Descripción de los Casos de Uso del Negocio

difficult since basically we are not that different from the insects, mites and ticks that we want to kill. This was clearly demonstrated in the previous section.

When it comes to killing plants and fungi, however, things look different. Both types of organism possess a number of characteristics with respect to anatomy (overall and cellular), physiology and biochemistry that differ from the animal organism and hence can be used as targets for the development of pesticidal compounds that kill plants or fungi specifically. This also means that, at least with regard to acute toxicity, we must expect the groups of herbicides and fungi-cides to be less problematic for man, his husbandry and wildlife. When it come to adverse effects of chronic exposure or to possible carcinogenicity things can be different, however.

Herbicides

A herbicide is a chemical compound that kills weeds.

Non-selective herbicides may be used to kill all veg-etation before cultivation and planting begin. Once the crop has emerged, selective herbicides must be used. These should target the troublesome weeds and leave other weeds and the growing crop unharmed.

Some have a residual action in the soil – allowing, for example, weed seedlings to be exposed to the chemi-cals as they emerge, which then kills them.

Contact herbicides destroy only the plant tissue in contact with the chemical. Generally, these are the fastest-acting herbicides. Systemic herbicides, on the other hand, are translocated through the plant from foliar application down to the roots, or from soil application up to the leaves. They can destroy a greater amount of plant tissue. Soil-applied herbicides are Soil-applied to the soil and are taken up by the roots of the target plant.

The first herbicide to be used was not only effec-tive, it was even selective in its mode of action, i.e.

it could (can) distinguish between monocotyledo-nous plants (monocots) and dicotyledomonocotyledo-nous plants (dicots). The agent was sulfuric acid and the rea-sons for its selectivity can be found in two factors:

● The monocots include among others the grasses, and as such all of the grains. Normally these plants have relatively highly waxed leaves.

The dicots include all of the so-called broad-leaved plants, whether herbs, bushes or trees.

The leaves of these plants normally have a less

developed wax layer but most often a leaf- covering hair layer.

● While the sulfuric acid runs off the wax layer of the monocot leaves thereby doing no harm, it sticks to the hairs of the dicot leaves, ultimately causing corrosion of the leaves and killing the plant.

Sulfuric acid never became a widely used herbicide for many reasons, so let us turn to the earliest syn-thetic organic compounds used as herbicides. These were the so-called yellow agents, such as DNOC (4,6-dinitro-o-cresol; Fig. 20.16).

Although these compounds could be used to kill broadleaved weeds in cereals, their overall selectiv-ity was still very limited indeed. Thus, they could be used not only as herbicides, but also as insecti-cides, acaricides and fungicides. This also meant that the toxicity towards warm-blooded animals was very high (oral LD₅₀ in the rat):

● DNOC, 25–40 mg kg⁻¹ BW; and

● dinoseb (2,4-dinitro-6-sec-butylphenol; DNBP), 50 mg kg⁻¹ BW.

For ruminants the oral doses leading to poisoning with methaemoglobinaemia and intravascular haemolysis for both compounds were to be found in the interval of 2–50 mg kg⁻¹ BW.

The compounds inhibit oxidative phosphoryla-tion in the mitochondria of cells. As a contact her-bicide, DNOC was used to control broadleaved weeds in cereals and to desiccate potato and leguminous seed crops before harvesting. Since it is strongly phytotoxic for broadleaved plants its use as an insecticide has been limited to dormant sprays, especially for such fruit trees as apples or peaches. In the USA, EPA cancelled the registration of DNOC as a pesticide agent starting in 1991.

DNBP, which was a less expensive and more effec-tive herbicide, had already begun to replace DNOC by the late 1980s. Today none of these compounds is registered in the USA or in Europe.

OH

NO₂

NO₂ Fig. 20.16. The structure of DNOC.

The second generation of herbicides (after yellow agents) comprised the chlorophenoxy herbicides, also termed hormone agents. These synthetic organic compounds exert the same effect on the plant cells as the natural plant hormone auxin (= indole-3-acetic acid; IAA). These so-called auxin herbicides are more stable in plants than the main natural auxin and show systemic mobility and selective action, preferentially against dicot weeds in cereal crops. Hence, dicots can be eradicated in a field of monocots such as a grain field. Although the finer details of the mechanism behind the phytotoxicity (with selectivity for dicots) have been investigated for decades, it is only recently that we have begun to get some understanding (which, however, will not be dealt with here). The first compound within this group was 2,4,5-T (2,4,5-trichlorophenoxyacetic acid). Developed in the late 1940s the compound was found to be effective in defoliating broadleaved plants. The compound seemed to be more or less an ideal herbicide since the acute toxicity to mammals (including man) was low, with an oral LD₅₀ of about 400 mg kg⁻¹ BW in mice and 500 mg kg⁻¹ BW in rats. Soon 2,4,5-T was used extensively in the USA.

New compounds within the same group were gradu-ally marketed such as 2,4-dichlorophenoxyacetic acid (2,4-D), 2-methyl-4-chlorophenoxyacetic acid (MCPA) and dichlorprop (Fig. 20.17). The latter, which possesses an asymmetric carbon and is there-fore a chiral molecule, appeared on the market in 1960s. Initially it was sold as a racemic mixture of

stereoisomers, but since then advances in asymmet-ric synthesis have made possible production of the enantiopure compound. Today, only the active R-dichlorprop (also called dichlorprop-p or 2,4-DP-p) and its derivatives are sold as pesticides.

The chlorophenoxy compounds soon became very popular around the world. However, in 1961 the US military took to using 2,4,5-T in the Vietnam war. A product called agent orange (a mixture of equal amounts of 2,4,5-T and 2,4-D) was sprayed from aircrafts over the jungle in order to remove the leaves of the trees to better be able to identify Vietnamese Vietcong partisans and possi-bly North Vietnamese soldiers from the air. The US military herbicide programme in South Vietnam took place between 1961 and 1971. Herbicides were sprayed in all four military zones of Vietnam.

More than 19 million gallons of various herbicide combinations were used.

After some years it was recognized that synthetic 2,4,5-T contained a number of impurities from the syntheses, compounds that gradually were revealed to be very toxic indeed to certain animal species such as the guinea pig. We are talking here about the so-called dioxins. The dioxins are a group of compounds with a basic skeleton structure of dibenzo-p-dioxin (two benzene rings joined by two oxygen bridges). The most toxic members of this family of chemical compounds are those with a high degree of chlorination, such as TCDD (2,3,7,8-tetrachlorodibenzodioxin). Also compounds from

2,4-D 2,4,5-T

MCPA Dichlorprop

Cl

Cl

Cl

Cl Cl

Cl Cl

Cl

O C O C

C

CH O

OH

CH₂ CH₂

CH₃

O C

O

OH

CH₂ O C

C

CH CH CH₃

Fig. 20.17. The structures of selected chlorophenoxy herbicides.

the group of dibenzofurans (which contain two benzene rings fused to one furan ring in the middle) are counted when we talk about dioxins generally in a toxicological context.

Gradually the chemical companies learnt to con-trol the syntheses in such a way that the formation of dioxins and dibenzofurans was minimized. Also a shift to some different members of the synthetic auxins (other than 2,4,5-T) meant a reduction in the toxic risk, since impurities other than TCDD were formed. In a number of European countries 2,4,5-T was never approved. However, huge amounts of, for example, 2,4-D were used in the 1980s and 1990s. After having solved the problems with the dioxin impurities these herbicides were regarded as just about perfect.

Nevertheless, at the end of the 1990s these com-pounds were found more and more frequently in groundwater, e.g. in European countries such as Denmark. Prior to this a number of evaluations based on models of soil composition and structure had denied that this could happen. In many coun-tries most of the compounds were now required to go through a new process of approval, resulting in several of the compounds being abandoned.

However, MCPA stayed in common use.

In 1970 USDA halted the use of 2,4,5-T on all food crops except rice, and in 1985 EPA termi-nated all remaining uses of this herbicide in the USA. The international trade of 2,4,5-T is restricted by the Rotterdam Convention, which covers pesticides and industrial chemicals that have been

banned or severely restricted for health or environ-mental reasons.

Examples of herbicides taking over after a reduc-tion in use of the chlorophenoxy herbicides include dicamba, triclopyr, prosulfocarb and glyphosate (Fig. 20.18). As an example from Europe, the Danish registered use of herbicides in 2006 amounted to 2500 t of which 42% was glyphosate followed by prosulfocarb (22%) and MCPA (12%).

Dicamba, a benzoic acid herbicide which can be applied to leaves or soil, controls annual and per-ennial broadleaved weeds in grain crops and grass-lands, and brush and bracken in pastures. It will kill broadleaved weeds before and after they sprout. Legumes are also killed by dicamba. In combination with a phenoxyalkanoic acid or other herbicide, dicamba is used in pastures, rangeland and non-crop areas (fence-rows, roadways and wastage) to control weeds. The reported oral LD₅₀ for dicamba in the rat ranges from about 750 to 1700 mg kg⁻¹ BW. However, dicamba is very irritating and corrosive and can cause severe and permanent damage to the eyes.

Triclopyr is a systemic, foliar herbicide. It is used to control broadleaved weeds while leaving grasses and conifers unaffected. The oral LD₅₀ of triclopyr in rats is about 700 mg kg⁻¹ BW.

Prosulfocarb has low acute oral toxicity in the rat (LD₅₀= 1820 mg kg⁻¹ BW in males and 1958 mg kg⁻¹ BW in females). The compound was re-evaluated in the EU in 2007, which resulted in the approval of an ADI of 0.005 mg kg⁻¹ BW. Fig. 20.18. The structures of selected modern herbicides.

Glyphosate, N-(phosphonomethyl)glycine, is a broad-spectrum systemic herbicide used to kill weeds, especially perennials. It is absorbed through the leaves, but may also be injected into the trunk, or applied to the stump of a tree. It was patented initially by Monsanto Company in the 1970s under the trade name Roundup^®. The US patent expired in 2000. Glyphosate is currently the most used herbi-cide in the USA, where some 2270–3630 t are used on lawns and yards and 38,500–40,800 t are used in US agriculture annually. The mode of action is to inhibit an enzyme involved in the synthesis of the amino acids tyrosine, tryptophan and phenylalanine.

It is absorbed through foliage and translocated to growing points. Because of this mode of action, it is effective only on actively growing plants; it is not effective as a pre-emergence herbicide. The toxicity to mammals is very low indeed with an acute oral LD₅₀ for the rat of 5600 mg kg⁻¹ BW.

In conclusion, the herbicides we use in our food production have changed from compounds show-ing an oral LD₅₀ to mammals of about 30 mg kg⁻¹ BW to compounds which in practice we classify as acutely non-toxic with LD₅₀ values ranging from approximately 1000 mg kg⁻¹ BW and upwards, to several grams per kilogram of body weight. In most cases relatively few compounds dominate the use.

Fungicides

The presently known fungicidal compounds that have or have had a use as fungicides can be broadly divided into the following four classes when it comes to their mode of action: (i) inhibitors of the electron transport chain; (ii) inhibitors of enzymes;

(iii) inhibitors of nucleic acid metabolism and pro-tein synthesis; and (iv) inhibitors of sterol synthesis.

The oldest fungicide in use – sulfur – disrupts elec-tron transport along the cytochromes and has been used as a non-systemic contact and protective fungi-cide (and acarifungi-cide) from long back in time, normally applied as sprays or a dust. It is relatively non-toxic to mammals, but can cause irritation of the skin and mucous membranes. Copper (i.e. Cu²⁺ ions) in the form of the so-called Bordeaux mixture (a mixture of copper sulfate with calcium hydroxide) has been used for more 150 years, originally especially in vineyards to control downy mildew. Later it was also used to control fungal diseases in potatoes, apples and hops.

Copper acts by causing a non-specific denaturation of proteins and enzymes necessary for the fungus.

Both of these fungicidal agents had to be applied to

the plant leaves before the attack of the fungus, i.e.

they protect the plant by killing the attacking fungus before it has penetrated the plant.

The mercury fungicides – starting with the inor-ganic salts such as the mercuric and the mercurous chlorides together with mercuric oxide, followed by the metallo-organic compounds such as methyl-mercury – also acted this way. All methyl-mercury com-pounds have been phased out of use today due to the extreme toxicity of this metal.

Although the fungicides used nowadays in the production of different food commodities in gen-eral show low acute as well as chronic toxicity, historically some of the most tragic epidemics of pesticide poisoning were due to fungicides. Mostly they occurred because of mistaken consumption of seed grain treated with such fungicidal com-pounds as organic mercury or hexachlorobenzene (HCB). For example, close to 4000 people in south-east Anatolia (Turkey) developed porphyria during 1955–1959 due to long-term ingestion of HCB, a fungicide added to wheat seeds. The expo-sures led to the development of bullae on sun-ex-posed areas, hyperpigmentation, hypertrichosis and porphyrinuria. The condition was called kara yara or black sore. Several breast-fed children under the age of 2 years whose mothers had ingested HCB-treated grain died from a disease known as pembe yara or pink sore. In a follow-up study of 252 patients some 20–30 years after exposure, many had dermatological, neurological and orthopaedic symptoms and signs. In Iraq an outbreak of poisoning happened in the winter of 1971/72 in a rural region where farmers con-sumed home-made bread made from seed wheat treated with a methylmercury fungicide. Analysis of the flour used gave an estimated amount of 1–4 mg of mercury per loaf of bread. The bread was eaten over a period from 2 weeks to 2 months depending on the family in question. Out of 49 children investigated clinically 2 years after the incident, 40 still had symptoms relating to the nervous system.

Indeed, fungicides that had to be applied prior the attack and that were effective only when present on the plant surface represented the sole means of protecting plants from fungal diseases until the emergence of the first systemic fungicides.

These finally started to appear in 1960 as a result of many attempts to develop compounds that were able to enter the plant and consequently could pre-vent penetration of the fungus from within or kill

fungus which had already penetrated. The first systemic fungicide for use in horticulture was the compound triamiphos (Wepsyn^®). Within the next approximately 10–15 years this was followed by others such as pyrazophos (Curamil^®), dimethiri-mol (Milcarb^®), oxycarboxin (Plant wax^®), beno-myl (Benlate^®) and thiabendazole (Tecto 60^®).

The present-day fungicide market is character-ized by being very diverse not only with regard to the number of compounds, but also the variation of their basic chemical structures. Major groups include benzimidazoles, dicarboximides, dithiocar-bamates, OPs, strobilurins and (tri)azoles. Most fungicides approved today have very low acute toxicity indeed, whether surface active or systemic, and whether belonging to one or the other chemi-cal group:

● Benzimidazoles (e.g. thiophanate-methyl): rat oral LD₅₀= 6640 mg kg⁻¹ BW.

● Dicarboximides (e.g. vinclozolin): rat oral LD₅₀> 10,000 mg kg⁻¹ BW.

● Dithiocarbamates (e.g. maneb): rat oral LD₅₀= 3000–8000 mg kg⁻¹ BW.

● OPs (e.g. fosetyl-Al): rat oral LD₅₀= 5400 mg kg⁻¹ BW.

● Strobilurins (e.g. azoxystrobin): rat oral LD₅₀> 5000 mg kg⁻¹ BW.

● (Tri)azoles (e.g. tebuconazole): rat oral LD₅₀= 1700 mg kg⁻¹ BW.

Now let us look into the possible chemical food safety problems with fungicides in today’s produc-tion of food commodities. In general fungicides are cytotoxic and many show positive results in one or more in vitro mutagenicity tests, a characteristic inherent to this group of biocides. Several chemical classes of fungicide – the imidazoles (imazalil), the triazoles (propiconazole, myclobutanil, tebucona-zole, triflumazole) and the morpholines (dimetho-morph) – inhibit sterol (ergosterol) production and affect membrane synthesis by inhibiting cyto-chrome P450 enzymes in the sterol pathways. The process of steroidogenesis seems to be highly con-served throughout living organisms, and indeed several fungicides (fenarimol and prochloraz) inhibit aromatase (converts C19 androgens to aro-matic C18 oestrogens) activity in mammals and affect mating behaviour. This being said, most fungicides still seem to pose few problems, the only exception possibly being the dithiocarbamates (Fig. 20.19).

Fig. 20.19. Dithiocarbamates (including polymeric dithiocarbamate fungicides = ethylenebisdithiocarbamates).

Within the group of dithiocarbamates we find some polymeric fungicides, i.e. the ethylenebis-dithiocarbamates (EBDCs), a group of non- systemic (surface-acting) fungicides that includes maneb, mancozeb and metiram. EBCDs are used on a wide range of crops worldwide including potatoes, cereals, apples, pears and leafy vegeta-bles. They control many fungal diseases such as blight, leaf spot, rust, downy mildew and scab.

Mancozeb is also used for seed treatment of cotton, potatoes, maize, safflower, sorghum, groundnuts, tomatoes, flax and cereal grains.

Degradation of these compounds will, on the plant at the field as well as during processing, give rise to the formation of the metabolite ethylenethio-urea (ETU). According to a report from the US EPA from 2002, there is sufficient evidence that three dithiocarbamates – maneb, mancozeb and metiram – can induce a common effect (thyroid cancer) by the formation of the common metabo-lite, ETU. These three chemicals should be grouped, as in the past, when conducting a risk assessment.

When measuring actual residues most authorities today also analyse for ETU.

The industrial development of new fungicides has resulted in a wide range of structural classes being used, representing both surface-active com-pounds and systemically acting agents which are taken up by the plant.

20.4 Persistent Organic Pollutants