoxyphenyl)-ethane in technical methoxychlor. Results of separation of other insecticidal ingredients are discussed. 

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