DDT-type insecticides

Chapter 31. Use of Chemical Relationships in Design of DDT-Type Insecticides... 

It is generally agreed that the contact-insecticidal activity of the DDT type of compound depends on at least one toxic component and the CC13 group or some other lipoid-soluble group for penetration. Beyond this point, there is a lack of agreement as to the exact mechanism by which the contact insecticides exert their action. 

Table III. It is obvious from the data in Table III that the housefly and the mosquito, in both the adult and larval stage, are susceptible to insecticides of the DDT type. However, the extravagant claims that DFDT is far superior to DDT as a contact insecticide against flies are not borne out by the results of controlled laboratory tests. The Peet-Grady testing technique used by Prill (92) would indicate that in the presence of added pyrethrins DDT is definitely superior to DFDT when applied as a spray. On the other hand, DFDT gave higher percentage kills than DDT when flies were placed under a Petri dish and held in contact with deposits of the compounds on glass surfaces. A comparison of the activity of these compounds against adult mosquitoes has not been reported.

MTD and MTD approaches will be discussed and compared using QSAR of insecticidal benzoylphenyl-ureas, DDT-type analogs and benzylchrysanthemates, herbicidal benzonitriles and nitrophenols, and plant-growth regulating phenoxypropionic acids. 

Insecticides of the "DDT-type" have potent excitatory activity in... 

Attempts to elucidate the structural requirements of bioactivity of DDT type compounds and to improve the properties of the parent molecule were initiated soon after the discovery of the insecticidal action of DDT. Early studies of Muller (1946) showed that a high degree of lipophilicity is desirable. On the basis of the relationship between the action of numerous DDT-analogues and their structures. 

The -thion organophosphates (those containing the P=S bond) are activated, not inactivated, by conversion to -oxon (P=0) derivatives. They are less stable than halogenated hydrocarbon insecticides of the DDT type therefore, they are less persistent in the environment Parathion is more toxic than malathion. It is very lipid-soluble and rapidly absorbed through the lungs and skin. The answer is (E). 

Propylene oxide is soluble in water and miscible with most organic solvents. It is found to be an excellent law-boiling solvent far cellulose acetate, nitrocellulose, adhesive compositions and vinyl chloride-acetate resins. It is also a solvent for hydrocarbons, gums and shellac. Same of its uses are as a solvent and stabilizer in DDT aerosol-type insecticides, and as a fumigant and food preservative. Since it is an acid acceptor, it is also used as a stabilizer for vinyl chloride resins and other chlorinated systems. 

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. 

Still another requirement is that the concentrated form of the insecticide readily combines with various types of vehicles. At the present time, pure DDT can be dissolved only in organic solvents, and it has been necessary to provide a 50% water-dispersible DDT powder when wrater was to be used as the vehicle. With the advent of 90% water-dispersible DDT, it may be possible to utilize this material in the preparation of both water suspensions and organic solutions of the chemical. The ideal chemical in this respect would be one that could be shipped as 100% dust, and could be readily diluted with inert dust, water, and organic solvents in the field. 

A number of official samples have been analyzed in the insecticide laboratory of the Production and Marketing Administration, which contained among the active ingredients 7-benzene hexachloride, DDT, and sulfur in the proportions of 3, 5, and 57%, respectively. The following detailed procedure was adopted after testing with authentic formulations of this type. 

Using the gas chromatography methods Sackmayereva et al. [30] obtained from spiked samples the four BHC isomers at 93-103.5% recovery. Both DDT and DDE were yielded in 85.6-94%, 90-93.2%, 90-102.4% and 92-105.8% from sediment. Purification on a Florisil column was used in determining chlorinated insecticides unstable at low pH (Aldrin, Dieldrin). The type and activity of Florisil influence the yield and accuracy of the method. Therefore, the activity of this adsorbent had to be verified and adjusted [36, 37],... 

The chief interest of petroleum chemists in the information presented here may be in the fact that, although the earlier kerosene type of petroleum derivatives mixture will produce insecticide fogs—and is still used successfully for them—an analysis of fog production and its use, from biological, physical, chemical, and mechanical standpoints, has resulted in the evolution of special fog oils which combine higher DDT solvency, specific insecticidal potency, greater safety, and greater chemical and physical stability with consequent limited and therefore more desirable particle size range, at known temperatures and pressures. 

Other types of insecticides have been developed and used over the past several decades. To discuss them all requires an entire course. There is an ASIDE which illustrates the halogenated hydrocarbons most of which are not in current use because of their effects on the environment (DDT) and toxicity to humans (Dieldrin/Aldrin). Estrogenic acivity of DDT led to fatal fragility in the eggs of certain predatory birds. All of these compounds are lipophilic and interfere with nerve signal transmission. 

Some pyrethroids have as many as eight different isomers, and there are several different types. Acute toxicity of a mixture of two isomers depends on the ratio of the amounts of isomers in the formulation. For example, the female rat s acute oral LD50 of permethrin increases from 224 mg/kg to 6,000 mg/kg as the proportion of transisomer increases from 20% to 80%. The route of exposure also is critical in assessing the acute toxicity of a synthetic pyrethroid. Like DDT and many other registered insecticides, naturally occurring pyrethrins and the synthetic pyrethroids are nerve poisons. 

The story of the discovery of the chlorinated hydrocarbon derived insecticides is one of outstanding achievement which deserves due recognition Indeed, the discovery within so few years of DDT, y-HCH ( y-BHC), the cyclodiene group and toxaphene, chlorinated insecticide types with distinct origins and synthetic principles, is truly remarkable ... 

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