Insecticides general discussion

Health and environmental impacts of nerve agents and related compounds (i.e., organophosphate insecticides) have been reviewed by O Brien (1960), Matsumura (1976), Dacre (1984), Carnes and Watson (1989), Watson et al. (1989), and Munro et al. (1994). A brief general discussion of the toxicology of nerve agents and related organophosphate pesticides is given below. 

The OP insecticides, as discussed elsewhere in this book, have as a primary mechanism of acute toxicity the inhibition of the critical and widespread nervous system enzyme AChE. However, the anticholinesterase (anti-ChE) potencies do not correspond with the acute toxicity levels (Chambers et a ., 1990), indicating that metabolism is an important factor in determining the overall toxicity level. The OP insecticides evolved from the chemical technologies of World War II, which were used to develop the anti-ChE nerve agents. The OP inseeticidc.s bear some chemical resemblance to these nerve agents but are generally less toxic, often require metabolic activation to display anti-ChE activity and therefore are slower to act, and usually have more complex chemical substituents. 

Herbicides, used to kill or damage a plant, are the most rapidly growing segment of pesticides. Prior to the 1930s, herbicides were non-specific and often very toxic to humans as well as other animals. In the 1930s, in parallel with the development of new insecticides, researchers discovered several chemicals that selectively killed plants. These chemicals are now widely used to increase food production and have been used in warfare. Herbicides come in a variety of chemical structures and mechanisms of action, so they will be discussed in only general terms. Interested readers are referred to the many web sites and extensive literature on herbicides (see below and the presentation). 

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. 

Upon release the formulated control agent is partitioned between the air, the forest vegetation and the forest floor. It is the post application loss of insecticides from conifers which is the subject of the following discussion. Volatilization has become increasingly recognized as a significant factor which limits the efficiency of pesticides and provides a major pathway to general environmental contamination. 

In order to evaluate the potential hazards chemical insecticides pose to forest environments, it is essential that adequate and reliable research data be generated on their environmental chemistry (distribution, persistence, movement, metabolic degradation, toxicity, fate, etc.). This paper gives a brief account of some laboratory and field research activities carried out at the Forest Pest Management Institute, Canadian Forestry Service to meet this requirement. Using two chemical insecticides which are extensively used now in forest insect control programs in Canada Viz aminocarb [Trade name, Matacil 4-dimethylamino-m-tolyl N-methylcarbamate] and fenitrothion [0,0-dimethyl 0-(3-methyl-4-nitrophenyl) phosphorothioate], studies conducted at the Institute to elucidate the environmental behavior and fate of forestry insecticides in general will be discussed. 

OPs are chemicals used in agriculture as acaricides, herbicides, and insecticides (Appendix 5-A-l). Because of their toxicity, several of these chemicals are being phased out from use parathion (ethyl) is an example. Many have been now classified by the United States Environmental Protection Agency (USEPA) as a restricted use pesticide (RUP) or a general use pesticide (GUP). Pesticide chemistry has taken a turn for the synthesis, manufacture, and use of still safer compounds. A list of OPs considered for banning has been identified by the USEPA (Table 5-3). The following pages will briefly discuss the uses and toxicity of different OPs. 

Data selection criteria have been discussed (Section 4.3). Maltby et al. (2005), in a discussion of the use of tropical or temperate species and lentic or lotic species, deemed it appropriate to include all in the 1 SSD assessment. However, if a substance has a specific mode of action (like a pesticide), it is generally agreed that the SSD should only include taxa from the most sensitive group (e.g., just arthropods for an insecticide). It is not appropriate to include unrelated insensitive taxa in such a distribution (e.g., algae in an SSD for an insecticide) since it will result in a bi- or trimodal distribution and thus a poor fit. 

There is a vast chemistry of organophosphorus compounds, and even for arsenic, antimony, and bismuth, the literature is voluminous. Consequently only a few topics can be discussed here. It must also be noted that we discuss only the compounds that have P—C bonds. Many compounds sometimes referred to as organophosphorus compounds that are widely used as insecticides, nerve poisons, and so on, as a result of their anticholinesterase activity, do not, in general, contain P—C bonds. They are usually organic esters of phosphates or thiophosphates examples are the well-known malathion and parathion, (EtO)2Pv(S)(0C6H4NO2). Compounds with P—C bonds are almost entirely synthetic, though a few rare examples occur in Nature. 

Figure 2) (8). These compounds are generally much less toxic to mammals and less phytotoxic than the related dicyanocarbonylphenylhydra-zones investigated by DuPont workers several years ago (9). Some of our hydrazones (Figure 2) show interesting insecticidal and acaricidal properties (not discussed in detail here). Many are potent uncouplers of oxidative phosphorylation in rat liver mitochondria. 

In keeping with the concept of the RIP discussed under Relative Inhibitory Potency , above, inhibition of lymphocyte and/or platelet NTE and possibly erythrocyte LysoPC hydrolase should be used in conjunction with inhibition of erythrocyte AChE and plasma butyryl-cholinesterase (BChE) to assess the likelihood that an exposure to OP compounds would produce cholinergic and/ or delayed neuropathic effects. Erythrocyte AChE inhibition has long been used as a biomarker of exposure to conventional nerve agents or OP insecticides (Lotti, 1995 Wilson and Henderson, 1992). BChE can be sensitive to both conventional and DN agents, and its inhibition could thus serve as a general biomarker for OP agents (Kropp and Richardson, 2007 Van der Schans et al, 2008). 

Carbamates generally act quickly. They are strongly toxic to a wide range of insect pests, but have a weak effect on the red spider mite. Some of them exhibit systemic characteristics. The duration of their action varies considerably. In a similar manner to the phosporic acid esters discussed later, they exert their action by paralysing the cholinesterase enzyme. During this process, the carbamate part of the molecule is attached to the esteratic site, and the aromatic part to the anionic site of the cholinesterase enzyme. As the distance between the esteratic and anionic sites is SO nm in the cholinesterase molecule, carbamate insecticides will be most efficient if the distance between the two groups to be bound to the two sites of the enzyme is also 50 nm. (Metcalf and Fukuto, 1965 1967 Fukuto et ai, 1967). 

While the structures of the organophosphorus nematocides discussed so far are largely similar to the general structural scheme of the derivatives known as insecticides, 0-ethyl-S,S-dipropyl phosphorodithioate (prophos, 13) takes a special place within the group because of its S,S-dithioate structure. 

The general synthesis of organic phosphorus compounds is discussed in detail in the section dealing with insecticidal compounds. 

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