Green rice leafhopper

This compound was essentially non-toxic to house flies by topical application but showed moderate activity against the green rice leafhopper and brown planthopper as a residual spray. These results raise the possibility of applying derivatization reactions for the synthesis of polymeric insecticidal materials which may act as slow release agents. 

Yamamoto, I., Takahashi, Y., and Kyomura, N., Suppression of altered acetylcholinesterase of the green rice leafhopper by N-propyl and N-methyl carbamate combinations, in Pest resistance to pesticides, Georghiou, G.P. and Saito, T., Eds., New York Plenum Press, 1983,579. 

Figure 1. Comparative inhibition (pI50) of acetylcholinesterase from susceptible (S) and two resistant (RN-N and RN-4) strains of green rice leafhopper by m-tolyl N-alkylcarbamates in which the jj-alkyl group varies from methyl to n,-butyl. Reproduced with permission from Ref. 37. Copyright 1983 Plenum Press.

This enzyme, purified from aphids (24). was very slow in hydrolyzing paraoxon compared to mammalian arylester hydrolases (Table III) however, it was twice as fast in recovery from paraoxon inhibition compared to a porcine carboxylester hydrolase (35) and >300 times faster than monomeric carboxylester hydrolase of rabbit liver (26). No qualitative differences were found in the enzyme, E4, isolated from resistant and susceptible aphids E4 was one of seven electrophoretic forms of hydrolases observed (34). Recovery indicates that resistance is due to both reaction with the insecticide and a very slow turnover, or catalysis. A similar mechanism of was observed with paraoxon in resistant green rice leafhoppers (37). 

Acetylcholinesterase. Altered acetylcholinesterase less sensitive to organophosphorus and carbamate insecticides has been observed in a wide variety of insects and mites (51). Acetylcholinesterase inhibiting insecticides phosphorylate or carbamylate the serine residue in the active site of the enzyme preventing vital catalysis of acetylcholine. Resistance due to reduced sensitivity to inhibition of this target enzyme has been found in house fly, mosquitoes, green rice leafhopper, and both phytophagous and predacious species of mites. 

In resistant green rice leafhopper, an acetylcholinesterase with reduced affinity for N-methyl carbamate insecticides had increased sensitivity to inhibition by longer N-alkyl groups however, the inverse relationship was observed against the susceptible enzyme (21). The optimal substitution appeared to be N-(n-propyl) and the use of this chemistry in a resistance breaking strategy was discussed. Unfortunately, N-(n-propyl)propoxur was inhibitory of neither susceptible nor resistant acetylcholinesterase of predatory mites, Amblyseius potentillae so that this type of vulnerability is not common to all resistant acetylcholinesterases (22)-... 

In a related study, ortho-substituted N-methyl arylcarbamates were assayed for binding to AChE derived from suseptlble (S) and resistant (R) green rice leafhoppers (135). The same series of compounds were explored on each enzyme using identical assay methods. 

Inhibition of Susceptible and Resistant Green Rice Leafhopper Acetylcholinesterase by iV-Methylcarbamate and Oxadiazolone Insecticides... 

Inhibition of AChE preparations from S- and R-strains of green rice leafhopper by aryl N-methylcarbamates (n 20) and aryl oxadiazolones (n = 81) were evaluated with pI50 as the end-point. Regression against binding and reactivity descriptors was used to explore differences in mechanistic behavior. 

One of the major pest Insects in rice fields is the green rice leafhopper (Nephotettix cincticeps). Recently acquired resistance to carbamate insecticides has greatly complicated control of this insect. Resistance can develop by a number of different mechanisms such as cuticle thickening to Impede transport, enhanced metabolic degradation or molecular changes within the target enzyme (AChE) (1 ). In the case of carbamate insecticides, resistance occurs mainly at the target enzyme which expresses reduced sensitivity to the inhibitors (2). 

Recently, another important class of chemicals based on the oxadiazolone ring have shown activity similar to the carbamates, but with surprising activity on carbamate-resistant green rice leafhopper (Structure, Table 2) (11-13). To Investigate the resistance mechanism in green rice leafhopper, we decided to explore the structure-activity relations of both insecticide classes against AChE preparations from susceptible and resistant insects. The difference in the substituent effects was expected to provide critical information at the molecular level of the receptor complex. 

AChE Preparation and Assay. Whole body enzyme preparations of susceptible(S) and resistant(R) strains of green rice leafhopper were prepared according to a standard method used for fly head AChE (16). The crude enzyme preparation was inhibited by serial dilutions of carbamate and oxadiazolone insecticides at 25"C for 15 minutes. 

OHTA ET AL. Inhibition of Green Rice Leafhopper Acetylcholinesterase... 

Whereas S. B. Soloway had originally assumed that the for insecticidal activity critical nitromethylene unit would have to be attached to a heterocycle, scientists at Takeda Chemicals were able to show that acyclic compoimds possessed good insecticidal properties as well. They developed a product to control two important pests in rice, both are cicada species the brown planthopper (Nilaparvata lugens) and the green rice leafhopper Nephotettix cincticeps) (Fig. 8.45). 

The brown planthopper (Nilaparvata lugens) (left) and green rice leafhopper (Nephotettix cincticeps) (right). 

G.R.L. (Green Rice Leafhopper), T.C. (Tobacco Cutworm), D.M. (Diamondback moth), G.C. (German Cockroach)... 

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