Insecticide knockdown

The edible parts of parsnips (Pastimea sativa L.), which have been consumed for centuries by humans without causing any obvious harm, were found to contain a chemical of insecticidal and strong synergistic nature (1). The insecticidal constituent, present at about 200 p.p.m., was isolated and identified as 5-allyl-l-methoxy-2, 3-methylenedioxybenzene or myristicin. Its toxicity to various insects [vinegar flies, houseflies, Mediterranean fruit flies, mosquito larvae, Mexican bean beetles, and pea aphids] was established and compared with pyrethrum and aldrin (Tables I and II). The knockdown effect, although definite, was not as great as that of pyrethrum. In tests... 

Sawicki and Elliott (31) re-examined the insecticidal activity of pyrethrin extracts and its four insecticidal constituents against four strains of houseflies, and checked the relative toxicity of pyrethrins I and II. This work confirmed the earlier results, showing that pyrethrin II was 1.3 to 1.6 times more toxic than pyrethrin I, but that the relative toxicities of pyrethrins I and II against the four strains of flies differed little. Resistance to knockdown but not to killing was associated in these strains with resistance to organophos-phorus and chlorinated insecticides. 

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. 

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... 

Table VI. The knockdown power of the organofluorine insecticides has been determined principally against members of the Diptera order and by a limited number of workers. There is general agreement that DFDT acts more rapidly than DDT, at least against those species with which they have been compared. Prill 92) found that twice the amount of DFDT compared to DDT was required to give the same knockdown against houseflies when tested by a space spray technique with added pyrethrins. The forced contact method of Fay and Buckner 27) revealed that without added pyrethrins DFDT was a more powerful knockdown agent than DDT.

A high degree of insecticidal activity per unit weight, effectiveness against many species of insects, prolonged residual effect combined with rapid knockdown, compatibility with various vehicles, and availability under wartime conditions are among the most important military characteristics of insecticides. No present insecticide is entirely satisfactory. 

Another extremely important military characteristic is prolonged residual effect combined with rapid knockdowm. Although, at the present time, we have some chemicals which give a comparatively long residual effect, and other chemicals which give a relatively quick knockdown, the military still require more prolonged residual effect and more rapid knockdown in their insecticides. 

At first, Geigy s biologists were puzzled because the insects sprayed with DDT did not die immediately. Accustomed to the quick action of pyrethrum and rotenone, they dismissed Muller s discovery as unimportant. They did not realize that DDT s long period of activity could be far more important than sheer speed. In the parlance of insecticides, DDT had slow knockdown but sure kill. Surfaces sprayed with DDT in 1941 and stored under dust-free laboratory conditions were still toxic to insects seven years later. 

The main devices used for mosquito protection in households have been mosquito coils, electric mosquito mats, and liquid vaporizers, all of them methods that vaporize insecticides into the air using heating by means of fire or electricity to control the insects. In recent years, new anti-mosquito products have been commercialized such as fan vaporizers, paper strip type emanators, and resin net type emanators which vaporize insecticides without heating. In all of these products pyrethroid insecticides are used as active ingredients because they are superior in what is called knockdown effect, where noxious insects are rapidly paralyzed and cannot bite, and have a high level of safety for humans. 

The mechanisms of resistance fall into two main categories. Many insects produce an increased level of detoxifying enzymes, such as esterases, that modify the insecticides to inactive metabolites very rapidly. Such a system is seen in aphids that are resistant to OP insecticides. In other cases it is the target site that is modified such that the insecticide (the enzyme inhibitor) no longer binds to the target and is, therefore, ineffective. This has recently been shown to occur in some aphids that are resistant to OP insecticides but the classical example is knockdown resistance (kdr) and super-kdr to pyrethroid insecticides shown by many insects but particularly house flies Musca domes tied). This resistance is thought to result from a modification of... 

AUethrin [584-79-2] (d 1.005—1.015, vp 16 mPa at 30°C) is the aUyl homologue of pyrethrin I, ie, R = allyl, —CH2CH=CH2. The synthetic product contains 75—95% of eight enantiomers, 70% (d=)-trans and 30% (d=)-cis acids esterified with (A)-cyclopentenolone alcohol. The relative insecticidal activities of the enantiomers are shown in Table 2. The rat LD5Qs of aUethrin are 920 (oral) and 11,000 (dermal) mg/kg. BioaUethrin [584-79-2] is the (+)-/rchrysanthemate ester of the (zt)-alcohol (RS) (R) which has enhanced knockdown activity (vp 2 mPa at 25°C, water solubility 4.6 mg/L). AUethrin is as effective as the natural pyrethrins against flies and mosquitoes but has a narrow spectmm of activity against other insect pests. 

Pyrethroids are synthetic esters produced to imitate or improve the activity of biological principles of the pyrethrum plant. They are powerful contact insecticides causing rapid knockdown of treated insects. The pyrethroids are extensively used in controlling insect pests on fruit trees, vegetables, and other field crops in space sprays and contact sprays to kill insects infesting homes, industrial locations, and nonfood processing areas and in protection of warehoused food. These compounds include fenvelerate [51630-58-1], (7), R = Cl flucythrinate [70124-77-5], (1) R = CHF20 allethrin [584-79-2] (8) cyfluthrin [68359-37-5] (9) cypermethrin [52315-07-8] (10) deltamethrin [52918-63-5] (11) permethrin [52645-53-1] (12) and tetramethrin [7696-12-0] (13). 

Aquatic Invertebrates. The initial impact of insecticides on most benthic invertebrates in streams is the inability to maintain their position on the streambed resulting in increased numbers being carried downstream in what is termed drift. Invertebrate drift in treated streams is easily sampled by holding a net in the current. Post-spray catches can be compared to both pre-spray diurnal patterns and catches at an untreated upstream station or in an untreated control stream. This method also can be used to document knockdown of streamside foliage-dwelling or arboreal insects if the net is set so as to sample the stream s surface. Impacts on stream invertebrates can also be determined by caging studies and by pre- and post-spray bottom fauna population assessments which either sample the numbers within a given area of natural streambed (e.g., Surber sampler) or on artificial substrates intentionally set in the stream before treatment to be colonized by resident populations. 

Aminocarb. Next to fenitrothion, aminocarb has been the insecticide used most extensively in forestry in Canada over the past decade. Application rates of up to 0.175 kg/ha have been found to have little or no impact on forest songbird populations or small mammal breeding activity, but do cause considerable knockdown of terrestrial arthropods, particularly at higher application rates (35, 36). Short-lived but fairly extensive honeybee mortality has been documented when aminocarb has been applied while active foraging was underway, but the overall colony vigor was not seriously effected (37 ). Aminocarb does not appear to cause bumble bee mortality at operational application rates, but does affect solitary bees ( 18). 

Soderlund, D.M. and Knipple, D.C., The molecular biology of knockdown resistance to pyrethroid insecticides, Insect Biochem. Molec. Biol., 33,563, 2003. 

This term was originally used to describe the incapacitation of insects such as mosquitoes by insecticides (Asher, K.R., Preferential knockdown action of cetyl bromoacetate for certain laboratory-reared resistant stains of houseflies. Bull. World Health Organ. 18, 615-611, 1958 Cohan, F.M. and Hoffmann A.A., Genetic divergence under uniform selection. II. Different responses to selection for knockdown resistance to ethanol among Drosophila melanogaster populations and their replicate lines. Genetics 114,145-164, 1986 Bloomquist, J.R. and Miller, TA., Sodium channel... 

Metcalf listed attempts to relate synergism to such diverse factors as stabilisation of droplet siiie, reduction [sic] of rate of knockdown, stimulation of flight activity, prevention of deterioration of the toxicant, increased penetration into the insect or formation of molecular complexes between synergist and insecticide. He added that recent investigations have made il very clear that virtually all examples of pronounced synergism arc related to interference by the synergist of the iji viYo metabolic detoxification of the insecticide . 

Insecticide Factor of synergism for kilt ai LC f using PBO Factor of synergism for knockdown at KC (4.0 mini using 0.5 ft PBO... 

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