Neonicotinoid insecticides

Hasegawa, K., Arakawa, M. and Funatsu, K. (1999) 3D-QSAR study of insecticidal neonicotinoid compounds based on 3-way partial least squares model. Chemom. Intdl. Lab. Syst., 47, 33 0. 

Imidacloprid is a widely used neonicotinoid insecticide that kills pests by targeting their central nAChRs. Levamisol is used to kill nematodes by acting on nAChRs in the worm s muscles. 

Many pesticides are not as novel as they may seem. Some, such as the pyre-throid and neonicotinoid insecticides, are modeled on natural insecticides. Synthetic pyrethroids are related to the natural pyrethrins (see Chapter 12), whereas the neo-nicotinoids share structural features with nicotine. In both cases, the synthetic compounds have the same mode of action as the natural products they resemble. Also, the synthetic pyrethroids are subject to similar mechanisms of metabolic detoxication as natural pyrethrins (Chapter 12). More widely, many detoxication mechanisms are relatively nonspecific, operating against a wide range of compounds that... 

Nicotine Acts on nicotinic receptor Neonicotinoids An insecticide in its own... 

Nicotine is a component of Nicotiana tabacum, the tobacco plant. It is toxic to many insects because of its action upon the nicotinic receptor of acetyl choline. It has served as a model for a new range of insecticides, the neonicotinoids, which also act upon the nicotinic receptor (Salgado 1999). 

Speaking generally, many laboratory studies have shown behavioral effects in vertebrates or invertebrates or both exposed to organochlorine, carbamate, OP, pyre-throid, and neonicotinoid insecticides. However, the critical questions are (1) to what extent have these effects been demonstrated at normal levels of exposure in the field and (2), if such effects have occurred in the field, have there been knock-on effects at the population level These issues will be returned to in Section 16.7. 

Neonicotinoids are potent broad-spectrum insecticides that exhibit contact, stomach and systemic activity. Acetamiprid, imidacloprid, nitenpyram, thiamethoxam and thiacloprid are representatives of the neonicotinoid insecticides (Figure 1). The mechanism of action is similar to that of nicotine, acting on the central nervous system causing irreversible blocking of postsynaptic nicotinic acetylcholine receptors (nAChR). Neonicotinoid insecticides are often categorized as antagonists of the... 

The LOD is an important criterion of the efficiency of an analytical method. It defines the smallest value of the concentration of a compound in the analytical sample. Detectable amounts of neonicotinoid insecticides range from 0.5 to 1 ng by HPLC. The LOQ ranges from 0.005 to 0.01 mg kg for vegetables, fruits and crops. 

The amount of neonicotinoids insecticide residue (R, mg kg ) in the sample is calculated by the following equation ... 

Wi = amount of neonicotinoid insecticide for Vj read from calibration curve (ng)... 

The definition of water residue for the neonicotinoid insecticides except for niten-pyram and thiacloprid is the parent molecule. For nitenpytram and thiacloprid both the parent and its metabolites are determined. These metabolites are 2-[A-(6-chloro-3-pyridylmethyl)-A-ethyl]amino-2-methyliminoacetic acid (CPMA) and... 

Solvent extraction. Extraction of neonicotinoid insecticides from water is a simple process involving saturation with sodium chloride and extraction with diethyl ether, dichloromethane or ethyl acetate. This extraction procedure will allow the simultaneous extraction of all neonicotinoids. 

Acetamiprid is a neonicotinoid insecticide with outstanding systemic activities and a broad insecticidal spectrum. Acetamiprid controls diverse soil and foliar insect pests infesting cotton, sugar beet, vegetables, fruits and other major food crops by both contact and stomach action... 

Neon—helium continuous lasers, 17 372 Neonicotinoid insecticides, 14 346 Neopentanal trifluoroborane, 4 144t Neopentanoic acid, 5 60-65 physical properties, 5 60t production from butylenes, 4 427 Neopentyl glycol, 12 670-672... 

The procedure is very easy to reproduce and the stereoselective hydroxylation of IMI with S. maltophilia CGMCC 1.1788 may be applied to some other neonicotinoid insecticides, such as thiacloprid (Table 12.1). 

The tobacco compound nicotine has been used as an insecticide for over 200 years. It is especially effective against sucking insects, such as aphids, and has excellent contact activity. Related compounds are neonicotinoids (e.g., imidacloprid), which have similar insecticidal activity, but are less toxic to mammals. Nicotine and imidacloprid mimic the action of acetylcholine, which is the major excitatory neurotransmitter in an insect s central nervous system. The action of acetylcholine is stopped by the enzyme acetylcholinesterase, which rapidly breaks down acetylcholine. Nicotine and imidacloprid are also neuroexcitatory, but do so persistendy, since they are not affected by acetylcholinesterase. Overstimulation of the nervous system often leads to convulsions, paralysis, and death. 

Neonicotinoids (also known as chloronicotinyls) are a new class of insecticides that are analogs of nicotine. Unlike nicotine, these insecticides are relatively nontoxic to mammals. Currently, there are several important insecticides in this class. 

Nauen R., Acetylcholine receptors as sites for developing neonicotinoid insecticides, in Biochemical Sites of insecticide action and resistance, Ishaaya, I., Ed., Berlin Springer-Verlag, 2001, p. 77. 

Imidacloprid will be used to illustrate the fate of neonicotinoid insecticides (Figure 8.32). Imidacloprid is reduced (1) and then becomes the triazine derivative (2). It can be hydroxv-lated at the imidazole ring to become themonohydroxylated derivatives (3 and 4). Reduction of parent imidacloprid to the nitroso derivative (5) occurs in some animal species. Imidacloprid is also cleaved to become the nitroiminoimidazololidine (6). Further metabolism involves olefin formation (7), glycine conjugation (8), and others. 

In a recent study, resistance to the neonicotinoid insecticide, imidacloprid, in the brown plant hopper, Nilaparvata lugens, was found to be due to a point mutation at a conserved position in two nAChR subunits. As a result, it reduced the receptor binding of imidacloprid (Liu et al., 2005). 

Yu and Nguyen (1996) showed that selection of a strain of diamondback moth (Plu-tella xylostella) with permethrin for 21 generations resulted in over 600-fold resistance to permethrin in this strain. The resistant strain was also cross-resistant to all pyrethroids tested, including bifenthrin, fenvalerate, esfenvalerate, A.-cyhalothrin, fluvalinate, and tral-omethrin. However, it remained susceptible to organophosphate, carbamate, cyclodiene, neonicotinoid, avermectin, and microbial insecticides tested. Biochemical studies indicated that pyrethroid resistance observed in this strain was most likely due to decreased target site sensitivity. 

Karmakar, R., Singh, S.B., and Kulshrestha, G., Persistence and transformation of thiamethoxam, a neonicotinoid insecticide, in soil of different agroclimatic zones of India, Bull. Environ. Contam. Toxicol, 76,400, 2006. 

Imidacloprid is a neonicotinoid insecticide that is registered for many uses, including grub and termite control, crop protection, and to control fleas and ticks on companion animals. Its insecticidal activity is attributed to nicotinic activity on post-synaptic receptors. 

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