Biophysical aspects of nervous activity in relation to studies on the mode of action of insecticides

The spread to the nervous system of topically applied neurotoxic insecticides is discussed. During spread different insecticides may distribute themselves between fluid and solid phases within the insect's tissues in greatly differing ratios. These differences are reflected in their concentrations in the haemolymph, which is probably the medium mainly responsible for distributing insecticides within the insect, and also the main source from which insecticides enter the nervous system. Evidence for spread via more restricted pathways, such as tracheae or the nerves themselves, is conflicting. The mode of action of neurotoxic insecticides is briefly reviewed, including the role of secondarily released neuroactive stress substances, and the extent to which resistance is expressed at the site of action is also discussed. Finally, the nervous system is examined for new sites of insecticidal action which, if attacked, could give greater specificity.     

Toxicology of insecticides to mammals

Many insecticides target structures or functions in non-target species, including mammals. This is particularly true of those that target the insect nervous system, such as the organochlorines, anticholinesterases and GABA antagonists. Another group of insecticides target structures or functions not present in mammals, and this group of insecticides has considerable target species specificity, but there are often potential targets in mammals. Octopamine is closely related to adrenaline and amitraz (an octopamine receptor agonist) and acts in mammals at α2-adrenergic receptors. Although there are potential targets in mammals for juvenile hormone mimics and ecdysone receptor agonists, there is no evidence that the mammalian toxicity of either group is related to their insecticidal activity. Nor do chitin synthesis inhibitors have high mammalian toxicity. Copyright © 2012 Society of Chemical Industry     

Metabolism of imidacloprid and DDT by P450 CYP6G1 expressed in cell cultures of Nicotiana tabacum suggests detoxification of these insecticides in Cyp6g1-overexpressing strains of Drosophila melanogaster, leading to resistance

BACKGROUND: With the worldwide use of insecticides, an increasing number of pest insect species have evolved target-site or metabolism-based resistance towards some of these compounds. The resulting decreased efficacy of pesticides threatens human welfare by its impact on crop safety and further disease transmission. Environmental concentrations of some insecticides are so high that even natural populations of non-target, non-pest organisms such as the fruit fly Drosophila melanogaster Meig. have been selected for resistance. Cyp6g1-overexpressing strains of D. melanogaster are resistant to a wide range of chemically diverse insecticides, including DDT and imidacloprid. However, up to now there has been no evidence that the CYP6G1 enzyme metabolises any of these compounds. RESULTS: Here it is shown, by heterologous expression in cell suspension cultures of Nicotiana tabacum L. (tobacco), that CYP6G1 is capable of converting DDT (20 microg per cell culture assay) by dechlorination to DDD (18% of applied amount in 48 h), and imidacloprid (400 microg) mainly by hydroxylation to 4-hydroxyimidacloprid and 5-hydroxyimidacloprid (58 and 19% respectively in 48 h). CONCLUSION: Thus, the gap between the supposed resistance gene Cyp6g1 and the observed resistance phenomenon was closed by the evidence that CYP6G1 is capable of metabolising at least two insecticides.     

Polarography of demeton,disulfoton and phorate

The polarographic response obtained when demeton-S, disulfoton and phorate, in 2% tetraethyl ammonium hydroxide, were investigated using a cathode ray oscilloscope polarograph depended considerably on the interval between making up solutions and measurement. Direct evidence to confirm previous suggestions that this type of behaviour is due to hydrolysis of the original insecticides to polarographically active SH-containing compounds was obtained by measuring the disappearance of the original insecticides from the solutions by gas-liquid chromatography and by investigating the behaviour of the hydrolysis product for demeton-S and disulfoton synthesised by an independent route. Measurements of electrocapillary curves and other lines of evidence indicate that the electrode process involves adsorption/desorption at the mercury drop. The results emphasise that conditions must be carefully standardised if polarography is used for estimating these insecticides.     

Spiders (Araneae) in the pesticide world: an ecotoxicological review

Being one of the most abundant and species‐rich groups of natural enemies occurring in all agroecosystems, spiders are variably affected by pesticide applications. Here, a review is given of research on spider ecotoxicology. More than 40 species of spiders and almost 130 pesticides (acaricides, insecticides, fungicides and herbicides) have been tested so far in the field or under laboratory conditions. Field studies show that the degree of population reduction following pesticide application is a function of a number of factors inherent to pesticides, crops and spider species (guilds). These studies also revealed indirect effects via habitat and prey disruption. Among laboratory studies, a number of papers have investigated only the direct lethal effect. A meta‐analysis of these data reveals that spiders are mainly affected by acaricides and insecticides, particularly neurotoxic substances. Currently, ecotoxicological research on spiders is focused more on direct sublethal effects on a variety of behavioural traits (locomotion, predation, web‐building, reproduction, development) and physiology. Yet a standardised approach to the evaluation of sublethal effects is lacking. A few studies have provided some evidence for hormesis in spiders. Future research should be more concentrated on sublethal effects and the estimation of long‐term changes in spider populations as a result of pesticide treatment. Copyright © 2012 Society of Chemical Industry     

A beekeeper's perspective on the neonicotinoid ban

Bees and agrochemicals have a long history. For example, the first volume of IBRA's journal Bee World in 1919 contains mention of poisoning of bees by spraying an orchard with lead arsenate. Bees being insects, it is self‐evident that the use of insecticides to control crop pests poses a risk to them. Bee poisoning incidents became a very serious problem in the 1960s and 1970s with spraying of, in particular, oilseed rape with organophosphorus compounds. The introduction of carbamates and then especially synthetic pyrethroids reduced these problems. Data from the Wildlife Incident Investigation Scheme show that in recent years there have been very few poisoning incidents in the United Kingdom that can be attributed to agricultural insecticides. The introduction of neonicotinoid insecticides has, however, been very controversial. Almost as soon as they were introduced in the 1990s, French beekeepers blamed colony losses on imidacloprid used on sunflowers and maize, but restrictions on its use did not lead to a reduction in losses or to a reduction in beekeepers' concerns. Acute pesticide poisoning incidents by neonicotinoids in Germany and Italy in 2008 further sealed their reputation. Despite laboratory evidence showing their harm, field experience remains equivocal, and many commercial beekeepers continue to move their colonies to oilseed rape crops for honey production. The neonicotinoid moratorium has undoubtedly led to the increased use of older insecticides, and the effect of this on bee populations is unknown and unquantified. Many beekeepers are currently confused by the conflicting evidence. © 2016 Society of Chemical Industry     

Mechanism of action of sodium channel blocker insecticides (SCBIs) on insect sodium channels

Sodium channel blocker insecticides (SCBIs) are a relatively new class of insecticides, with a mechanism of action different from those of other classes of insecticides that target voltage-gated sodium channels. These compounds have no effect at hyperpolarized membrane potentials, but cause a voltage-dependent, nearly irreversible block as the membrane potential is depolarized. The mechanism of action of SCBIs is similar to that of local anesthetics (LAs), class I anticonvulsants and class I antiarrhythmics. In this article, we review the physiological actions of these compounds on the whole animal, the nervous system and sodium channels, and also present the results from recent studies that elucidate the receptor site of SCBIs.     

Efficacy of systemic insecticides on the gall wasp Quadrastichus erythrinae in wiliwili trees (Erythrina spp.)

BACKGROUND: Erythrina gall wasp (EGW), Quadrastichus erythrinae Kim, was first found on Oahu Island, Hawaii, in April 2005. Its rapid spread and infestation in wiliwili trees (Erythrina spp.) have brought an urgent need to suppress its population. Little is known about the control of EGW in wiliwili trees.RESULTS: Among the systemic insecticides abamectin, dinotefuran and imidacloprid, applied via trunk injections or soil drenches, injections of imidacloprid showed better control of EGW in wiliwili trees. All the imidacloprid injection treatments had varying levels of effectiveness against EGW, but not the soil drench. The levels of imidacloprid were higher in lower canopies than those in the other parts within a tree. Imidacloprid remained detectable 1 year after treatment. The trees injected with the products IMA-jet and Merit 200 SL via Arborjet had lower infestation severity ratings for the entire growth season and carried more imidacloprid than those with Imicide via Mauget or Pointer via Wedgle injections.CONCLUSION: The results indicate that, among the three insecticides tested, imidacloprid is most effective against EGW in the trees. Concentrations of imidacloprid in the leaves need to be 4 mg kg(-1) or higher for good EGW control.     

A Rapid Method for Screening Insecticides in the Laboratory

An efficient method for rapidly mass-screening insecticides for use against sap-feeding virus vectors is presented with a case study of 30 chemicals. The method permits large numbers of insecticides to be tested simultaneously and relatively inexpensively in a sequence of laboratory bioassays. The sequence is designed to find the most effective pesticide at the lowest concentration giving control without phytotoxicity. The system was derived to test candidate insecticides to control tomato yellow leaf curl virus vectored by the tobacco whitefly, Bemisia tabaci Gennad., the most serious pest of greenhouse and field tomatoes in the Middle East. Although the insecticides were all more efficacious in the laboratory than in the field, bioassay results were highly correlated with results from field trials, giving high confidence that the screening process selected only the most efficacious insecticides. Most of the insecticides accepted by the screening process have since been adopted by vegetable growers in Israel. The method is not intended to eliminate field efficacy trials, but to reduce the number of trials and treatments that need to be performed, thereby reducing costs. The method provides for the optimization of application rates which will contribute to the expected life of insecticides before resistance develops, and will also help to reduce environmental contamination. In addition, the method is suitable for estimating relative efficacy for pesticide benefits assessments, a required part of the (re-)registration process for pesticides in some countries. Although developed for screening insecticides against virus-transmitting sap-feeding insects, the method could be modified to assess the efficacy of insecticides in controlling other insect pests.     

杀虫剂分子靶标烟碱型乙酰胆碱受体研究进展

昆虫烟碱型乙酰胆碱受体(nicotinic acetylcholine receptors,nAChRs)广泛分布于昆虫中枢神经系统,是杀虫剂作用的主要靶标。目前昆虫中该受体的天然亚基组成尚不完全明确。果蝇的任意α亚基与脊椎动物的一个β亚基共表达是目前最好的异源表达模型,但仍然急需新的研究工具,研究表明一些与受体相关的蛋白质影响着表达。胞内磷酸化的调节作用为今后受体药理学特性的研究提供了新方向。受体亚基上一些关键氨基酸在新烟碱杀虫剂对受体的选择作用中起重要作用。在对吡虫啉抗性的褐飞虱种群中找到了与抗性相关的突变位点,这为新烟碱类杀虫剂靶标不敏感性研究提供了直接证据。对昆虫烟碱型乙酰胆碱受体的分子多样性、功能表达、胞内调节机制、受体与杀虫剂的选择作用及其抗性分子机理等的研究进展进行了综述。     

Is insecticide-induced release of insect neurohormones a secondary effect of hyperactivity of the central nervous system?

Organochlorine and organophosphorus insecticides, in the concentration range of 1–10 μM, induce the release of hyperlipemic (adipokinetic) hormone from the isolated corpora cardiaca of the locust Locusta migratoria. Treatment of locusts with these insecticides in vivo also provokes the release of hyperlipemic hormone. The insecticide-induced release of hormone in vivo was found to precede the onset of poisoning symptoms. The insecticides tested in this study modulate the electrical activity of the isolated corpora cardiaca at doses lower than those required to have similar effects on the central nervous system. The results indicate that the insecticide-induced release of hormone may be mediated by the action of insecticides directly on neurosecretory cells.     

The genetic basis of insecticide resistance in the house fly: Evidence that a single locus plays a major role in metabolic resistance to insecticides

Genetic evidence indicates that insecticide resistance in insects is controlled by relatively few genes. In the house fly, Musca domestica L., major resistance genes include one for decreased uptake of insecticides, three for changes at the target sites of insecticide action, and a single gene for metabolic resistance to multiple types of insecticides. The latter gene, which is located on chromosome II, interacts with minor genes located on other chromosomes. The product of the major gene for metabolic resistance appears to be a receptor protein which recognizes and binds xenobiotics, including insecticides and plant defense substances, and then induces synthesis of appropriate detoxifying enzymes.     

新烟碱类杀虫剂抗药性研究进展

新烟碱类杀虫剂是一类新开发的杀虫剂。研究表明，害虫野外种群对其敏感性差弄较大，现已有多种害虫对吡虫啉和啶虫脒产生了抗性。初步研究显示，马铃薯叶甲对吡虫啉抗性以不完全隐性的常染色体遗传；抗性似不稳定，交互抗性谱随虫种而变化，抗性形成可能与多功能氧化酶和酯酶有关。合理轮用和高剂量杀死策略是治理其抗性的有效措施。     

Topical, residual and ovicidal contact toxicity of three reduced-risk insecticides against the European corn borer, Ostrinia nubilalis (Lepidoptera: Crambidae), on potato

BACKGROUND: The goal of the research was to gather efficacy data required to introduce reduced-risk insecticides in sustainable control programs for European corn borer, Ostrinia nubilalis Hubner, on potato. RESULTS: Laboratory tests confirmed that sprays of indoxacarb and novaluron at recommended field rates are as effective as spinosad against neonate larvae of O. nubilalis. However, there is evidence that higher rates would enhance the inhibition of chitin synthesis by novaluron. The three insecticides showed ovilarvicidal activity when applied to O. nubilalis egg masses 2 days prior to black head stage. The ovicidal activity of spinosad and novaluron was almost twice that of indoxacarb. At the recommended field rates, the residues of the three insecticides displayed contact toxicity to O. nubilalis larvae. Spinosad residues 16 h old or less provided the highest immediate (24 h after exposure) contact mortality, followed by indoxacarb and then by novaluron. Also, residues of spinosad had faster contact efficacy than indoxacarb, which had faster efficacy than novaluron. CONCLUSION: Spinosad, indoxacarb and novaluron have ovicidal properties, which could enhance O. nubilalis management programs. However, the contact residual toxicity is limited in duration and would likely only play a minor role in O. nubilalis control.     

Pesticide Use and Mycotoxin Production in Fusarium and Aspergillus Phytopathogens

The major mycotoxigenic species of Fusarium and Aspergillus phytopathogens have been identified in this review. Since fungicides are widely used to control crop diseases caused by these fungi, it is pertinent to assess efficacy with respect to mycotoxin production. In both laboratory studies with pure cultures of phytopathogens and field trials with crop plants, the overall evidence concerning the effectiveness of fungicides is contradictory and in certain cases somewhat unexpected. In particular, at sub-lethal doses of a number of fungicides including carbendazim, tridemorph, difenoconazole and tebuconazole with triadimenol, mycotoxin production from Fusarium phytopathogens may increase. Furthermore, the efficacy of propiconazole and thiabendazole in the control of deoxynivalenol production from F. graminearum is not consistent. Evidence has been presented to suggest, for the first time, that fungicide-resistance in F. culmorum may be accompanied by a more persistent pattern of mycotoxin production. The limited evidence on the effects of fungicides on mycotoxin production in Aspergillus species is also conflicting. Under laboratory conditions, miconazole and fenpropimorph have been shown to increase aflatoxin production from A. parasiticus. Moreover, fenpropimorph increased production of the more toxic aflatoxin B1. Since fungal infection of plant products is often preceded by insect damage, there is interest in the effectiveness of insecticides to reduce infestation, infection and mycotoxin contamination. Additionally, insecticides may be effective in their own right, causing a direct effect on mycotoxin synthesis. The bulk of the evidence relates to effects on aflatoxin (AF) components B1, B2, G1 and G2. Under laboratory conditions, AFB1 production was most resistant to inhibition by insecticides, followed by AFG1, AFG2 and AFB2. This pattern of inhibition was particularly consistent for the organophosphorus insecticides. In one field study, Bux and carbaryl were considerably more effective than naled in reducing AFB1 contamination of maize kernels. It is concluded that if pesticide control is to be more effective in the future, additional criteria may be required in developing evaluation protocols for candidate compounds. In particular, the issue of fungicide-resistance in relation to mycotoxin production needs to be addressed in a concerted programme of research. Additionally, the potential of breeding and selecting cultivars resistant to disease caused by toxigenic fungi needs to be exploited in a parallel search for an environmentally acceptable solution to the question of mycotoxin contamination of plant products.     

The cuticle,growth and moulting in insects: The essential background to the action of acylurea insecticides

The functions, structure and biochemistry of the insect cuticle in relation to the moulting cycle are briefly reviewed as an introduction to the actions of insecticides that act on the cuticle, particularly acylureas. The symptoms of poisoning with diflubenzuron (DFB) and other acylureas are consistent with ultra-structural and biochemical evidence that these insecticides inhibit the formation of chitin microfibrils in newly synthesised cuticle. It is probable that DFB acts at a late stage in chitin biosynthesis, perhaps inhibiting chitin synthase (CS) itself. However, the results of studies using cell-free preparations of CS have not, on the whole, supported this hypothesis. A number of alternative suggestions as to the mode of action of DFB are reviewed. Among the most attractive of these is the possibility that DFB may inhibit the transmembrane transport of chitin synthesis precursors from their site of production within the epidermal cells to the site of the final poly condensation reaction, presumably at the apical membrane of the epidermal microvilli.     

Insect hormones and growth regulators

The endocrine system that controls development and reproduction in insects has many unique features that could provide targets for insecticides with novel modes of action. Such insecticides, which act by disrupting the growth and development, are often called insect growth regulators. Three components of the endocrine system are discussed in this context, and an example of the specific chemical disruption of one of them, which could form the basis for the development of such insecticides, is described.     

Toxicity of indoxacarb and spinosad to the multicolored Asian lady beetle, Harmonia axyridis (Coleoptera: Coccinellidae), via three routes of exposure

The use of selective insecticides may improve conservation of natural enemies and therefore contribute to the success of integrated pest management (IPM) programs. In this study, the toxicity of two commonly used selective insecticides, indoxacarb and spinosad, to the multicolored Asian lady beetle, Harmonia axyridis (Pallas), was evaluated. Third instars and adults of H. axyridis were exposed to indoxacarb at 50 and 100% of the field rate (FR), to spinosad at 100% FR and to water (untreated check) under laboratory conditions via three routes of exposure. Treatments were applied directly on insects (i.e., topical application), on Petri dishes (i.e., residues), or on soybean aphids, Aphis glycines Matsumara (i.e., treated prey). Mortality of exposed individuals in each life stage was recorded 2 and 7 days after treatment. Logistic regression indicated that indoxacarb at 100% FR, followed by indoxacarb at 50% FR, was more insecticidal than spinosad to third instars. Mortality was higher when H. axyridis were exposed to both insecticides via residues followed by treated prey. Indoxacarb at 100 or 50% FR was insecticidal to adults. Adults were tolerant to spinosad via all routes of exposure. The present results suggest that indoxacarb may decrease H. axyridis field populations by causing mortality to larvae and adults via all routes of exposure. Implications of the toxicity of indoxacarb to H. axyridis within an IPM context and possible reasons for the differences in susceptibility of H. axyridis for each route of exposure are discussed.     

新烟碱类杀虫剂毒理学研究进展

以吡虫啉和啶虫脒为代表的新烟碱类杀虫剂具有卓越的内肖活性及很高的残留活性。放射配基结合实验表明，新烟碱类杀虫剂对多种昆虫乙酰胆碱受体具有高亲和力，乙酰胆碱是其竞争性结合抑制剂，电生理学研究表明，该类杀虫剂与乙酰胆碱类似，可诱导瞬时内向电流，并可作用于药理学性质不同的昆虫烟碱型乙酰胆碱受体（nAChR)亚型，分子生物学研究进一步表明，该类杀虫剂主要作用于昆虫nAChRα亚基。     

Comparison of catalytic properties and inhibition kinetics of two acetylcholinesterases from a lepidopteran insect

Acetylcholinesterase (AChE) is the primary target of organophosphate (OP) and carbamate (CB) insecticides. Many insect species have been shown to have two different AChE genes. The amino acid identity between the two lepidopteran AChEs is lower than 40%, and potential differences in enzymatic function have not been characterized. In this study, the cDNAs encoding two AChEs (Boma-AChE1 and Boma-AChE2) from Bombyx mandarina were sequenced, and the corresponding proteins were heterologously expressed to compare their enzymatic properties and interactions with insecticides in vitro. Both of these enzymes had high specific activities for acetylthiocholine iodide. Studies on substrate and inhibitor specificities confirmed that both enzymes belong to AChE. Insecticide inhibition assays indicated that Boma-AChE1 was more sensitive than Boma-AChE2 to eight of the nine insecticides tested. However, Boma-AChE2 was more sensitive than Boma-AChE1 to one of the OP insecticides, heptenophos. The results suggested that two AChEs from a lepidopteran insect have distinct catalytic properties and responses to different inhibitors.