231 12 Pyrethroid Insecticides 12.1 BACKGROUND The insecticidal properties of pyrethrum, a product prepared from the dried and powdered heads of owers belonging to the genus Chrysanthemum, have long been recognized. First introduced into Europe in the middle of the 19th century, early sources were the region of the Caucasus and the Adriatic coast. Subsequently, the major source of commercial pyrethrum was the species Chrysanthemum cinerari- aefolium grown in East Africa. In the course of time the insecticidal ingredients of pyrethrum, the pyrethrins, were chemically characterized. Six pyrethrins were identied, all of them lipophilic esters (Figure 12.1). They are formed from two acids—chrysanthemic acid and pyrethric acid—in combination with three bases: pyrethrolone, cinerolone, and jasmolone. A serious limitation of pyrethrins as commercial insecticides is their instability. On the one hand they are photolabile and have only limited life when applied to surfaces, for example, plant leaves, exposed to direct sunlight. On the other, they are readily biodegradable and often have only a short “knockdown” effect on tar- get insects unless they are synergized with compounds such as piperonyl butoxide that will repress their oxidative metabolism. The important point is that they have served as models for the development of the synthetic pyrethroids, one of the most widely used types of insecticide at the present time. The rst synthetic pyrethroids, compounds such as allethrin and bioallethrin, were not sufciently photostable to have great commercial potential (Leahey 1985). Subsequently, a series of compounds were discovered that had greater stability, which came to achieve great commercial success. Included among these are permethrin, cypermethrin, deltamethrin, fenval- erate, cyuthrin, cyhalothrin, and others. Their widespread introduction during the 1970s came on the heels of the environmental problems associated with the persistent organochlorine insecticides. Although the synthetic pyrethroids have sufcient meta- bolic stability to be effective insecticides, they are, nevertheless, readily biodegrad- able by vertebrates and do not tend to be biomagnied in food chains. At the time of their introduction, they were seen to be environmentally friendly insecticides, which, for some purposes, were effective alternatives to organochlorine insecticides. 12.2 CHEMICAL PROPERTIES The structures of some pyrethroid insecticides are shown in Figure 12.1. They are all lipophilic esters showing some structural resemblance to the natural pyrethrins. They can all exist in a number of different enantiomeric forms. Permethrin, cyper- methrin, and deltamethrin, for example, all have three asymmetric carbon atoms © 2009 by Taylor & Francis Group, LLC 232 Organic Pollutants: An Ecotoxicological Perspective, Second Edition and, consequently, eight possible enantiomers (Leahey 1985; Environmental Health Criteria 94; Environmental Health Criteria 95; Environmental Health Criteria 97). Their enantiomers fall into two categories, cis or trans, depending on the stereochem- istry of the 1 relative to the 3 position of the three-membered ring of the acid moiety. Thus, there are four possible cis enantiomers and four possible trans enantiomers for each of these three compounds. Commercial products are usually racemic mixtures of different enantiomers. A notable exception is deltamethrin, which is marketed as a single cis-isomer (Environmental Health Criteria 97). Fenvalerate differs from the other pyrethroids featured in Figure 12.1 on account of the structure of its acid moi- ety. Nevertheless, it has similar biological properties to the other pyrethroids. The properties of some pyrethroids are given in Table 12.1. Taking the pyrethroids, apart from fenvalerate, they are solids with low water solubility, marked lipophilicity, and low vapor pressure. Fenvalerate is a viscous liquid with an appreciable vapor pressure. Being esters, the pyrethroids are subject to hydrolysis at high pH. They are sufciently stable to heat and light to be effective insecticides in the eld. 12.3 METABOLISM OF PYRETHROIDS The metabolism of permethrin will be taken more generally as an example of the metabolism of pyrethroids (Figure 12.2). The two types of primary metabolic attack are by microsomal monooxygenases and esterases. Monooxygenase attack involves Chrysanthemic acid Pyrethric acid Pyrethroids Permethrin Cypermethrin O H H HCO 2 H CH 3 CO 2 C Acids Pyrethrins Pyrethrolone Bases HO O H Jasmolone HO O H Cinerolone HO cis-Deltamethrin Fenvalerate H 3 1 2 H Br Br H HCO 2 H O O O Cl Cl CN Cl O O O CN Cl O O CN S RR O O O OH Cl FIGURE 12.1 Structure of pyrethrins and pyrethroids. © 2009 by Taylor & Francis Group, LLC Pyrethroid Insecticides 233 different forms of cytochrome P450 and yields metabolites with hydroxyl groups sub- stituted in both the acidic and basic moieties. The principal metabolites formed by primary oxidation are compounds 1 and 2 in the gure. Hydroxylation occurs on a methyl group of the acid moiety and on a free para ring position in the basic moiety. Esteratic hydrolysis of permethrin yields metabolites 4 and 5. Metabolite 4 is a base, TABLE 12.1 Properties of Some Pyrethroid Insecticides Compound Water Solubility μg/mL @ 20 or 25°C log K ow Vapor Pressure #Pa @ 20 or 25°C Permethrin (racemate) 0.2 6.5 1.3 × 10 −6 Cypermethrin (racemate) 0.009 6.3 1.9 × 10 −7 alpha Cypermethrin (2 cis isomers) 0.005–0.01 5.16 1.7 ×10 −7 Deltamethrin <0.002 5.43 2 × 10 −6 Fenvalerate 0.002 6.2 3.7 × 10 −5 Note: #1 Pascal (Pa) = 0.0075 torr (i.e., mms of Hg). Source: Data from Environmental Health Criteria 82, Environmental Health Criteria 94, Environmental Health Criteria 95, Environmental Health Criteria 97, and Environmental Health Criteria 142. CH 3 CH 3 Cl Cl O OH O C O C CCH OH HO CH 2 OH O HO CH 2 O CH 2 O HO CH 2 OH H 2 O CH 3 Cl Cl O C CCH OH CH 2 OH CH 3 Cl Cl O 1 C O C CCH O O CH 2 CH 3 CH 3 O 2 O 2 Cl Cl C CCH O O 2 O 2 H 2 O H 2 O O CH 2 CH 3 CH 3 Cl Cl trans-Permethrin CCH O 3 4 5 2 6 7 FIGURE 12.2 The metabolism of trans-permethrin. © 2009 by Taylor & Francis Group, LLC 234 Organic Pollutants: An Ecotoxicological Perspective, Second Edition and metabolite 5 is an acid. The oxidative metabolites 1 and 2 are also subject to ester- atic hydrolysis. Hydrolysis of oxidative metabolite 1 yields again the base, metabo- lite 4, whereas hydrolysis of oxidative metabolite 2 yields again the acid, metabolite 5. In addition to these, oxidative metabolite 1 yields the hydroxy base, metabolite 6, whereas oxidative metabolite 2 yields the hydroxy acid, metabolite 3. Thus, taken together, the esteratic hydrolysis of metabolites 1 and 2 yields, on the one hand, the same two metabolites that arise from the hydrolysis of permethrin itself and, addition- ally, two further metabolites (3 and 6) that contain hydroxyl groups that were intro- duced by oxidative attack upon the parent compound. In summary, metabolites 4 and 5 are the products of esteratic hydrolysis of permethrin; metabolite 4 is also generated by the hydrolysis of metabolite 1 and metabolite 6 by the hydrolysis of metabolite 2. Metabolites 3 and 6 contain additional hydroxy groups introduced by oxidative attack. The hydroxyl groups are then available for conjugation with glucuronide, sul- fate, peptide, etc., depending on species. In both insects and vertebrates the excreted products are mainly conjugates. There has been some controversy over the relative importance of oxidation and esteratic hydrolysis in primary metabolic attack. The strong potentiation of toxicity of certain pyrethroids to insects by piperonyl butoxide (PBO) and other P450 inhibi- tors (see Chapter 2, Section 2.5) suggests the dominance of oxidation over hydroly- sis as a detoxication mechanism. However, the interpretation of metabolic studies has sometimes been complicated by the shortage, even the apparent absence, of primary oxidative metabolites such as those shown in Figure 12.2. One problem has been iden- tication and quantication of conjugates that can be rapidly formed from the various metabolites containing hydroxy groups, in both in vivo and in vitro studies on insects. When trying to elucidate the metabolic regulation of toxicity, a difculty had been establishing the metabolic pathways by which hydroxylated metabolites such as com- pounds 3 and 6 were formed. Did hydroxylation occur before or after hydrolytic cleav- age of the ester bond? In most cases, available evidence strongly suggests that oxidation predominates over hydrolysis as a primary mode of metabolic attack. In insects, the marked synergistic action of P450 inhibitors such as PBO and ergosterol biosynthesis inhibitors (EBIs) (see Chapter 2, Section 2.6) is not consistent with esterase attack, the dominant mechanism of primary metabolism of pyrethroids. Further, the products of esteratic cleavage are strongly polar in character and are hardly ideal substrates for the hydrophobic active centers of cytochrome P450s. It should also be mentioned that the primary products of oxidative attack are more polar than the original insecticides, and are likely, on that account, to be better substrates for esterase attack (cf. OP hydrolysis, Chapter 10). Such a mechanism can explain an observation made by several workers studying microsomal metabolism of pyrethroids—that switching on P450 oxidation by addition of NADPH can increase the rate of hydrolysis (Lee et al. 1989). 12.4 ENVIRONMENTAL FATE OF PYRETHROIDS Pyrethroids are extensively used in agriculture, so agricultural land is often con- taminated by them. They can also reach eld margins and hedgerows through spray drift. Because of their high toxicity to aquatic organisms, precautions are taken to prevent their entering surface waters, which can be a consequence of spray drift or © 2009 by Taylor & Francis Group, LLC Pyrethroid Insecticides 235 soil run off. In normal agricultural use, it is important that they are not applied too close to surface waters including ditches and water courses. Their use in sheep dips, to control ectoparasites, has raised concern over the safe disposal of unused dipping liquids; unused dips should not be discharged into adjacent water courses. Two major factors determining the environmental fate of pyrethroids are marked lipophilicity and rapid biodegradation by many animals. Even sh can degrade them reasonably rapidly. When pyrethroids reach soils or aquatic systems, they become strongly adsorbed to the colloidal fraction—mineral particles and associated organic matter. Consequently, if they do reach surface waters, their initial concentrations in water fall rapidly because of adsorption to this colloidal material. In most aquatic organisms, they are metabolized rapidly enough to limit the degree of bioconcentration that occurs. Bioconcentration studies with sh have shown bioconcentration factors (BCFs) in the steady state ranging from 50 to several thousand, depending on spe- cies, age, etc. These are, of course, concentrations in tissue relative to concentrations in water, and they are, in the main, considerably below the values predicted from the high K ow of the insecticides (Environmental Health Criteria 97; Environmental Health Criteria 142). The lower values are considerably below the BCFs reported for persistent organochlorines (OCs) such as dieldrin and p,pb-DDE, which usually exceed 1000 (see Chapter 5, Section 5.3.3). The major factors responsible for this are believed to be (1) reasonably rapid metabolism by many sh (Chapter 4, Table 4.1) and (2) strong adsorp- tion to colloids in some test systems that contain suspended material such as sediment. In some studies, the pyrethroid concentrations measured in water by chemical analysis included considerable amounts of insecticide in the adsorbed state, which was not read- ily available to the sh. Here, the concentration of pyrethroid determined by chemical analysis considerably overestimated the levels that sh were effectively exposed to, and consequently underestimated the BCFs that were achieved (Leahey 1985). In a laboratory study of the persistence of ve pyrethroids in soil, the rates of loss followed the order fenpropathrin>permethrin>cypermethrin>fenvalerate>delta methrin (Chapman and Harris 1981). Microbial degradation was an important factor determining their rate of disappearance. Half-lives of deltamethrin determined in two German soils were found to be 35 days in a sandy soil but 60 days in a sandy loam. Hill and Schaalje (1985) showed that deltamethrin applied in the eld underwent a biphasic pattern of loss—an initial rapid loss being succeeded by a slower rst-order degradation. This is essentially similar to the pattern of loss of another group of hydrophobic insecticides—the OC compounds—except that the latter are eliminated much more slowly, especially in the later stages of the process (see Chapter 4, Section 4.2, and Chapter 5). Both types of insecticide have pK ow in the range 5–7, but OCs are metabolized much more slowly than pyrethroids by soil microorganisms. Pyrethroids can also persist in sediments. In one study, alpha-cypermethrin was applied to a pond as an emulsiable concentrate (Environmental Health Criteria 142). After 16 days of application, 5% of the applied dose was still present in sediment, falling to 3% after a further 17 days. This suggests a half-life of the order of 20–25 days—comparable in magnitude to half-lives measured in temperate soils. The general picture, then, is that pyrethroids are reasonably persistent in soils and sediments but not to the same degree as OC compounds such as dieldrin and p,pb-DDE. They do undergo bioconcentration from water by sh and other aquatic © 2009 by Taylor & Francis Group, LLC 236 Organic Pollutants: An Ecotoxicological Perspective, Second Edition organisms. However, because of their ready biodegradability, they are not biomag- nied with movement through the upper trophic levels of food chains in the way that persistent OCs are. There is, however, concern that residues in sediments may continue to be available to certain bottom feeders long after initial contamination, and that some aquatic invertebrates of lower trophic levels, which are decient in detoxifying enzymes, may bioconcentrate/bioaccumulate them to a marked degree. 12.5 TOXICITY OF PYRETHROIDS Pyrethroids, such as p,pb-DDT, are toxic because they interact with Na + channels of the axonal membrane, thereby disturbing the transmission of nerve action potential (Eldefrawi and Eldefrawi 1990, and Chapter 5, Section 5.2.4 of this book). In both cases, marked hydrophobicity leads to bioconcentration of the insecticides in the axonal membrane and reversible association with the Na + channel. Consequently, both DDT and pyrethroids show negative temperature coefcients in arthropods; increasing temperature brings decreasing toxicity because it favors desorption of insecticide from the site of action. Pyrethroids show very marked selective toxicity (Table 12.2). They are highly toxic to terrestrial and aquatic arthropods and to sh, but only moderately toxic to rodents, and less toxic still to birds. The selectivity ratio between bees and rodents is 10,000- to 100,000-fold with topical application of the insecticides. They therefore appear to be environmentally safe so far as terrestrial vertebrates are concerned. There are, inevitably, concerns about their possible side effects in aquatic systems, especially on invertebrates. A eld problem that has emerged is the synergistic action of certain EBI fungicides upon pyrethroids. Some combinations of EBIs with pyrethroids are highly toxic to bees, with synergistic ratios of the order 10–20 (Pilling 1993; Colin and Belzunces 1992; Meled et al. 1998). There have been reports from France and Germany of kills of bees in the eld attributable to synergistic effects of this kind, following the use of tank mixes by spray operators. The enhancement of the toxicity of lambda cyhalothrin TABLE 12.2 Toxicity of Some Pyrethroids Compound LD 50 Rat (mg/kg) LD 50 Birds (mg/kg) 96 h LC 50 Fish (μg/L) LC 50 Aquatic Invertebrates (μg/L) Permethrin 500 >13,000 [4] 0.6–314 0.018–1.2 Cypermethrin 250 >10,000 [1] 0.4–2.8 0.01–5 Fenvalerate 451 >4,000 [3] 0.3–200 0.008–1 Deltamethrin 129 4,000 [1] 0.4–2.0 5 (Daphnia) Note: Mean values given for birds; the number of species tested is given in brackets. Source: Data from Environmental Health Criteria 82, Environmental Health Criteria 94, Environmental Health Criteria 95, and Environmental Health Criteria 97. © 2009 by Taylor & Francis Group, LLC Pyrethroid Insecticides 237 to bees by the EBI fungicide prochloraz has been demonstrated in a semi eld trial (Bromley-Challenor 1992). The synergistic action of EBIs has been attributed, largely or entirely, to inhibition of detoxication by cytochrome P450 (see, for example, Pilling et al. 1995). Questions are now being asked about possible hazards to wild bees and other pollinators posed by pyrethroid/EBI mixtures. 12.6 ECOLOGICAL EFFECTS OF PYRETHROIDS 12.6.1 P OPULATION DYNAMICS Because of the high toxicity of pyrethroids to aquatic invertebrates, these organ- isms are likely to be adversely affected by contamination of surface waters. Such contamination might be expected to have effects at the population level and above, at least in the short term. In one study of a farm pond, cypermethrin was applied aeri- ally, adjacent to the water body (Kedwards et al. 1999a). Changes were observed in the composition of the macroinvertebrate community of the pond that were related to levels of the pyrethroid in the hydrosoil. Diptera were most affected, showing a decline in abundance with increasing cypermethrin concentration. Chironimid lar- vae rst declined and later recovered. Harmful effects on macroinvertebrate communities have also been demonstrated in mesocosm studies, and will be discussed briey here for comparison with eld stud- ies. In one study, cypermethrin and lambda cyhalothrin were individually applied to experimental ponds at the rates of 0.7 and 1.7 g a.i./ha and the results subjected to mul- tivariate analysis (Kedwards et al. 1999b). Treatment with pyrethroid caused a decrease in abundance of gammaridae and asellidae but a concomitant increase in planorbi- dae, chironimidae, hirudinae, and lymnaeidae. Gammaridae were found to be more sensitive to the chemicals than asellidae, their numbers remaining depressed until the termination of the experiment (15 weeks) with both treatments. This may have been because they inhabit the sediment surface where there would have been relatively high levels of recently adsorbed pyrethroid, whereas the asellidae are epibenthic, burrowing into the hydrosoil, where lower levels of insecticide should have existed, at least in the short term. In a wide-ranging study of the impact of pyrethroids used to control pests of cotton, three different pond systems in Great Britain and the United States were employed in seven separate experiments (Giddings et al. 2001). Results from meso- cosm studies were compared with those from related eld studies that also utilized toxicity data for the insecticides (Solomon et al. 2001). The different taxa showed the following range of sensitivity to cypermethrin and esfenvalerate, measured in terms of abundance: amphipods, isopods, midges, mayies, copepods, and cladocerans (most sensitive) ranging to sh, snails, oligochaetes, and rotifers (least sensitive). Values for lowest-observed effect concentrations were derived from this investigation. Considering evidence from both eld and mesocosm studies, it may be concluded that certain groups of aquatic macroinvertebrates are sensitive to pyrethroids and that there can be changes, in the short term, at the population level and above with exposure to environmentally realistic concentrations of them. It should be possible to pick up effects of this kind in natural waters using ecological proling, for example, the River Invertebrate Prediction and Classication System (RIVPACS). There is © 2009 by Taylor & Francis Group, LLC 238 Organic Pollutants: An Ecotoxicological Perspective, Second Edition a need here for combining ecological proling with chemical analysis, to facilitate the detection of chemicals that cause changes in community structure in natural waters. 12.6.2 POPULATION GENETICS The continuing use of pyrethroids in agriculture has led to the emergence of resistant strains of pests. One of the best-studied examples is the tobacco budworm (Heliothis virescens), a very serious pest of cotton in the southern United States (McCaffery 1998). Indeed, the resistance problem has sometimes been severe enough to threaten a loss of control over the pest. A study of a number of resistant strains from the eld has revealed two major types of resistance mechanism. Some individuals possess aberrant forms of the target site, the Na + channel. At least two forms are known that confer either “kdr” (<100-fold) or “super kdr” (>100-fold) resistance, which is the consequence of the presence of insensitive forms of the Na + channel protein (McCaffery 1998, and Chapter 4, Section 4.4 of this book). This type of resistance has been found in a number of species of insects, includ- ing Musca domestica, Heliothis virescens, Plutella xylostella, Blatella germanica, Anopheles gambiae, and Myzus persicae. Kdr has been attributed to three different changes of single amino acids of the voltage-dependent sodium channel, and super kdr to changes in pairs of amino acids, also located in the sodium channel (Salgado 1999). Interestingly, it appears that earlier selective pressure by dichlorodiphenyl trichloroethene (DDT) raised the frequency of kdr genes in the population before pyrethroids came to be used. Thus, some “pyrethroid resistance” already existed before these insecticides were applied in the eld. The other major mechanism of pyrethroid resistance found in some eld strains of Heliothis virescens was enhanced detoxication due to a high rate of oxidative detoxi- cation, mediated by a form of cytochrome P450 (McCaffery 1998). Some strains, such as PEG 87, which was subjected to a high level of eld and laboratory selec- tion, possessed both mechanisms. Other example of pyrethroid resistance due to enhanced detoxication may be found in the literature on pesticides. 12.7 SUMMARY Pyrethroid insecticides were modeled upon naturally occurring pyrethrins, which were once quite widely used as insecticides but had the disadvantages of being photo- chemically unstable and susceptible to rapid metabolic detoxication. Pyrethroids are more stable than pyrethrins and, like DDT, act upon the voltage-dependent sodium channel of the nerve axon. They are lipophilic but are readily biodegradable by most organisms of higher trophic levels. Although they can undergo bioconcentration in the lower trophic levels of aquatic food chains, unlike OC insecticides, they are not prone to biomagnication in the upper trophic levels of either aquatic or terrestrial food chains. They are, however, strongly adsorbed in soils and sediments where they can be persistent. Pyrethroids are much more toxic to invertebrates than to most vertebrates. They can have serious effects upon aquatic invertebrates, at least in the short term. They can be © 2009 by Taylor & Francis Group, LLC Pyrethroid Insecticides 239 synergized by inhibitors of cytochrome P450, such as EBI fungicides, and so there are potential hazards associated with the use of mixtures of these two types of pesticides. Resistance to pyrethroids has developed in a number of pest species due to both insensi- tive forms of the target site (sodium channel) and/or enhanced metabolic detoxication. FURTHER READING Environmental Health Criteria 82 [Cypermethrin], 94 [Permethrin], 95 [Fenvalerate], 97 [Deltamethrin], and 142 [Alphacypermethrin] are all valuable sources of information on the environmental toxicology of pyrethroids. Leahey, J.P. (Ed.) (1985). The Pyrethroid Insecticides—A multiauthor work that covers many aspects of the toxicology and ecotoxicology of the earlier pyrethroids. © 2009 by Taylor & Francis Group, LLC . CH 2 O CH 2 O HO CH 2 OH H 2 O CH 3 Cl Cl O C CCH OH CH 2 OH CH 3 Cl Cl O 1 C O C CCH O O CH 2 CH 3 CH 3 O 2 O 2 Cl Cl C CCH O O 2 O 2 H 2 O H 2 O O CH 2 CH 3 CH 3 Cl Cl trans-Permethrin CCH O 3 4 5 2 6 7 FIGURE 12. 2 The metabolism of trans-permethrin. © 2009 by Taylor & Francis Group, LLC 234 Organic Pollutants: An Ecotoxicological Perspective, . as dieldrin and p,pb-DDE. They do undergo bioconcentration from water by sh and other aquatic © 2009 by Taylor & Francis Group, LLC 236 Organic Pollutants: An Ecotoxicological Perspective, . 2009 by Taylor & Francis Group, LLC 232 Organic Pollutants: An Ecotoxicological Perspective, Second Edition and, consequently, eight possible enantiomers (Leahey 1985; Environmental Health