Contact insecticide

A contact insecticide with the trade name Sevin . White solid, m.p. 142 C. It is prepared by reaction of I-naphthol with methyl isocyanate or with phosgene and a base. 

CaH24N.,03P2,(Me2N)2P(0)0P(0)(NMe2)z. It is not highly toxic to insects when used as a contact insecticide, but is readily absorbed by the roots and leaves of plants and translocated in the sap, so that the plant becomes toxic to species feeding on it. 

Nicotine is used as a contact insecticide for aphids attacking fmits, vegetables, and ornamentals, and as a fumigant for greenhouse plants and poultry mites. Nicotine sulfate is safer and more convenient to handle and the free alkaloid is rapidly Hberated by the addition of soap, hydrated lime, or ammonium hydroxide to the spray solution. Nicotine sprays commonly contain 0.05—0.06% nicotine, and nicotine dusts, 1—2% nicotine. 

Thiiranes (77) show some juvenile hormone activity, but the epoxide is often more active. Thiophosphates of 2-mercaptomethylthiirane are strong contact insecticides 2-chloromethylthiirane and4-vinyl- 1,2-epithiocyclohexane are nematocides. Severalthiirane 1 -oxides are reported to be insecticides, molluscicides and herbicides (68USP3413306). 1,2-Epithio-1,2,3,4-tetrahydronaphthalene is a mild herbicide. 

The biochemistry or mode of action of pyrethrum is not as well known as its chemistry. There are several theories of the toxic action of pyrethrum. Lauger et al. (26) consider that a highly effective contact insecticide must possess a toxic component (toxaphore) and must have groups attached which absolutely insure pronounced lipid solubility. They consider in the case of pyrethrins that in the cyclopro-... 

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. 

STAHL, for instance, was able to demonstrate that on irradiation with long-wavelength UV light the naturally occurring contact insecticides pyrethrin I and II, cinerin I and II and jasmolin I and II present in Chrysanthemum cinerariifolium are converted to inactive pyrethrin oxides by the incorporation of oxygen [7]. 

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. 

Although a lipoid-soluble group characterizes many contact insecticides, simple oil-solubility of a compound is not always a criterion of activity. Busvine (14) tested a series of DDT analogs and found that solubility in oil was not essential to activity. Kirkwood... 

There are several theories concerning the mechanism by which the toxic component of a contact insecticide exerts its action. The toxicity has been credited to the condensed chlorobenzene system, which is also lipophilic in character (70). 

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.

Methyl bromide has been identified as an ozone-depleting substance and is being gradually removed from world markets. Current legislation and plans call for the elimination of methyl bromide in most industrial countries by 2005, with possible exemptions for quarantine (UNEP, 1996). Currently there is an extensive search worldwide for products that are alternatives to methyl bromide (Kawakami, 1999). These alternatives are broadly defined and include components of management plans such as sanitation, monitoring, contact insecticides, heat treatments, and modified atmospheres, in addition to new fumigants (Batchelor, 1998). 

Systemic insecticides are superior to contact insecticides in many ways. On account of their absorption and transfer in the plant they can deal with insects which might normally remain hidden or protected from the direct spray of a contact insecticide. Many systemic insecticides are specific to certain pests. For example, owing to the insecticide being contained within the plant, parasitic non-phytophagous insects are less likely than the pests to absorb the insecticide. And so ecological selectivity1 comes into play. 

The use of systemic insecticides has also opened up the possibility of combating virus diseases by controlling disease-carrying insects (cf. p. 171). Contact insecticides are again less efficient here as the insects are usually in hidden positions. 

Uses Widely used as a pesticide in the U.S. until it was banned in 1972 chemical research nonsystemic stomach and contact insecticide. 

Uses A contact insecticide and fumigant used against Diptera and Culicidae in homes and against Coleoptera, Homoptera, and Lepidoptera in fruit, cotton, and ornamentals. 

N.A. Pyrethrins, cinerins, palmitic, linoleic acid, sesquiterpene lactones.100 107 118 Externally used as a contact insecticide. 

Synthetic pesticides Various systemic and contact insecticides, molluscicides, acaricides and fungicides, pyrethroids Absent Exceptional Common Common... 

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