The long term use of many insecticides is continually threatened by the ability of insects to evolve resistance mechanisms that render the chemicals ineffective. Such resistance poses a serious threat to insect pest control both in the UK and worldwide. Resistance may result from either an increase in the ability of the insect to detoxify the insecticide or by changes in the target protein with which the insecticide interacts. DDT, the pyrethrins and the synthetic pyrethroids (the latter currently accounting for around 17% of the world insecticide market), act on the voltage-gated sodium channel proteins found in insect nerve cell membranes. The correct functioning of these channels is essential for normal transmission of nerve impulses and this process is disrupted by binding of the insecticides, leading to paralysis and eventual death. Some insect pest populations have evolved modifications of the sodium channel protein which prevent the binding of the insecticide and result in the insect developing resistance. Here we review some of the work (done at Rothamsted Research and elsewhere) that has led to the identification of specific residues on the sodium channel that may constitute the DDT and pyrethroid binding sites.

https://iubmb.onlinelibrary.wiley.com/doi/epdf/10.1080/15216540701352042

DDT, pyrethrins, pyrethroids and insect sodium channels.

Pyrethroids1 are among the most potent insecticides known. They are synthetic compounds structurally derived from pyrethrin I, one of the six active components of pyrethrum, which is an extract from the dried flower heads of Chrysanthemum cinerariaefolium. The natural pyrethrins have excellent insecticidal properties and low mammalian toxicity, but are of limited use because of their low photostability and high biodegradability (Wouters and van den Bercken 1978). However, pyrethroids are relatively stable, have a high toxicity to a wide spectrum of insects (Elliott 1976), are relatively nontoxic to mammals (Elliott 1976), and have tremendous agricultural potential (Harris and Turnbull 1978). Moreover, pyrethroids are much less persistent than the organochlorine insecticides, such as DDT and dieldrin, and apparently do not accumulate in the environment.

Effects of synthetic pyrethroid insecticides on nontarget organisms.

Natural products (NPs) have long been used as pesticides and have broadly served as a source of inspiration for a great many commercial synthetic organic fungicides, herbicides and insecticides that are in the market today. In light of the continuing need for new tools to address an ever-changing array of fungal, weed and insect pests, NPs continue to be a source of models and templates for the development of new pest control agents. Interestingly, an examination of the literature suggests that NP models exist for many of the pest control agents that were discovered by other means, suggesting that, had circumstances been different, these NPs could have served as inspiration for the discovery of a great many more of today's pest control agents. Here, an attempt is made to answer questions regarding the existence of an NP model for existing classes of pesticides and what is needed for the discovery of new NPs and NP models for pest control agents.

Natural products for pest control: an analysis of their role, value and future

This review has examined a number of issues surrounding the use of carboxylesterase activity in environmental monitoring. It is clear that carboxylesterases are important enzymes that deserve increased study. This class of enzymes appears to have promise for employment in environmental monitoring with a number of organisms and testing scenarios, and it is appropriate for inclusion in standard monitoring assays. Given the ease of most activity assays, it is logical to report carboxylesterase activity levels as well as other esterases (e.g., acetylcholinesterase). Although it is still unclear as to whether acetylcholinesterase or carboxylesterase is the most "appropriate" biomarker, there are sufficient data to suggest that at the very least further studies should be performed with carboxylesterases. Most likely, data will show that it is optimal to measure activity for both enzymes whenever possible. Acetylcholinesterase has the distinct advantage of a clear biological function, whereas the endogenous role of carboxylesterases is still unclear. However, a combination of activity measurements for the two enzyme systems will provide a much more detailed picture of organism health and insecticide exposure. The main outstanding issues are the choice of substrate for activity assays and which tissues/organisms are most appropriate for monitoring studies. Substrate choice is very important, because carboxylesterase activity consists of multiple isozymes that most likely fluctuate on an organism- and tissue-specific basis. It is therefore difficult to compare work in one organism with a specific substrate with work performed in a different organism with a different substrate. An attempt should therefore be made to standardize the method. The most logical choice is PNPA (p-nitrophenyl acetate), as this substrate is commercially available, requires inexpensive optics for assay measurements, and has been used extensively in the literature. However, none of these beneficial properties indicates that the substrate is an appropriate surrogate for a specific compound, e.g., pyrethroid-hydrolyzing activity. It will most likely be necessary to have more specific surrogate substrates for use in assays that require information on the ability to detoxify/hydrolyze specific environmental contaminants. The use of carboxylesterase activity in TIE protocols appears to have excellent promise, but there are further technical issues that should be addressed to increase the utility of the method. The main concerns include the large amount of nonspecific protein added to the testing system, which can lead to undesirable side effects including nonspecific reductions in observed toxicity, decrease in dissolved oxygen content, and organism growth. It is probable that these issues can be resolved with further assay development. The ideal solution would be to have a commercial recombinant carboxylesterase that possessed elevated pyrethroid-hydrolysis activity and which was readily available, homogeneous, and inexpensive. The availability of such an enzyme would address nearly all the current method shortcomings. Such a preparation would be extremely useful for the aquatic toxicology community. Further work should focus on screening available esterases for stability, cost, and activity on pyrethroids, with specific focus on esterases capable of distinguishing type I from type II pyrethroids. It would also be beneficial to identify esterases that are not sensitive to OP insecticides. Many esterases and lipases are available as sets to test chemical reactions for green chemistry, enabling large-scale screening. Other potential approaches to increase the utility of the enzyme include derivatization with polyethylene glycol (PEG) or cyanuric acid chloride to increase stability and reduce microbial degradation. It is also possible that the enzyme could be formulated in a sol gel preparation to increase stability. It is likely that the use of carboxylesterase addition will increase for applications in sediment TIEs. Carboxylesterases are an interesting and useful enzyme family that deserves further study for applications in environmental monitoring as well as to increase our understanding of the fundamental biological role(s) of these enzymes. There are, of course, other enzymes that show high esterase activity on pyrethroids but are not technically carboxylesterases in the alpha/beta-hydrolase fold protein family. These enzymes should also be examined for use in TIE protocols and "esterase" arrays as well as for general applications in environmental monitoring. One can envision the creation of a standardized screen of enzymes with esterase activity to (1) identify environmental contaminants, (2) estimate the potential toxic effects of new compounds on a range of organisms, and (3) monitor organism exposure to agrochemicals (and potentially other contaminants). This approach would provide a multibiomarker integrative assessment of esterase-inhibiting potential of a compound or mixture. In conclusion, much is still unknown about this enzyme family, indicating that this area is still wide open to researchers interested in the applications of carboxylesterase activity as well as basic biological questions into the nature of enzyme activity and the endogenous role of the enzyme.

Applications of Carboxylesterase Activity in Environmental Monitoring and Toxicity Identification Evaluations (TIEs)

Natural pyrethrins, also called "pyrethrum" or "'pyrethrum extract", have long been known as insecticides. They are extracted from the flowers of Chrysanthemum cinerariaefolium Vis which contain six insecticidally active components, of which pyrethrin I and pyrethrin II are the most active (Head 1973, Elliott & Janes, 1973). Pyrethrins are widely used for domestic purposes and for the protection of stored food, but for general pest control in agriculture they are applied on a limited scale only. The total world production does not exceed about 350 tons per yr: cf. organophosphates, about 100,000 tons (Glynne-Jones, 1973: Fest & Schmidt. 1973). Their Limited use is due in part to economic reasons. Other factors restricting their use are their low photostability and high (bio) degradability, and pyrethrins are usually applied in combination with metabolic synergists. Apart from these problems, the pyrethrins possess excellent insecticidal properties combined with remarkably low mammalian toxicity. Their insect (topical) to mammalian (oral) toxicity ratio is much better than for the majority of other organic insecticides (Elliott, 1977). In mammals and other animals pyrethrins are rapidly metabolized and practically no residues are left in the biosphere. These favourable properties have stimulated the search for synthetic insecticides based on the structure of the pyrethrins. The production of synthetic pyrethroids on a commercial basis started with allethrin and cyclethrin around 1950 (O'Brien, 1967). These early compounds, however, still lacked sufficient stability and they are less effective against many species of insects than the natural product. At the moment the total yearly production of allethrin is some 100 tons. In the last decennium great progress has been made, notably by Ellion and co-workers, in the development of more stable and highly active pyrethroids and in the near future more widespread application of these compounds may be expected. Because of the complex molecular structure, the most important features of the pyrethroid molecule will be described briefly before the mode of action is discussed. Excellent reviews on the structural requirements of insecticidal active pyrethroids can be found elsewhere (Elliott, 1971, 1976a, b, 1977; Elliott eI al., 1974a). Most of the information presented in the next section is derived from these reviews. GENERAL FEATURES OF THE PVRETHROID MOI.ECLI,I."

Action of pyrethroids.

Although structure modifications of natural pyrethrin constituents have disclosed a variety of potent synthetic analogues, all known examples are cyclopropanecarboxylate esters, a grouping that appeared to be essential for insecticidal activity Some new substituted 2-phenylalkanoates, whose biological activities are of a similar nature and potency to those of conventional pyrethroids, are now reported 5-Benzyl-3-furylmethyl and 3-phenoxybenzyl 3-methyl-2-phenylbutyrates and their analogues are potent insecticides Activity is increased on the introduction of appropriate groups into the 3 and/or 4-positions of the aryl ring and the (S)-2-phenylalkanoates are far more active than their (R)-enantiomorphs Structure/activity relationships are compared with those for conventional pyrethroids Some of the new series compare favourably with typical insecticides in tests against Musca domestica, Spodoptera litura and Plutella xylostella

2-arylalkanoates, a new group of synthetic pyrethroid esters not containing cyclopropanecarboxylates†

(1974). A New Class of Pyrethroidal Insecticides; α-Substituted Phenylacetic Acid Esters. Agricultural and Biological Chemistry: Vol. 38, No. 4, pp. 881-883.

A New Class of Pyrethroidal Insecticides; α-Substituted Phenylacetic Acid Esters

A novel process for preparing cis-chrysanthemic acid comprising (1) hydrolysis of trans-δ-halogenodihydrochrysanthemic acid or ester thereof under neutrally kept condition with an alkaline earth metal carbonate, (2) conversion of the obtained trans-δ-hydroxydihydrochrysanthemic acid ester into a mixture of cis-chrysanthemic acid and iso-cis-chrysanthemic acid by reacting with a strong base, and (3) separation of the cis-chrysanthemic acid from the mixture by selective conversion of the iso-cis-chrysanthemic acid with water in the presence of an acid catalyst into cis-dihydrochrysanthemolactone, followed by separation of the cis-chrysanthemic acid from the lactone with a caustic alkali and an acid. The present process is very useful for converting (-)-trans-chrysanthemic acid which is obtained, as a main remainder of racemic chrysanthemic acid prepared industrially, after (+)-trans-chrysanthemic acid was used for preparation of (+)-trans-chrysanthemic acid ester having a high insecticidal activity, into (+)-cis-chrysanthemic acid which is industrially valuable for preparation of insecticides.

Toshiko Honda

Papers

2-arylalkanoates, a new group of synthetic pyrethroid esters not containing cyclopropanecarboxylates†

A New Class of Pyrethroidal Insecticides; α-Substituted Phenylacetic Acid Esters

Process for preparing cis-chrysanthemic acid