ORGANIC POLLUTANTS: An Ecotoxicological Perspective - Chapter 12 docx

Metabolite 4 is a base, TABLE 12.1 Properties of Some Pyrethroid Insecticides Compound Water Solubility μg/mL @ 20 or 25°C log Kow Vapor Pressure #Pa @ 20 or 25°C Permethrin racemate Cy

12.1 BACKGROUND

The insecticidal properties of pyrethrum, a product prepared from the dried and powdered heads of flowers 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 identified, 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 first synthetic pyrethroids, compounds such as allethrin and bioallethrin, were not sufficiently 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, cyfluthrin, 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 sufficient meta-bolic stability to be effective insecticides, they are, nevertheless, readily biodegrad-able by vertebrates and do not tend to be biomagnified 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 deltacyper-methrin, for example, all have three asymmetric carbon atoms

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

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 sufficiently stable to heat and light to be effective insecticides in the field

12.3 METABOLISM OF PYRETHROIDS

The metabolism of permethrin will be taken more generally as an example of the

are by microsomal monooxygenases and esterases Monooxygenase attack involves

Chrysanthemic acid

Pyrethric acid

Pyrethroids

Permethrin

Cypermethrin

O

H H

H CO2H

CH3CO2C

Acids

Pyrethrins

Pyrethrolone

Bases

H

Jasmolone HO O

H

Cinerolone HO

cis-Deltamethrin

Fenvalerate

H

3 12 H

Br Br

H

H CO2H

O O

O

Cl

Cl

O O

O

CN

CN

S

O

O H

Cl

FIGURE 12.1 Structure of pyrethrins and pyrethroids.

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 figure 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 Kow

Vapor Pressure

#Pa @ 20 or 25°C

Permethrin

(racemate)

Cypermethrin

(racemate)

alpha Cypermethrin

(2 cis isomers)

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.

CH3

CH3

Cl Cl

O

OH O

C

O C

C CH

OH

HO CH2

OH

O

HO CH2

O

CH2

O HO

CH2OH

H2O

CH3

Cl

Cl

O

C

C CH

OH

CH2OH

CH3

Cl

Cl

C

O C

C CH

O

O

CH2

CH3

CH3

Cl Cl

C

C CH

O

O2

O2

H2O

H2O

O

CH2

CH3

CH3

Cl Cl

trans-Permethrin

C CH

O

2

6 7

FIGURE 12.2 The metabolism of trans-permethrin.

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 metabometabo-lite 2 yields again the acid, metabometabo-lite

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

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

iden-tification and quaniden-tification 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 difficulty 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 field 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

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 fish 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 fish 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

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)

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 fish Here, the concentration of pyrethroid determined by chemical analysis considerably overestimated the levels that fish were effectively exposed to, and consequently underestimated the BCFs that were achieved (Leahey 1985)

In a laboratory study of the persistence of five 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 field underwent a biphasic pattern of loss—an initial rapid loss being succeeded by a slower first-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

4.2, and Chapter 5) Both types of insecticide have pKow 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 emulsifiable 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

organisms However, because of their ready biodegradability, they are not biomag-nified 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 deficient in detoxifying enzymes, may bioconcentrate/bioaccumulate them to a marked degree

12.5 TOXICITY OF PYRETHROIDS

the axonal membrane, thereby disturbing the transmission of nerve action potential

cases, marked hydrophobicity leads to bioconcentration of the insecticides in the

both DDT and pyrethroids show negative temperature coefficients 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 fish, 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 field 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 field 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)

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.

to bees by the EBI fungicide prochloraz has been demonstrated in a semi field trial (Bromley-Challenor 1992) The synergistic action of EBIs has been attributed, largely

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 D YNAMICS

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 first declined and later recovered

Harmful effects on macroinvertebrate communities have also been demonstrated in mesocosm studies, and will be discussed briefly here for comparison with field 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, chironimiplanorbi-dae, 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 field 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, mayflies, copepods, and cladocerans (most sensitive) ranging to fish, snails, oligochaetes, and rotifers (least sensitive) Values for lowest-observed effect concentrations were derived from this investigation

Considering evidence from both field 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 profiling, for example, the River Invertebrate Prediction and Classification System (RIVPACS) There is

a need here for combining ecological profiling with chemical analysis, to facilitate the detection of chemicals that cause changes in community structure in natural waters

12.6.2 P OPULATION G ENETICS

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 field has revealed two major types of resistance mechanism Some individuals possess

that confer either “kdr” (<100-fold) or “super kdr” (>100-fold) resistance, which

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 field

The other major mechanism of pyrethroid resistance found in some field 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 field 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 biomagnification 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

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.