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Pesticides: origins and challenges.

Agriculture depends to a great extent on pesticide use for managing pests. These range from insects which damage or consume crops, to weeds which compete for moisture and nutrients, to fungi which grow on seeds preventing germination, to rodents which eat stored food. Pest is defined in Canadian legislation as "any injurious, noxious, or troublesome insect, fungus, bacterial organism, virus, weed, rodent, or other plant or animal pest and includes any injurous, noxious or troublesome organic function of a plant or animal".

Pesticide is a generic term of a product which is intended to control a pest. Pesticides which control insects are insecticides; those to control weeds are herbicides; those for fungi, fungicides; and so on. Pesticides are also classified by the chemical class to which they belong; they may be organochlorine compounds (OCs), organophosphorus compounds (OPs), carbamates, pyrethroids, phenoxy acids triazines, dinitroanilines, sulfonylureas, juvenile hormone analogs, insect growth regulators, microbial toxins, etc.

Pesticides may also be classified according to the ease with which they degrade in the environment. Thus, some persist and may be accumulated in the food web. Others are readily degraded photochemically by sunlight, or by hydrolysis under suitable conditions of moisture and pH, by oxidation or reduction, and by metabolism in plants, animals, or microorganisms. Degradation products are usually more water soluble, and therefore more easily excreted by animals than the pesticides themselves.

Pesticides are categorized by their selectivity for the target organism. Those affecting a wide range of organisms are broad-spectrum pesticides, and those controlling only a very specific group are narrow-spectrum pesticides. The basis of pesticide use may be physiological or ecological sensitivities which make the pest vulnerable.

The hazard to the pesticide applicator, the environment, and the consumer leads to a fourth way of classifying pesticides. The most hazardous (Restricted) may be sold to or used by only specially-trained operators. Pesticides which are least hazardous to the applicator, and which can safely be used around dwellings, are sold in hardware stores and pharmacies, and are classified Domestic. Those which can be used by commercial operators and farmers, but which are not generally available to the consumer are Commercial. Table 1 shows this classification system.

[TABULAR DATA OMITTED]

Why are Pesticides Controversial?

Pesticides are by nature toxic to some life forms and therefore represent a potential hazard to human health. The risk posed by pesticide use varies widely. Immediately life-threatening products form only a small prt of the pesticides available. The public has often expressed concern about health threats related to the propensity of pesticides to cause cancer in test animals. However, a quantitative assessment of these risks must be made in the context of similar risks from involuntary exposure to other chemical contaminants, eg., from indoor air pollution, gasoline vapour blow-by in self-serve gas stations, or second-hand tobacco smoke. Pesticide residues in foods at levels permitted in Canada threaten human health much less than all of the above. Enforcement of Canadian regulations is important to maintain this safety.

Pesticide use impacts natural ecosystems. By reducing or removing one or more ecosystem members, the use of a pesticide can disrupt that system, causing other changes. The death of target insects will affect their predators. If the predator population is insufficient to prevent economic damage to a crop, pesticide use becomes attractive. Removal of the food source, though, will force the predators to look elsewhere for food, or to face starvation.

Non-target organisms are also exposed to pesticide, and the effects on birds, fish, beneficial plants and fungi, and beneficial insect species must be considered to ensure that the ecological price of using the pesticide is acceptable.

The impact of pesticide use on natural enemies of pest populations can outweigh the benefits of reducing the pest population. When the food supply of natural enemies is reduced, their population must decline, often catastrophically. As the pest population recovers (and it is usually impossible to eradicate a pest population), it will face fewer natural enemies than before and may return in larger numbers.

There is a choice of pesticides. Broad-spectrum pesticides were developed first. One of the first was salt, used against plant, animal, and fungal pests. Organic chemical pesticides such as DNOC and DDT preceded the insect growth regulator methoprene and the discovery of allelopathic chemicals in some plants. Thus, although broadspectrum pesticiees are more familiar and are generally cheaper, costs other than the purchase price should be considered.

There are two methods of pest control: chemical control methods, and biological control methods. Neither is perfect: chemical methods tend to be ecologically disruptive, and biological control methods are often not immediately effective. In practice, a combination of the two, together with cultural methods such as carefully coordinated tillage, provide the most sophisticated integrated pest management (IPM) approach favoured by many progressive pest management workers today. IPM requires an understanding of the relationships between pesticide use, natural enemy populations, and ecosystem sensitivities.

Agriculture involves manipulation of the natural environment to produce food and fibre. The challenge facing us is how to carry out agriculture in a manner which can be sustained for centuries. The chemistry involved in addressing these challenges is a field of study which is both broad and professionally rewarding.

History of Pesticides

Since the beginning of recorded history, pest control has used (in roughly chronoligcal order) superstition and social practices, plant extracts with presticidal action, inorganic chemical preparations, and products from the synthetic organic laboratories of the chemical industry. The history of pest management stretches over several millenia; a few highlights are described here.

A method first mentioned in the Geoponika (a 6-7th century collection of agricultural practices from Greek and Roman times), was used until the late 19th century for getting rid of field mice. The farmer was to write on a piece of paper:

"I adjure the mice taken in this place, that you do me no injury yourself, nor suffer another to do it; for I give you the ground [the field or area]; but if I again take you on this spot I take the Mother of Gods to witness I will divide you into seven parts."

The paper was to be taken to the infested field before dawn and placed under a stone with the writing visible (see Figure 1). No mention is made of the degree of mouse literacy required for pest management success.

The ferocious nature of an animal was sometimes used without the living animal actually having to be present. Pliny noted that mice could be kept from stored grain by sprinkling it with the ashes of a cat or weasel, or with the water in which one of these had been boiled. He warned, however, that the odour of this preparation might taint bread made from the grain.

The use of salt as a soil sterilant dates from biblical times.

"And Abimelech fought against the city all that day; and he took the city, and slew the people that was therein and beat down the city, and sowed it with salt" (Judges 9:42).

Xenophon (4th Century BC) and the Romans (146 BC) used salt on the fields of vanquished enemies, believing that crops could never again be grown there. The 9th century Arabic writer, Ibn Qutayba, noted that equal parts of salt and duck excrement would kill vegetable crops. In the 17th century, several authors observed that salt had herbicidal effects but in some instances, it would also strengthen grains and pulses. The concept of the application rate (dose) determining the overall benefit or damage was noted in Scottish writing from the 18th century. During this period, salt was used as an insecticide in preparations of botanical extracts combined with brine. Similarly, combinations of salt with both inorganic and biologically-derived material were employed to control fungi on seed grain.

Fate and Type of Pesticide

Table 2 shows the general trend from inorganic and botanical pesticides to botanical and organic chemical ones. A parallel trend exists in the environmental behaviour of these pesticides, the early inorganic materials remaining in their original form, while the synthetic organic pesticides of the last 20 years or so are degradable over a period of days to weeks.

In general, the inorganic pesticides listed can change oxidation state, but cannot degrade. Botanical and synthetic organic pesticides usually degrade, the rate of degradation determining whether the pesticide will persist. If a pesticide persists long enough, it can migrate to other parts of the environment. Volatilization of a pesticide allows it to be moved as drift to contaminate nearby land or distant ecosystems. Runoff and movement within the soil can transport residues to ground and surface waters. Thus, longer-lived pesticides may affect forms of life and ecosystems never considered when the pesticide was originally registered for a specific pest. Shorter-lived pesticides can undergo chemical, photolytic, and/or metabolic degradation before translocation becomes a problem; however,

[TABULAR DATA OMITTED]

volatilization and runoff may occur shortly after the pesticide has been applied. The fate of pesticides is determined to a great extent by their physical and chemical properties.

Table 3 lists a diverse and historically representative selection of pesticides as a framework within which to discuss their comparative environmental degradation rates.

Salt: Common salt, sodium chloride, dissociates completely into sodium cations and chloride anions. Both occur naturally; however, saline water and soil are less able to support agriculture and saline water is not suitable for human and some aquatic life. Salt is persistent, but hardly foreign to the environment. It poses a problem if present in excess. The fact that it cannot degrade limits its capacity to be detoxified. Only dilution reduces the severity of contamination.

Arsenicals: Arsenic compounds undergo reduction and oxidation, but the arsenic remains. Thus, although arsenic is derived from natural sources, it cannot degrade to nontoxic material. Where arsenicals have been used on soil, residues remain for many years.

Thiram: The fungicide thiram (bis[dimethylthiocarbomyl] disulphie) is used to control Botrytis species on lettuce, ornamentals, soft fruit, and vegetables; rusts on ornamentals; and as a seed treatment. It is hydrolyzed over a period of weeks to the volatile products carbon disulphide and dimethyl amine.

Pentachlorophenol: Pentachlorophenol is an effective fungicide which has herbicidal properties. It readily under goes photdegradation in aqueous solution by sunlight. However, in soil and on wood, it is often not directly exposed to sunlight. Pentachlorophenol contains chlorinated dibenzo-p-dioxins in the parts-per-million to hundreds of parts-per-million range, formed as accidental by-products during synthesis. These dioxins are notirously stable except to environmental photodegradation; however, their extremely low water solubilities prevent their rapid partitioning into water. Pentachlorophenol persists itself under various conditions, and its contaminants are even more persistent. Only its low cost of production and its wide use as a wood preservative and an anti-sapstain agent by the lumber industry has enabled it to survice environmental scrutiny.

DDT: Dichlorodiphenyltrichloroethane (DDT) has had a long history of saving human lives, particularly through its effectiveness against mosquitoes carrying malaria and yellow fever. This efficacy prompted a creative American chemist to compose the following limerick:

A mosquito was heard to complain

That a chemist had poisoned his brain.

The cause of his sorrow

Was paradichloro-

Diphenyltrichloroethane.

DDT has been extensively used for over 45 years to protect crops against insect pests. Its current use for this purpose is largely in the developing world.

DDT fell into disfavour because of its failure to degrade to innocuous products in the environment. It was banned in the US in 1973 because it was considered to be a carcinogen; however, birds of prey (such as the peregrine falcon) accumulated residues of the metabolite DDE and suffered hormonal effects which weakened the shells of their eggs, thus threatening their ability to reproduce. DDT degrades metabolically to DDE, which in birds and fish, partitions to fatty tissue and is concentrated over time. Metabolic degradation beyond DDE is slow, requiring a period of many years. In the environment, DDT is resistant to photochemical and microbiological degradation.

The drastic reduction in the use of DDT in the early 1970s in Canada resulted from concern about its effects on wildlife. In less well-off parts of the world, DDT is used because of its low cost and continued efficacy against insect pests. A major issue accompanying continued DDT use, however, is the development of resistance (a biological phenomenon limiting the long-term use of most pesticides) within insect populations so that the effective dose required increases dramatically over time. Increaseduse levels can be tolerated if only the cost of synthesis is considered; however, long range transport of chlorinated pesticides and related compounds has been well documented, and residues of DDT and its metabolites are present in Arctic food webs, and in Inuit propulations who hunt and fish. The human population in the Arctic thus receives persistent OC insecticides which have been used to benefit people half a world away.

2,4-D: The herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) is the most commonly used herbicide in western Canada. It is very effective in the control of broadleafed weeds in grains; some 4300 tonnes of 2,4-D are used each year on the Prairies alone. Both ester and amine salt formulations have been used, with the esters falling out of favour because of their volatility and concomitant drift problems. Esters of longer chain alcohols are less volatile, but amine salt formulations have little volatility. The latter are now used almost exclusively.

2,4-D is excreted unchanged by mammals within 24 hours. The degradation of 2,4-D in soil proceeds quite quickly, largely by microbiological metabolism. The resulting chlorophenol is also readily metabolized the same way; thus, 2,4-D in soil does not create a long-term residue problem. 2,4-D is soluble enough in water that it does not readily associate with soil organic matter, and it has been detected in ground water under sandy soils. When 2,4-D or 2,4-dichlorophenol are introduced into surface waters, microbial populations capable of degrading it are not readily available. Photolytic degradation is effective in degrading residues which are exposed to sunlight, but residues in water systems persist at law levels for months to years.

Malathion: The phosphorothioate insecticide malathion (O[2-(diethyl succinyl]) O,O-dimethyl phosphorothioate) is perhaps the least acutely toxic organophosphorus pesticide in common use. While it can inhibit cholinesterase activity in the human nervous system, it is relatively easily metabolized and degraded. It must undergo oxidation to form the more insecticidal oxon form. At the same time, it is vulnerable to hydrolysis and is readily metabolized and degraded within hours to days of application.

Carbaryl: Carbaryl (O-[1-naphthyl] N0methyl carbamate) is a member of the carbamates which degrade to methylamine, carbon dioxide, and the parent phenol or alcohol. This insecticide is systemic, ie., it can travel within a treated plant from the point of exposure to other parts of the plant. In the environment, it undergoes hydrolysis plus ring and methyl group hydroxylation, and is therefore not persistent.

Trifluralin: The dinitroaniline herbicide trifluralin (N,N-di-n-propyl, 2,6-dinitro 4-trifluoromethyl anilin) is peculiar in that it is not readily taken up by plants, nor is it susceptible to biological degradation. On the other hand, it undergoes ready reduction of the nitro groups is anaerobic (flooded) soils in a few days. In aerobic soils, it is much longer lived, undergoing oxidative dealkylation over a period of months to years, depending on conditions. Under dry conditions it appears to degrade even more slowly. In soils having substantial organic matter content, sorption prevents trifluralin from being fully active. Under these conditions, several times that usual dose may be required to control grassy weeds. Over the long term, particularly following dry years, slow release from the sorption sities may cause residue persistence problems for farmers wishing to grow a rotation of crops.

Methoprene: Methoprene (isopropyl (E,E)-(RS)-11-methoxy-3,7,11-trimethyldodeca-2,4-dienoate) is a synthetic juvenile hormone analogue very effective in the control of mosquite larvae. It is quite quickly broken down in the environment following use; therefore, the timing of its application is crucial to ensure its presence when developing mosquitoes are at their most sensitive stage. An unsaturated isoprenoid fatty acid ester, methoprene is soon oxidised and hydrolyzed. Compared to early anti-mosquito pesticides such as DDT, methoprene is expensive to produce, but an environmental bargain.

Abamectin: The macrocyclic lactone insecticide/fungicide abamectin is one of a new family of anthelmintics called avermectins used to control intestinal parasites in cattle and sheep. The remainder of its structure is reminiscent of an oligosaccharide. It survives passage through the gut of the animal where it controls parasitic nematodes. Mammalian toxicity is very low, and it is susceptible to hydrolytic degradation at the lactone linkage.

Conclusion

The group of pesticides discussed above shows the variety of chemical families which has been utilized over the history of pest control. It is also evident that, far from acting similarly (in environmental terms) pesticides have widely varying properties causing them to impact pests and other life-forms in many ways. (Additional information on solubilities, partition coefficients, vapour pressures, and photochemical

Table 3

A Historical Selection of Pesticides with

Comments on Their Persistance and

Environmental Fate
Pesticide Environmental Fate Rating (a)
NaCl elements remain in system R
As and Cu elements remain in system R
thiram hydrolysis to [CS.sub.2] and D
 diethylamine
pentachlorophenol photochemical degradation W/C
DDT dehydrohalogenation R
 reductive dehalogenation R
2,4-D microbial metabolism, D/W
 hydroxylation
malathion oxidation, hydrolysis D
carbaryl hydrolysis, hydroxylation D
trifluralin oxidative N-dealkylation, D/R
 reduction
methoprene oxidation, metabolic D
 processes
abamectin hydrolysis, oxidation R/D
(a) R = residue remains for years; D = degrades to innocuous
products within days to weeks; W = residues remain in
contaminated water for years; C = persistent contaminants
(dioxins).


quanum yields reinforce this conclusion.) Clearly, only some pesticides accumulated in food chains, but these are of concern to organisms at the top of the food chain, including humans in the Arctic. Many pesticides now available do not accumulate, but are excreted quickly, or are degraded after use rapidly enough to be of little environmental concern.

A combination of the ingenuity of the chemist in conjunction with that of the entomologist, the weed scientist, the soil scientist, the plant or animal pathologist, and the ecologist is enabling the development of integrated pest management systems which are increasingly environmentally friendly. In agriculture, which necessarily involves the manipulation of the natural environment to yield a sustained food supply, the enlightened use of the best pesticides available will cause few serious direct environmental impacts.
COPYRIGHT 1991 Chemical Institute of Canada
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Author:Webster, G.R. Barrie
Publication:Canadian Chemical News
Date:Aug 1, 1991
Words:3077
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