Selective insecticide

Synthetic Pyrethroid Insecticides. Elucidation of the chemical stmctures of the naturally occurring pyrethmm esters, their rapid and selective insecticidal action, and their high cost stimulated the search for effective synthetic derivatives (13,17,21). Since the 1940s, stmctural optimisation has produced an array of broad-spectmm insecticides with activity 10- to 20-fold greater than other types of insecticides, and with extended residual action. These synthetic pyrethroids have become one of the most important classes of insecticides with world aimual production estimated at 6000 t (21). 

The development of malathion in 1950 was an important milestone in the emergence of selective insecticides. Malathion is from one-half to one-twentieth as toxic to insects as parathion but is only about one two-hundredths as toxic to mammals. Its worldwide usage in quantities of thousands of metric tons in the home, garden, field, orchard, woodland, on animals, and in pubHc health programs has demonstrated substantial safety coupled with pest control effectiveness. The biochemical basis for the selectivity of malathion is its rapid detoxication in the mammalian Hver, but not in the insect, through the attack of carboxyesterase enzymes on the aUphatic ester moieties of the molecule. 

Insect Growth Regulators. These compounds (40—45), unlike most conventional insecticides, interfere with biochemical processes that are unique to arthropods eg, molting, ecdysis, and formation of the chitinous exoskeleton. Therefore, they are selective insecticides with very low mammalian toxicity. 

Mortensen,ME. 1986. Management of acute childhood poisonings caused by selected insecticides and herbicides. Pediatric Toxicology 33(2) 421-445. 

Nation, J.L., F.A. Robinson, S.J. Yu, and A.B. Bolten. 1986. Influence upon honeybees of chronic exposure to very low levels of selected insecticides in their diet. Jour. Apicult. Res. 25 170-177. 

Snyder et al. [253] compared supercritical fluid chromatography with classical sonication procedures and Soxhlet extraction for the determination of selected insecticides in soils and sediments. In this procedure the sample was extracted with carbon dioxide modified with 3% methanol at 350atm and 50°C. An excess of 85% recovery of organochlorine and organophosphorus insecticides was achieved. These included Dichlorvos, Diazinon, (diethyl-2-isopropyl-6-methyl 4-pyrimidinyl phosphorothioate), Ronnel (i.e. Fenchlorphos-0,0 dimethyl-0-2,4,5-trichlorophenyl phosphorothioate), Parathion ethyl, Methiadathion, Tetrachlorovinphos (trans-2-chloro-l-(2,4,5 trichlorophenyl) vinylchlorophenyl-O-methyl phenyl phosphoroamidothioate), Endrin, Endrin aldehyde, pp DDT, Mirex and decachlorobiphenyl. 

Snyder et al. [23] compared supercritical fluid extraction with classical sonication and Soxhlet extraction from the analysis of selected insecticides in soils. 

Kodavanti PRS, Mehrota BD, Chetty SC, et al. 1988. Effect of selected insecticides on rat brain synaptosomal adenylate cyclase and phospodiesterase. J Toxicol Environ Health 25 207-215. 

Kodavanti PR S, Mehrotra BD, Cherry SC, et al. 1989c. Inhibition of calmodulin-activated adenylate cyclase in rat brain by selected insecticides. Neurotoxicology 10(2) 219-228. 

The same sort of balance must be maintained in the use of insecticides. DDT furnished an excellent example in Florida. It killed the young scale insects, but it also killed so many of the parasites and predators that terrific scale populations resulted, which were difficult to bring under control. Obviously, selective insecticides are needed, which kill only injurious insects. Lacking this perfect answer, however, insecticides are used which are relatively less damaging to the friendly insects as compared to their effect on deleterious insects. 

Amidases can be found in all kinds of organisms, including insects and plants [24], The distinct activities of these enzymes in different organisms can be exploited for the development of selective insecticides and herbicides that exhibit minimal toxicity for mammals. Thus, the low toxicity in mammals of the malathion derivative dimethoate (4.44) can be attributed to a specific metabolic route that transforms this compound into the nontoxic acid (4.45) [25-27]. However, there are cases in which toxicity is not species-selective. Indeed, in the preparation of these organophosphates, some contaminants that are inhibitors of mammalian carboxylesterase/am-idase may be present [28]. Sometimes the compound itself, and not simply one of its impurities, is toxic. For example, an insecticide such as phos-phamidon (4.46) cannot be detoxified by deamination since it is an amidase inhibitor [24],... 

The identification of new natural product structural classes active against insects will attack the pest-resistance problem and provide environmentally safer insecticides. The marine environment, with its chemical diversity, clearly holds an enormous potential to provide leads for the development of insecticidal agents. Many marine-derived structural classes have not yet been examined for their insecticidal activity. Further SAR studies of these marine natural products could lead to new and more selective insecticidal agents. 

Malathion has two carboxyester linkages, which are hydrolyzable by carboxylase enzymes to relatively nontoxic products, as shown in reaction 18.7.1. The enzymes that accomplish this reaction are possessed by mammals, but not by insects, so that mammals can detoxify malathion, whereas insects cannot. The result is that malathion has selective insecticidal activity. For example, although malathion is a very effective insecticide, its LD50 for adult male rats is about 100 times that of parathion, reflecting the much lower mammalian toxicity of malathion than those of some of the more toxic organophosphate insecticides, such as parathion. 

J.J. van Daalen, J. Meltzer, R. Mulder, K. Wellinga, Selective insecticide with a novel mode of action, Naturwissenschaften 59 (7) (1972) 312-313. 

It also is used for the control of internal parasites affecting domestic animals and livestock. Trichlorfon is available in dust, emulsifiable concentrate, granular, fly bait, and soluble powder formulations with the percentage active of ingredient ranging from 40% (soluble powder) to 98% (technical). Trichlorfon is a selective insecticide, meaning that it kills selected insects.28... 

The similarities in the influences of the different parameters in the two subseries suggested that the optimum compound for development should still contain the p-Cl-aniline moiety, so that diflubenzuron was ultimately selected as the final benzoylphenyl urea derivative to be developed as a new selective insecticide. 

Crystal Blooms. A preliminary examination of crystal blooms of selected insecticides on various species of conifer bark (32) soon demonstrated a wide variation and high unpredictability in crystallization patterns. Because of this variation and the difficulty of bioassay with irregular bark surfaces, fiberboard panels were chosen as the test surface. Fiberboard is not a substitute for bark and does not simulate bark except in a general way and so was used mainly for convenience in studying the potential toxicity of crystal blooms and the broad aspects of crystallization patterns. 

Next, pirimicarb, a selective insecticide that kills sap-sucking aphid pests but does not affect the useful predators such as ladybirds (ladybugs) that eat them. It contains a pyrimidine ring—a benzene ring with two nitrogen atoms. 

These developments indicate potential new modes of selective insecticidal action. Through a better understanding of the physiological processes, the basis of screening can be broadened and the increased knowledge of the diverse biochemical pathways suggests new approaches to the design of pesticides. 

Figure 5. Electrical resistivity measurements for selected insecticide formulations as a function of aqueous dilution (1000 Hz,20 C).

Rotenone is present in the roots of Derris spp., from Malaya and the East Indies, and Lon-chocarpus spp. Rotenone is a selective insecticide with some acaricidal properties for control of garden insects, and lice and ticks on animals. Rotenone is very toxic to fish, so it can be used to control undesirable fish species (to eradicate them from lakes, streams, and reservoirs). Its oral LD50 in rats is 132 mg/kg. 

With the preceding discussion as background, it would be easy to understand how the selective toxicity of insecticide occurs, showing that some species are more susceptible to the toxicants than others. In fact, we would not be able to develop selective insecticides if it were not for the species differences that have evolved. Metabolism is not the only mechanism for selectivity, however. Ecological selectivity, which involves particular behavioral characteristics of the target pest, is also important, e.g., the use of insecticide bait against some species, or systemic insecticides against others. 

Insecticide selectivity is an important consideration in integrated pest management (IPM) programs. If we can use selective insecticides that are effective against pest species but relatively safe for beneficial insects, we would preserve these natural enemies and hence reduce insecticide usage. Yu (1988) determined 13 insecticides representing three... 

Table 9.12 Relative selectivity of selected insecticides toward beneficial insects...

More basic research concerning the biochemistry of beneficial insects is urgently needed to provide a biochemical basis for designing selective insecticides for use in IPM programs. 

The use of selected insecticide mixtures should retard resistance development because it should be more difficult for an insect to develop several adaptations simultaneously. The concept of joint use of insecticides assumes that the mechanisms of resistance to each member chemical exist in such low frequencies that they do not occur together in any single individual in the population. Thus, insects that may survive one of the chemicals are killed by the other. This approach delays resistance in laboratory experiments and has been, at least temporarily, successful in a few field cases, particularly with certain organo-phosphate combinations (Hopkins and Moore, 1980). The use of insecticide mixtures is not without problems. Resistance to both compounds used in mixtures has sometimes developed rapidly. Cross-resistance also occurs among some of the pesticides. 

It has been shown for avermectin B that the hydroxyl group at C-5 and a disaccharide moiety are essential for antiparasitic, mitocidal and selective insecticidal activities. Chemical modification of the macrolide-type avermectins resulted in ivermectin which was commercialized to be used in agriculture and animal health care as well as in human medicine. 

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