Persistent insecticides

M.p. I08-5 C. Ordinary DDT contains about 15% of the 2,4 -isomer, and is prepared from chloral, chlorobenzene and sulphuric acid. It is non-phytotoxic to most plants. It is a powerful and persistent insecticide, used most effectively to control mosquitoes in countries where malaria is a problem. It is stored in the bodies of animals and birds. 

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. 

Disturbance of nerve action Persistent insecticides potential... 

It is well established that important photochemical reactions are mediated by humic material in the aquatic environment (Zepp et al. 1981a,b), and that these are particularly signihcant for hydrophobic contaminants. Partial reductive dechlorination of the persistent insecticide mirex associated with... 

Residue analytical chemistry has extended its scope in recent decades from the simple analysis of chlorinated, lipophilic, nonpolar, persistent insecticides - analyzed in the first Si02 fraction after the all-destroying sulfuric acid cleanup by a gas chro-matography/electron capture detection (GC/ECD) method that was sometimes too sensitive to provide linearity beyond the required final concentration - to the monitoring of polar, even ionic, hydrophilic pesticides with structures giving the chemist no useful feature other than the molecule itself, hopefully to be ionized and fragmented for MS or MS" detection. 

Phosphonothioate Esters of Phenols, Phosphonates with a single P—C bond are highly toxic and persistent insecticides but have not been used extensively because some compounds produce delayed neuropathy leading to irreversible paralysis in higher animals, including humans. Such compounds specifically inhibit an enzyme, neurotoxic esterase, that is responsible for the growth and maintenance of long nerve axons (31,32). 

Pesticides are also a major source of concern as water and soil pollutants. Because of their stability and persistence, the most hazardous pesticides are the organochlorine compounds such as DDT, aldrin, dieldrin, and chlordane. Persistent pesticides can accumulate in food chains for example, shrimp and fish can concentrate some pesticides as much as 1000- to 10,000-fold. This bioaccumulation has been well documented with the pesticide DDT, which is now banned in many parts of the world. In contrast to the persistent insecticides, the organophosphorus (OP) pesticides, such as malathion, and the carbamates, such as carbaryl, are short-lived and generally persist for only a few weeks to a few months. Thus these compounds do not usually present as serious a problem as the earlier insecticides. Herbicides, because of the large quantity used, are also of concern as potential toxic pollutants. Pesticides are discussed in more detail in Chapter 5. 

It was often observed that, even after heavy usage of persistent insecticides like HCHs and DDTs in India in the previous three decades, the residues in coastal sediments remained constant and lower than the levels that can not be normally expected after such heavy use (Sarkar and Sen Gupta, 1987 Pandit et al., 2001, 2002). Logically, based on all the... 

From the literature cited it can be concluded that about 90% of the recoverable DDT residue will reside in the top 3 inches of the soil or at cultivation or plow depth. Certain persistent insecticides can be removed from the soil through volatilization. The extent of volatilization is determined in part by the vapor pressure of the insecticide, by displacement from the soil particle by other substances, by the presence and type of cover crop, and by the "turning over of the soil. Insecticides from treated areas can enter the surface water sorbed onto eroded soil particles and/or desorbed from the soil by water. 

Experimental reproductive effects. Mutation data reported. A persistent insecticide that is toxic to non-target species. It can bioaccumulate. See also CHLORINATED HYDROCARBONS, ALIPHATIC. 

The existence of multiple AChE isoenzymes has several consequences. First, it increases the chances of an insect having one that is, or by a minor genetic change can be rendered, insensitive. The molecular redundancy combined with a selection pressure in the form of persistent insecticide applications would facilitate target site resistance development. Second, it could be a factor in the frequent lack of target site cross resistance between OPs and carbamates, and even between different OPs. Third, it would facilitate the disappearance of the insensitive form(s) in the absence of a selection pressure. This would especially easily explain observed instability of resistance if the form(s) with decreased affinity for the inhibitors also have decreased affinity for the neurotransmitter. Insensitivity to the inhibitor may be accompanied by a reduced rate of neurotransmitter hydrolysis (56. 28), but this is not always the case. It seems that the reduced rate of neurotransmitter hydrolysis does not impair survival, at least in laboratory cultures of insects. It is unclear what impact such reduced rates have in field populations. 

Woodwell G.M., Wurster C.F., Isaacson P.A. (1967) DDT residues in an east coast estuary a case of biological concentration of a persistent insecticide. Science 156, 821—4. 

West, T.F. and Campbell, G.A. 1950. DDT and Newer Persistent Insecticides. Chapman Hall Ltd., London. 632 pp. 

Table 8.3 Vapor Pressure and Water Solubility of Some Persistent Insecticides...

Resistance to insecticides was recorded as a problem just a few years after the introduction of the newer persistent insecticides, whereas resistance to herbicides and fungicides developed much later. Figure 9.1 shows an approximate graph of the development of resistance. 

In general, insecticides can be grouped according to whether they are persistent (i.e., long lasting) or nonpersistent (i.e., they break down quickly in the environment into relatively harmless compounds). DDT (dichlorodiphenyltrichloroethane) is probably the best-known persistent insecticide. Even though it is actually less potent than many other insecticides on the market, DDT lasts for years in the environment and becomes more concentrated in the tissues of animals as it works its way up the food chain. 

Many natural products and biologically active compounds contain cyclopropane rings we shall feature just a few. First, a most important natural insecticide, a pyrethrin from the East African pyrethrum daisy, and its synthetic analogue decamethrin, one of the most important insecticides in agriculture. Very low doses of this highly active and non-persistent insecticide are needed. 

Eventually it began to become clear that the prodigious use of DDT had harmful side effects. Aryl halides are usually highly stable compounds that are only slowly destroyed by natural processes. As a result they remain in the environment for years they are what we now call persistent insecticides or hard insecticides. The U.S. Environmental Protection Agency banned the use of DDT beginning in 1973. 

A disposable amperometric immunosensor was also developed for sensitive detection of chlorpyrifos-methyl, a persistent insecticide widely used in farming and horticulture. The device was fabricated by modiflcation of the screen-printed carbon electrodes with nanocomposites made by skillful doping of bovine serum albumin conjugated chlorpyrifos-methyl and platinum colloid into silica sol-gel. The immobilization of BSA-Ag conjugates on the nanocomposite retained their immunoactivities, which allowed the immobilized BSA-Ag to effectively capture unbound Ab-HRP in the detection solution. A linear response to chlorpyrifos-methyl concentration ranging from 0.4 to 20 ng/ml was reported and good results were obtained for the detection of the insecticide in treated soil and grape samples [53]. 

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