Insecticides, carbaryl

Naphthol is mainly used in the manufacture of the insecticide carbaryl (59), l-naphthyl A/-methyicarbamate/ iJ-2j5 - (Sevin) (22), which is produced by the reaction of 1-naphthol with methyl isocyanate. Methyl isocyanate is usually prepared by treating methylamine with phosgene. Methyl isocyanate is a very toxic Hquid, boiling at 38°C, and should not be stored for long periods of time (Bhopal accident, India). India has developed a process for the preparation of aryl esters of A/-alkyl carbamic acids. Thus l-naphthyl methylcarbamate is prepared by refluxing 1-naphthol with ethyl methylcarbamate and POCl in toluene (60). In 1992, carbaryl production totaled > 11.4 x 10 t(35). Rhc ne-Poulenc, at its Institute, W. Va., facihty is the only carbaryl producer in United States. 

The insecticide carbaryl can be produced by several routes, some of which do not use methyl isocyanate, or which generate only small quantities of this toxic material as an in-process intermediate (Kletz, 1991b). One company has developed a proprietary process for manufacture of carbamate insecticides which generates methyl isocyanate as an in-situ intermediate. Total methyl isocyanate inventory in the process is no more than 10 kilograms (Kharbanda and Stallworthy, 1988 Manzer, 1994). 

The widely used insecticide carbaryl (FD50=270 mg/kg) transforms in oxidizing processes into 5-oxynaphthyl-N-methylcarbamate, a substance that is as toxic as carbaryl itself (FD50=297 mg/kg) [30, 33]. One of the metabolites of the fungicide benomyl, the methyl ester carbamino acid (BMK, carbendazim), is also toxic to fungi [33]. 

Studies were initiated at Iowa State University in 1977 to determine if pesticides would be contained and degraded when deposited in water/soil systems. Although the addition of known amounts of the selected pesticides was controlled, the physical environment was not temperature, humidity, wind speed, etc. were normal for the climate of Central Iowa. Four herbicides and two insecticides were chosen on the basis of three factors. Firstly, they represented six different families of pesticides. The four herbicides, alachlor, atrazine, trifluralin, and 2,4-D ester, represent the acetanilides, triazines, dinitroanilines, and phenoxy acid herbicides, respectively. The two insecticides, carbaryl and para-thion, represent the carbamate and organophosphorus insecticides, respectively. Secondly, the pesticides were chosen on the basis of current and projected use in Iowa Q) and the Midwest. Thirdly, the chosen pesticides were ones for which analytical methodology was available. 

Dispositional interactions are those in which one chemical affects the disposition of the other, usually metabolism. Thus, one chemical may increase or inhibit the metabolism of another to change its toxicity. For example, 2,3-methylenedioxynaphthalene inhibits cytochrome P-450 and so markedly increases the toxicity of the insecticide carbaryl to flies (potentiation) (see chap. 5). Another example, which results in synergy, is the increased toxicity of the organophosphorus insecticide malathion (see chap. 5) when in combination with another organophosphorus insecticide, EPN. EPN blocks the detoxication of malathion. Many chemicals are either enzyme inhibitors or inducers and so can increase or decrease the toxicity of other chemicals either by synergism or potentiation (see chap. 5). 

The insecticide carbaryl is hydrolyzed by liver enzymes to 1 -naphthol (Fig. 4.47). This compound also undergoes extensive metabolism by other routes. 

P 15.4 Photolysis or Hydrolysis Which Process Is More Importantfor the Elimination of the Insecticide Carbaryl from a Shallow Water Body ... 

DP Jonhson, HA Stansbury. Adaptation of sevin insecticide (carbaryl) residue method to various crops. J Agric Food Chem 13 235-327, 1965. 

The most common adsorption systems consist of silica gel or alumina adsorbents in association with an organic solvent system. The adsorbent can exert a considerable influence on the separation of compounds. Alumina and silica gel, for example, have significantly different properties and can result in quite different separations. Activation of the adsorbent also influences sample retention. The presence of water on the adsorbent decreases the adsorbent activity due to blockage of active sites. If large quantities of water are present, a partition system may be set up which may extensively change the retention times due to the different chromatographic principle involved. Table 2.1 compares results obtained for the separation of the insecticide carbaryl (Sevin) and its hydrolysis product 1-naphthol on alumina and silica gel. Comparisons between activation and deactivation are made. The results show that separation of the two components is reversed with the two adsorbents examined. In most cases, activation of the plates caused the/ f values to increase relative... 

The oxidation of naphthalene was one of the earliest examples of an epoxide as an intermediate in aromatic hydroxylation. As shown in Figure 7.3, the epoxide can rearrange nonenzymatically to yield predominantly 1-naphthol, or interact with the enzyme epoxide hydrolase to yield the dihydrodiol, or interact with glutathione S-transferase to yield the glutathione conjugate, which is ultimately metabolized to a mercapturic acid. These reactions are also of importance in the metabolism of other xenobiotics that contain an aromatic nucleus, such as the insecticide carbaryl and the carcinogen benzo(a)pyrene. 

The choice of an insecticide is somewhat narrower because of the criteria that must be met in forest use. Nonetheless, a number of the organophosphate and carbamate insecticides have been found to meet the necessary standards. For example, the insecticide carbaryl has been employed for control of gypsy moth and some of the related lepidopterous insects of the forest. 

The amounts volatilized in 10 days were extremely low in most cases. Dicamba was the only herbicide exhibiting significant volatilization and that only in the presence of high water evaporation. Apparently the insecticide carbaryl, has the potential to volatilize significantly from the soil with or without water evaporation, particularly when it is present near the soil surface. The reference insecticide, lindane, will appreciably volatilize when it is present near the soil surface, but its volatility decreases markedly when present within the entire 0 - 10 cm depth. Volatilization of the other insecticides will be essentially insignificant due either to their rapid degradation rate or low Kh-... 

In Section 17.3, rodenticidal thioureas, insecticidal thiocyanates, and fungicidal dithiocarbam-ates were discussed. Sulfur is a common constituent of other classes of insecticides. These prominently include the organophosphate insecticides discussed in Chapter 18. Mobam (Figure 17.10) is a contact insecticide of the carbamate type, closely related in structure and function to the well-known carbamate insecticide carbaryl (see Figure 15.8). Mobam has been found to have a relatively high toxicity to laboratory mammals and is considerably more toxic than carbaryl. 

Insecticidal carbaryl undergoes the following metabolic reaction ... 

The quantity and quality of food in the diet on which insects are reared may affect their size and survival capacity. Variegated cutworm larvae fed peppermint leaves were more tolerant of the insecticides carbaryl, acephate, methomyl, and malathion than larvae fed snap bean leaves. Increased tolerance for carbaryl and methomyl was also observed in larvae of the alfalfa looper and cabbage looper when they were fed peppermint plants instead of their favored host plants alfalfa and broccoli. Furthermore, fall armyworm larvae fed corn leaves became less susceptible to the insecticides methomyl, acephate, methamido-phos, diazinon, trichlorfon, monocrotophos, permethrin, and cypermethrin than those fed soybean leaves. Although nutrition may play some role in these cases, it was found that enhanced insecticide tolerance caused by these host plants was mainly due to plant allelochemicals, which induced detoxification enzymes in the insects (Yu, 1986). 

Hydroxylation Hydroxylation can occur with an aliphatic or aromatic carbon atom. DDT and the carbamate insecticide carbaryl are known to be hydroxylated by cytochrome P450 monooxygenases as shown in Figures 8.3 and 8.4. Microsomal hydroxylation usually results in detoxification. 

In another dermal penetration study, it was shown that the absorption of the insecticide carbaryl was enhanced by the solvent acetone and the surfactant sodium lauryl sulfate. The synergist PO significantly increased the absorption of carbaryl when added to the acetone-carbaryl rnixJ31 This study points out both the enhanced absorption of pesticide by individual inert additives as well as the further enhanced mixture effect. 

The study dealt with the stability and recovery of insecticides from milk (1 ) and one of Che compounds was the carbamate insecticide carbaryl. These studies were being conducted in 1959 just after Che introduction of this material which was touted insec-ticidally as the new DDT without the persistence problems. 

Figure 1. Hydrolytic pathway of metabolism of the insecticide carbaryl Once considered the only route of metabolism, radio-tracer studies later showed that oxidation, hydrolysis and conjugation reactions resulted in over a dozen metabolites being formed by some organisms.

There are a lot of examples of fatal accidental massive poisonings, such as the Bhopal catastrophe caused by a leak of 4,000 tons of methylisocyanate, used in manufacturing of the insecticide carbaryl, from the Union Carbide facility in... 

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